Multicentered Bonding and Quasi-Aromaticity in Metal-Chalcogenide Cluster Chemistry

Journal ~)/" Cluster Science, Vol. 6, No. 3, 1995

Zhida Chen ~' 2

Re«eived December 30, 1994

On the basis of the Iocatized molecular orbital (LMO) theory, the bonding schemes Ihr the following types of cluster compounds are briefly reviewed in this paper: the linear [Et4N][C1,FeS,MoS2Cu(PPh3), ] cluster, triangular trinuclear [M3(lt3-)()(l~--Y)3] ~'+ (M=Mo, W; X=O, S; Y=O, S, Se) clusters, triangulated polyhedral clusters: closo-boranes B,,Hù 2-, octahedral [Co6(CO)14] 4~~ , [Ni2Co4(CO)14] 2 and [Cod/t3-X)s.L«,]" (X=S, Se; L= PPh 3, PEt~, CO; n=0,1), as weil as quasi-aromatic cluster ligands in cubane-type [Mo3S4.MLù] 14«q)+ (M=Mo, W, Fe, Ni, Cu, Sn, Sb: L = ligand) and sandwich-type [ Mo3S a . M. 8 4 Mo3] s + (M = Mo, Sn, Hg). We pur elnphasis upon the characteristics of multicentered bonding in these ctuster molecules, and, especially, point out existence of a novel species of quasi- aromatic cluster compounds.

KEY WORDS: Multicentered bonding; quasi-aromaticity; Iocalized MO; cluster: bonding.

INTRODUCTION

As a result of the determination of molecular geometrical configuration for diborane B2H6, the three-centered two-electron (3c-2e) hydrogen bridged bonding B-H-B in this molecule was first recognized, Diborane provides, indeed, a prototypal example of 3c-2e bonding, diflEring from the ordinary two-centered two-electron (2c-2e) bondings in the conventional valence theory. It is obvious that the concept of the 3c--2e bonding has gone beyond the limitation of the classical valence theory, in which the pair of bonding electrons is located essentially in between the pair of adjacent atoms. It is beyond any doubt that the idea of multicentered bonding is an

~The Rare Earth Research Center, Department of Chemistry, Peking University, Beijing 100871, China. 2 To whom all correspondence should be addressed. 357

1040-7278/95/0900-0357507,50/0 ! 1995 Plenum Publishing Corporalion 358 Chen important development in modern chemical valence theory. So far, it is already well-known that the stoichiometries of the boranes, fi'om the simplest B2H«, to the complex higher boranes, together with the number of electrons available, do not permit a single Kekule structural formula for these compounds. Several types of 3c-2e bonding, such as the 3c-2e hydrogen bridged bonding B-H-B and other 3c-2e boron bonding have been suggested in bonding schemes for the series of boranes. In other compounds with electron deficiency, such as methyl bridged organometallic compounds, there is also certain valence electron delocalization compared with 2c-2e bondings. Also, their bonding schemes cannot be limited to the classical valence theory, similar to the case of boranes. As a matter of thct, there exist extensive multicentered bondings in cluster compounds. It is interesting to note that in certain cluster compounds the nmlticentered bonding electrons join up each other and are bound to delocalize further in the whole molecule, resulting in quasi- aromaticity ~~~. Such compounds exhibit unusual thermodynalnic stability, unusual chemical reactivity and unusual spectral properties. In this paper, bonding schemes of such cluster compounds will be briefly reviewed on the basis of quantum chemical energy-localized molecular orbital (LMO) calculations at certain semi-empirical approximation level ~z ~. Herein we put emphasis upon characteristics of multicentered bondings and their possible quasi-aromaticity for these cluster compounds.

LINEAR TRINUCLEAR CLUSTER COMPOUNDS In our research on the Chemical modeling of the active center of nitrogenase in biological nitrogen fixation, certain linear trinuclear Mo-Fe-S cluster compounds have been considered as reactive fi'agments tbr the syntheses of modeling cluster compounds ~2~. Thus a number of linear trinuclear Mo-Fe-S cluster compounds have been synthesized and characterized. Moreover, a variety of heterotrimetallic linear cluster compounds with the core [M'S2MS2M"] (M=Mo, W; M'=Fe; M"=Cu, Ag) have been recently obtained via condensation-redox reactions with the simple anion (MS«) 2 (bi= Mo, W) as starting materials ~3~. The versatile reactivities of (MS4) 2 anion (M=Mo, W) in these synthetic reactions have been extensively investigated. As an example, the cluster anion [C12FeS2MoS2Cu(PPh3)2] - (Fig. 1)in [Et4N][C12FeS2MoS2Cu(PPh3)2] ~4~ is herein discussed. From the point of view of the LMO theory, Ihr the tetrahedral (MoS4) 2 anion, besides the four (Mo-S) cr bondings, there is some delocalization of electrons fi'om the lone pairs on the S atoms to the Mo atom, with each S atom contributing two lone electron pairs, tbrming MoS ~ bonding. When the "building-blocks" Cu(PPh3)3CI and MetaI-Chalcogenide Cluster Chemistry 359

$4 P2 01 A $I o ks Y- k'(o Cu Fe Ph

Ca) (b) Fig. 1. Geometrical configuration (a) and bonding scheme (b) of the cluster anion [ CI2FeS2MoS2Cu( PPh3)2] -.

[S2MoS2FeC12] 2 t41 (or FeC12 and (MoS4) 2-) are assembled in the solvent, one of the bonding 7z-electron pairs between the Mo and S atoms is further delocalized into the "intruder" Fe and Cu atoms respectively, forming three-centered Fe-S'-Mo and Mo-S-Cu bondings, instead of the direct S'-Fe and S-Cu bonding (Fig. 1). The Fe-Mo and Cu-Mo distances have been found to be 2.769 A and 2.786 A, respectively~4( However, the LMOs calculated for this cluster anion show ~5~ that there exists only one Fe-Mo a bond without any direct Cu-Mo bond. Thus the multicentered bondings mentioned above are obviously responsible for the shorter Cu-Mo distance. Meanwhile the LMO bonding scheme is in good agreement with the following sequence of experimental frequencies of the Fourier transform infrared spectra(S~:

Mo-S > Mo-S' > Fe-C1 > Fe-S' > Cu-S

TRIANGULAR TRINUCLEAR CLUSTER COMPOUNDS

Mo3S4(d[p) 4 , H20 During the past few decades, a number of trinuclear cluster compounds with Mo atoms in the IV oxitation state forming a discrete equilateral triangle have been reportedt6( It should be pointed out that in recent years systematic crystal structure analyses and chemical reactivity studies for a series of typical trigonal pyramidal cluster compounds have been carried out by Lu and bis co-workers. Thus, they have been able to establish a new concept of quasi-aromaticity for the puckered six- membered ring [Mo3(/t-S)3] 6+ which exhibits benzene-like behavior ~7~ in [M%(it~-Sc)(It-Sb)3(/l-dtp)(x-dtp)3(OH2) ] (I) (Fig. 2), where dtp denote {S2P(OEt)2} ~, and S,., Sc,, and X denote the capping S atom, the 360 Chen

Mo Mo Mo

S~ S~ S~

~c Fig. 2. Molecular configuration of the cluster molecule [Mo3(i«3-S)(lt-S)3(/t-dtp) (x-dtp)3(H_,O) ] (dtp=[S2P(OEt2) ]) and quasi-aromaticity of the puckered [M03S~] ring as the most essential part of the [ MO384] 4~ core. H atoms are not shown. bridging S atoms and the chelating dtp ligands, respectively. Structurally, the six Mo-(it-S) bonds have an average length of 2.281 A intermediate between the Mo-S single-bond length of 2.44 Ä and the Mo = S double- bond length of 2.08 A whereas the six bond angles fall into two sets of 96.6 ° ihr /(I«-S)-Mo-(II-S) and 74.3 ° for L(Mo-(IL-S)-Mo). With regard to its chemical reactivity, the reactivity sequence for ligand substitution reactions is H20>/l-dtp>z-dtp»(/«3-S)>(/~-S), with the [Mo3S3] ring kept intact and practically invariant in the course of these reactions. There are two types of addition reactions: simultaneous addition of three S atoms with the formation of three (/«-$2); addition of one metal atom alone to tbrln, for instance, a cubane-like [(Mo3S4)Cu] 5+ or a sandwich-type [(MOBS4)2Mo] s+, with the entire [Mo3S4] 4+ core entering into these reactions as a whole. On the other hand, oxidation ofone of the three (/t-S) atoms is at once accompanied by the rupture of two adjoining Mo--(/L-S) bondings and the formation of two new Mo = O bondings. The resem- blances to CöH~, in all these respects are indeed very striking. From the localized bonding point of view ~~~, for the well-known aromatic planar six-membered ring C6H~, , each of the three 7>electron pairs is localized on three adjoining C atom triads, thus forming three adjoining 3c-2e (p-p-p) zr bondings (Fig. 3). Upon comparing the aromatic and quasi-aromatic C6H6, C3N3H3, B~N3H«, and (B30«,)) ...... with the out-plane non-aromatic ret~rence molecule P3N3CI(, it is shown that a sufficiently large interaction energy between the three (p-p-p) or (d-p-d))z-bondings is necessary for the formation of a closed continuous 7>conjugated system around the ring. Furthermore, of the 19 occupied LMOs for the [Mo~S4] 4+ core of the cluster molecule Mo3S4(dtph. H20 (Fig. 4), the 13 lower-energy LIMOs correspond to the 4 lone electron pairs on the 4 S atoms, the 3 Metal-Chalcogenide Cluster Chemistry 361

C«H~

CMO LMO ~ c~ e,, "(~_'c') <~, et~

cqC C) ~-'-- <22>

Fig. 3. CMOs and LMOs of the benzene molecule CöH«~.

Mo-Sc a bondings and the 6 Mo--S b o- bondings of the valence bond theory. Besides, just like the case of C(,H 6, there are 3 3c-2e 7r bondings, with each Sb atom contributing one lone pair of electrons and one p__ AO perpendicular to the MoSbMO plane while each of the two adjoining Mo atoms contributes one vacant d AO, thus lbrming a 3c-2e (d-p-d) bonding shaped like a Dewar "island," although these 3 "islands" are not at all isolated fi'om each other. This bonding LMO has the plane MoSbMO as a nodal plane. On the side farther away from the atom Sc, the electron clouds from the 3 "islands" superpose themselves to form a conical cloud distribution on account of their cooperative effect, leading to a chemical active center of the molecule at the site of the incomplete cubane-type cluster. On the other hand, such an eflEct does not exhibit itself to any appreciable extent on the other side of the nodal plane, and thus gives rise to three more or less discrete patches of electron cloud concentrated over the three "islands" (cl/ the MO "CT" contour graphs in Fig. 5), forming three other active centers of the cluster compound. It should be noted that just like the case of CöH~,, each pair of adjoining "islands" shares the same d AO of the common Mo atom with a rather large interaction energy of 362 Chen

-0.050 a.u. Moreover, on account of the cooperative effect of the adjoining ùisland," this nonplanar puckered [Mo3S3] ring forms a large three- dimensional closed, completely continuous conjugated lr-electron system (similar to the [C~,] ring in Co, H(, to a good extent), yet with unsymmetri- cal electron cloud distributions with respect to each of the three MoSu Mo planes (thus quite difl'erent from the planar [C(,] ring in C«,H~~). In addition, the remaining 3 higher-energy LMOs are seen to correspond to 3 3c-2e (Mo Mo) a bond (ct: Fig. 4). This leads at once to a clear-cut quantum chemical interpretation of the benzene-like behavior of the cluster naolecule Mo3S4(dtp)4.H,O with regard to its bond length and bond angle structural parameters as well as its ligand substitution, addition and oxidation reactions. (1) the formation of a large three-dimensional closed, completely continuous conjugated 7t-electron system in the nonplanar puckered [Mo3S3] cluster ring in the cluster molecule confers upon this ring additional chemical stability, leading to the strengthening of this cluster compound and shortening of the six essentially equivalent Mo-S b bond lenghts, (2) intactness of this [Mo3S3] ring in ligand substitution reactions, (3) rupture of this large conjugated 7r-electron system and the [Mo3S3] ring as weil, following oxidation of one of the three Su atoms; (4) two different types (M:S) of addition reactions because of the unsym- metrical electron cloud distribution around the puckered [Mo3S3] ring (simultaneous addition of 3 S atoms to lbrm 3/z-S2 ligands vs. addition of only one M. such as Mo, W, Fe, Ni, cu, Sn, Sb). It should be noted that in the addition product [Mo~S7] 4+ there is no longer any 3c-2e bonding located on (Mo-Sb Mo), entirely difl'erent from the case of [ Mo3S4] 4~ cluster core discussed above, It may serve as another evidence for the nature of quasi-aromaticity in the nonplanar puckered [Mo383] ring of certain [ MO 3S 414 clusters. In other words~ this quantum-chemical picture of the 3c-2e (Mo Sh-Mo) (d-p-d) 7: bondings has led us to

<:~ ~( \C g ) ~ (Mo--Mo)

«(Mo Mo} Ol@~ß-- aIC H

o'(C~ C) ~,{S, } o:Me S<: »{q~~ »,{ «,« j

Fig. 4. LMOs of Caft«,t's. the [Mo3S4]a* cluster core. Metal-(halcogenide Cluster Chemistry 363

;, :%, : :l MA_°",', ' ''b ' ',

ù_3;-- "-;.C.' M°I~"° -

:' :" .-';s~:~_o.25

" "" "" 2"1~"~'-O.20 "~.. .. - --0 45 -040 (a) Mo S~~Sb MO~ Mo % (b} Fig. 5. "CT" of ~-LMOs Ihr the [Mo (tt~- S)(it- Sb] 4 cluster corc at heights of (a) 0.0! A. (b) 1.39 A {at the [S~] plane), with line (from innermost to outermost) corrcsponding to values of wavefunction: 0.65, 0,60, 0.55, 0,50, 0.45. and 0.40. The positions of atoms shown in the diagram are the projection of the cluster core onto the sections. visualize the [Mo3S»] ring as et rather stable structural sublevel in the cluster molecule and to account for its quasi-aromaticity as well.

[M3(~.I3-X)(~I-Y)3] 4+ (~/~~/= Mo, W,; X=O, S; Y=O, S, Se)

It is noteworthy that numerous discrete triangular trinuclear cluster compounds with [M3(ll~- X)(ll- Y)3] 4+ cores (,4,1= Mo, W; JC Y=O, S, Se) and various peripheral ligands have been shown to consist of three Mo or W atoms tbrming an essentially equilateral triangle with one capping X atom each above the [M3] plane and three bridging Y atoms below it, thus making a distorted octahedral coordination environment around each Moor W atom with M M bonds ~s "'. It should be noted that some mixed metal clusters with [M3 ,M',(ll3 -- X)(tl -- Y)3] 4+ core (M, M' = Mo, W; X, Y= O, S; n = 0, 1, 2, 3) and mixed oxo/sulfido species of Mo/W clusters [MO,,S 4 ,]4+ (M=Mo, W) have also been reported ~9~1~'~4~ to have isostructures as [Mo3(I~3-S)(Iz-S)3] 4+, constituting the most important structural type of discrete triangular trinuclear cluster compounds with M-M bondings. The LMOs of the kind of cluster compounds with [M3(II)--X)(I~--Y)3] 4+ cores reveal some common bonding feature resembling the prototype cluster compound [Mo3S4(dtp)4.H20]. The

SXO ~~ 3-2 364 Chen

LMOs for the cluster cores [M0304] 4~, [M038cO3] 4+, [M03Oc83] 4+, and [Mo384] 4~ of Cs2[Mo304(C204)3(H20)3] .4H20-1/2H,,C204 (15~ Ba[ MO3ScO3(Hnta)3 ] • 10H20(Ijl, [ Mo3OcS3(dtp)4]. C3H3N2 IH) [Mo3S4(/12-dtp)(z-dtp)3. H20] (Ic') compounds respectively, are shown in Fig. 6. These four cluster cores have different bridging and capping chalcogen atoms, although they have quite similar electronic structures in the core frameworks. Of the 19 occupied LMOs, the LMOs for the two lower levels correspond to the four lone pairs of electrons on the bridging and capping atoms. The LMOs of the next two higher levels correspond obviously to the three a(Mo-X) and the six a(Mo-Y). It is important that for all these cores there exist 3 3c-2e (Mo-I;Mo) ;r bondings, as described above, with each bridging Y atom contributing one pair of electrons from the lone pair occupying the p: AO perpendicular to the Mo-YMo plane and each of the two Mo atoms contributing its unique vacant 4d AO, thus forming a (d-p-d) ?z-bonding (cf. Fig. 4). The delocalization of the lone pair electrons of the bridging atom onto the two adjoining Mo atoms depends mainly on the size and electronegativity of the bridging atom itself, and the degree of delocalization tor a bridging S atom is larger than that for a bridging O atom due to the larger atomic volume and smaller elec-

[Mo~O«'] 4~ [Mo~SO~] ~* [Mo~OS3] 4» [Mo~S«}'*

o'{ Mo»Mo) o I Mo-Mo} -I.20 o (Mo-Mo) cr {Mo--Mo) . _ ...... Mo Mo « / Mo Mo Mo Mo -~ ~'t{\ ,,} o~{Mo--S~) --I .40 MO Mo 7r ( \cJ ) Sù

o ~'("~/~, .... - -~ .... ~" « ~~' o ~, Mo-S , .." " o-(Mo--S.) . o-(Mo--S() ~. e(Mo--O.).-" / o(Mo-Ob)/ z ~~ ~ k{S~) a(Mo--Ob) ." ...... Yuc 1,60 P~ .... ~ / X(Sb) "" ;k(S,) a(Mo-O() / , • // J

-b80 J / x • x / / • / ; \ . k{O«) • / klO, ) ,' X(Ob)

X(Où) 200

Fig. 6. LMO energy levels of [M0304] 4 , [Mo3ScO3] 4~ , [Mo30«$3] 4~ , and [Mo3S4] 4~ (subscripts b, c denote bridging and capping atoms, respectively). MetaI-Chalcogenide Cluster Chemistry 365 tronegativity of the S atom. For the next highest level, the LMOs correspond obviously to the 3 2c-,2e (Mo Mo) a bondings. Meanwhile, Fig, 6 shows clearly how the energy levels of these cluster cores change with the diflkrent bridging and capping atoms. In general, the LMO energy levels for these four cluster cores are raised to some extent as the number of S atoms is increased. In particular, the LMO energies for the (Mo Mo) er bondings are only inappreciably raised as the bridging O atoms are replaced by the S atoms. On the other hand, the increasing trends of the LMO energies for the (Mo-Y-Mo) re bondings and the (Mo-Y) er bondings are much more apparent, arising essentially from the difference in electronegativity for the valence orbitals of the constituent O and S atoms. The Mulliken bond order decreases in general as the bond length increases. However, for the Mo-Y atom pairs in [Mo304] 14+, [M038cO3] 4~, [M03Oc83] 4 and [Mo3S4] 4~ , the Mulliken bond orders are 0.447, 0.449, 0.573, and 0.565 (in unit e), respectively, while their bond lengths are 1.921, 1.917, 2.282, and 2.283 (in unit A) in the same order. It may be seen that though the bond lengths of Mo-(/l-S) in [MO3OcS3] 4+ and [Mo384] 4+ are quite larger than those of Mo--(ll-O) in [Mo304] 4+ and [MO38cO3] 4+, the Mulliken bond orders of Mo-(ll-S) are unusually large compared to those of Mo-(/l-O). This change in Mulliken bond orders following the replacement of the bridging O atoms arise indeed from the large delocalization of the lone pair electrons of the bridging S atoms into the vacant 4d AO in each of the two adjoining Mo atoms, resulting in stronger bonding in the (Mo--S»-Mo) re bondings relative to the (Mo-O»-Mo) re bondings. Even though the cluster cores, [MoßO4] 4+, [M03ScO3] 4+, [M030~S~] 4+, and [M03S4] 4+ all have identical bonding schemes, different bridging atoms make these cluster cores exhibit more or less different degrees of quasi-aromaticity. The large delocalizability of the lone pair- electrons in a bridging S atom gives rise to a greater stability of the local delocalized (Mo-S»-Mo) re bontting; yet on the other hand, it enhances inter-LMO interaction for the three (Mo-S»-Mo) re bondings. Both the interaction energy and the depopulation of the bridging atom show consistently that the three re bonds with bridging S atoms form a more stabi- lized re-coniugated system around the [Mo3S3] ring than in the case of bridging O atoms. Furthermore, the more labile rt-electrons around the [Mo383] ring give rise to a larger diamagnetic susceptibility compared with the [Mo303] ring. This conclusion is in good agreement with Shibahara's experimental measurements ~iv~ made for Ba[Mo3SO3(Hnta)3] • 10H,O and CaLs[Mo3S4(Hnta)2(nta)] • 12H20. It is quite apparent that the size and electronegativity of the bridging atoms, which play a more decisive role in the delocalizability of the lone pair re-electrons in the 366 Chen

bridging atom, enable the [Mo~ Y3] ring (Y=O, S) to exhibit different degrees of quasi-aromaticity in the cluster compounds containing the cores [Mo3(ll3-X-)(/l-Y)»] 4+ (X, Y=O, S).

The Effect of Terminal Ligands

The LMO analysis for [Mo3S4(CN)9] 5- in Ks[Mo3S4(CN)9], and [Mo3S4(H20)9] 4+ in [Mo3S4(H,O)9](CH3CöH4SO3) 4 as weil as the latter cluster core [M03S4] 4+ can account, at least qualitatively, for the effect of terminal ligands on the quasi-aromaticity of the puckered six- membered [M03S3] ring. It is well known that the anion CNis a strong- field ligand, whereas the H20 molecule is an intermediate-field ligand. The Mo-CN Mulliken bond order (0.6%) in the former is ahnost twice as large as Mo-OH 2 (0.36e) in the latter. The interaction between the Mo atoms and the terminal ligands causes delocalizaton of the lone pair electron from the terminal ligands to the Mo atoms, leading to reduction of these Mo atoms. The net charges on the Mo atoms in [Mo384] 4+, [Mo3S4(H20)9] 4+, and [Mo3S4(CN)9] 5- with 0.885e, 0.377e, and 0.052e, respectively, show that in the case of strong-field ligan& the Mo atoms are reduced to lower oxidation states, and thus enable these Mo atoms to accept lone pair electrons from the bridging S atoms, to a smaller extent in the formation of the 3c-2e (Mo-S»-Mo) zr bonds, compared with those in the higher oxidation states. This results in a smaller degree of delocalization of the lone pair electrons on the bridging S atoms in [Mo3S4(CN)9] 5 than that the in [Mo3S4(H20)9] 4+. It is obvious that the quasi-aromaticity of the puckered [M03S3] ring decreases with increasing interaction between the cluster core [Mo~S4] 4+ and the terminal ligands. It is weil known that W and Mo are elements of the same group in the periodic table and have much similar atomic radii, ionization energies, and electronegativities. For the series of [Mo3O,,S 4 ..... ]4+ and [-W3OnS 4 ...... ]4q (11=0, 1, 2, 3), their electronic structures and bonding pictures resenable each other to a good extent. There are also 3c-2e (W-Sc-W) 7t bondings in the cluster core [W30,$4_,] 4~ , forming a closed and completely continuous conjugated system around the puckered [W3 Y3] (Y= O, S) ring. This accounts ihr the nature of quasi-aromaticity of the cluster compounds with the core [ I'V~(IL3-X(p - Y)3] 4~ ()(, Y=O, S). MetaI-Chalcogenide Cluster Chemistry 367

TRIANGULATED POLYHEDRAL CLUSTER COMPOUNDS

Polyhedral Closo-Boranes

It is weil known ~~s~ that the closo-boranes B,,H,, 2 are thermally and chemically quite stable, especially to a variety of substitution reactions. Consequently, it is possible to carry out a wide range of substitution reactions without degradation of the boron cage t?amework. Many reagents used in aromatic organic substitution chemistry can also be used to carry out a similar substitution in the closo-boranes. Much of the chemistry of thses closo-boranes parallels or is lbrmally rather analogous to substitution chemistry in aromatic organic chemistry. These closo-boranes may be regarded indeed as aromatic clusters. From the LMO point of view, the origin of aromaticity in C6H 6 lies in the zr-electron delocalization of ordinary double bonds on the C6 ring, tbrming 3 3c-2e ~r-bondings. Aromaticity of the closo-B,, H,, 2- lies beyond any doubt in the delocalization of its framework bonding electrons. As a result of electron deficiency, the framework bonding electrons in closo- borane and their derivatives are generally localized on the triangular faces of the B,, polyhedron. As typical examples we may cite B4C14 and 1,5-C2B3H s, in which their three-centered framework bondings entirely fill the faces of the B 4 and C2B 3 polyhedra, and are quite weil localized (Fig. 7). However, further deficiency of tYamework bonding electrons leads to fewer three-centered bondings than the number of triangular faces in the B,, polyhedron, so that delocalization of these three-centered bondings to adjacent faces without direct l~amework bonds occurs in larger B,, polyhedra. In general, the more deficient are the framework bonding electrons, the more delocalized are the three-centered B-B-B bondings. Of course, lklrther delocalization of such a three-centered bonding to the adjacent B atom will certainly lead to form a new type of ~bur-centered B-BB-B bonding. It should be noted that though no preferred LMOs

Fig. 7. LMO(BIB2B 3)ofB4CI4. 368 Chen arrangements are appropriate for symmetries of the closo-B,, H,, 2 (n > 5 ), the localized treatment Ihr molecular orbitals provides a reasonable description tbr the delocalization of the ffamework bonding electrons. This leads us to a deeper insight into the nature of aromaticity in the closo- boranes and their derivatives.

Octahedral [Co6(CO)14] 4- and [Ni2C04(CO),4] 2-

From Hoffmann's isolobal analogy point of view, some transition metal clusters consisting of triangulated polyhedral frameworks of metal atoms taust be closely related to the closo-boranes ~1'( Thus the concept of aromaticity may tilrther be extended to three-dimensional triangulated polyhedral metal cluster chemistry ~~'j 2o( The isoelectronic series of hexanuclear carbonyl cobaltates Co6(CO)I«, [Co«,(CO)~5] 2 , [Co«,(CO)H] 4 , and [Ni2Co4(CO)14] 21211 with regular octahedral skeletal structures can be referred to as a typical example of 6-atom, 86-electron clusters containing an essentially octahedral arrangement of metal atoms (Fig. 8). The skeletal bonding scheine in all these species is filled by a total of 86 cluster electrons. Upon application of the polyhedral skeletal electron pair theory to these species, there are 14 skeletal bonding electrons to hold their 6 metal atoms together. Obviously, these species are lbrmally analogous to the octahedral closo-boranes B«,H~,2 , and may therefore be regarded as aromatic clusters. The LMO analysis ihr [Co~~(CO)H] 4 and [Ni2Co4(CO)14] 2 leads us to the lbllowing chemical bonding picture~22( Besides the 6 a(C, -, M)

,C coi Co1 C 0 SS4(2.~

P5 Co3 p3 Co5 • @

$6 Co6

)F'6 ~a~ (b} Fig. 8. Geometrical configurations of the «luster cómpounds: (a) clustcr anion [Co«»(J«~-CO)~(CO)«] 4 , (b}Co6(/«3-S)s(PPh3h,. MetaI-Chalcogenide Cluster Chemistry 369 carbonyl donation bondings {where C, denotes the C atom in terminal CO group) and the 12 ~z(M--,C,) back-donation bondings (M=Co, Ni), there are the following skeletal bondings: the 8 o'(C»--+M3) bridging bondings (where C» denotes the C atom of the bridging CO group), the 6 n(M--+ {C»4}) metal back-donation bondings and 11 a(C» *-M-+C») metal back-donation bondings (Fig. 9). One of the last designated bondings is further delocalized among the M atom and the four adjacent capping CO ligands may thus be regarded as a result of resonance between the two (Co, ~ M ~ C») a bondings. In our calculations it is t'ound that in these two cluster anions the metal atom skeletal bonding originates mainly from the interaction between the metal atoms and the capping CO ligands, instead of direct metal-metal bondings. The LMO analysis gives a bonding scheme different from those of both the classical valence bond theory and the polyhedral skeletal electron pair theory. The ibrmer allocates 11 metal- metal bondings to the 12 octahedral edges, while in the latter, 7 skeletal electron pairs are assigned to the metal atom skeleton. Experimentally it has been demonstrated ~21~ that in the [Co6(CO)14] 4 cluster anion the distances of Co-C, and (C-O), for the terminal CO ligands are 1.70 A and 1.17 A respectively, different from Co-C» of 1.89-2.31 Ä and (CO)» of 1.21 A For the capping CO ligands. While our theoretical calculations lead to the Mulliken bond orders of (C-O), 1.01le and (C-O)~, 0.922e, these

a b

A~ cb-" J"- c0

c d

e Fig. 9. Chemical bonding scheme between the Co atom and the carbónyl ligands in the [Có«,{CO)I4] 4 chlster anion. (a) a(C«-,Co} bonding, (b} n{Co-,C,} bonding, (c) a{ C~, -, I Cõ 3} } bonding, {d ) fr( Co -, { C»4 ~ ) bonding, ( e } a( Có -, { C«,2 } ) bonding. 370 Chen

indicate consistently a more efficient back d-+p donation from the Co atoms to the capping CO ligands than in the case of terminal CO ligands. It appears that the bonding description on the basis of the LMO theory is quite reasonable. In the [Co6(C0)14] 4- cluster anion, there are three-centered, tbur- centered, and five-centered bondings, implying the existence of local electron delocalizations. The (C» ~ {Co3}) a bonds are indirectly involved in the Co-Co interaction, localized 93.85% on the tbur bonding atoms with an average interbond interaction energy of 0.179 a.u., large enough to allow the bonding electron pair throughout the entire octahedral faces, leading to a skeletal electron delocalization. A similar situation also exists in [Ni2Co4(CO)H] 2 . It is interesting to note that as mentioned above, the resonance between two (C» ~-M---,C») a bondings leads to lbrm a (Co---, {Cb4}) a bonding. This electron delocalization may contribute to the stability of the octahedral metal atom skeleton. Therefore, the origin of the stability for the two cluster anions is complicated by the formation of metal-~ligand back donation bondings, unlike the octahedral closo- B~,H62- anion. It is beyond any doubt that rich information of chemical bonding has been hidden in the isolobal analogy.

Electron-Rich Octahedral [Co6([L[ 3 --X)8 • L6] ~+ (X= S, Se; L : PPh3, PEt3, CO; n=0,1) In the past decade, polynuclear posttransition metal-chalcogen cluster compounds have aroused great interest, mainly under the stimulus to achieve synthetic models either for active sites of iron-sulfur proteins or for heterogeneous metal sulfide catalysts. Recently, several compounds each with a [Co¢,(//3-X)s-L(,] ''+ (X=S, Se; L=PPh» PEt3, CO; 11=0, 1) cluster have been synthesized, characterized, and theoretically studied ~23~. Such a cluster compound has an inner core consisting of an octahedron of Co atoms with all the faces symmetrically capped by triply bridging X(S, Se) atoms, while each Co atom is additionally linked to a terminal ligand L (Fig. 8). These [Co6(/t3--X)8.L6] n+ cluster compounds and hexanuclear carbonyl cobaltate [Co6(/«3-CO)s(CO)6] 4- are isostructural, but are by no means, isoelectronic. Obviously, the tbrmer contains 98 valence electrons, i.e., 12 electrons more than the 86 valence electrons of the latter, required by the electron count on the basis of the polyhedral skeletal electron pair theory. Other electron count rules, such as the 18-electron and the topological rules ~-~4~, are also unable to explain such excess electrons in these [ Co6(ILB -- X)s. L6 ]" + cluster molecules. It is interesting to note that the neutral species of the [C06(//3 -X)8. L6] ''~ cluster compounds can undergo one-electron charge MetaI-Chalcogenide Cluster Chemistry 371

[C,o6( U3-S)8(PF13)6] [C06(p3-S)8{PH31611+ o~ /3 A(Co) ù - , A(co)

A(co)~\~ « ///~ (co,..co)A(c°)" o)~

er{Co6} A(Co)A(C " \ ~cc.o-col

,~(s) \

«cco*pl \\ ,,

L J, \ \ o-(c.o-s) o.-(P~) a-(P~} Fig. 10. Level diagram and diagrammatical expression of the a{C06} bonding and 7r(CoCo ) bonding for Co«,(p~- S)s ( PPh3)«-

transfer while leaving the unchanged geometry of the inner framework and small changes in the structural parameters. It is beyond any doubt that there exists a certain skeleton electronic delocalization in the electron-rich cluster molecules, leading to their special chemical stability. Thus, the [Co«~(It3--32")s.L6] ''+ 01=0, 1) cluster compounds can be referred to another typical example of quasi-aromatic electron-rich metal clusters with regular octahedral skeleton structures. From the LMO point of view ~25~ for [C06(ll3-S)s(PPh3)6] ''+ (n = 0, 1 ), the cluster skeleton bonding of these two isostructural molecules consists of the edge-localized 2c-2e (Co-S) cr bondings plus a pair of skeleton electrons delocalized on the whole cluster core, resulting in an extra stability of the cluster core. The one-electron oxidation for the neutral molecule gives rise to an open-shell electronic configuration with a one- electron (Co-Co) n bonding, which further resonates anaong the three diagonal lines of the {C06} octahedron (Fig. 10).

Comparision of [Co6(~t3-S)8(PPh3)6] with [Co6(~3-C0)8(C0)6] 4-

[Co6(/z3-S)s(PPh3)6] (II) and [Co6(]13-C018(C0)6] 4 (III) are two isostructural hexanuclear cobalt compounds with octahedral cluster cores, 372 Chen but their electronic structures are quite difl'erent from each other. In these two molecules, the 6 Co atoms are held together by the 8 bridging S atoms or carbonyl groups, tbrming the cluster cores. However, there is not any direct Co-Co bonding. Nevertheless these bridging atoms or groups play an important role in stabilizing the clusters. It is interesting to note that these two cluster molecules adopt different fashions of bridge bonding. As shown above, in the molecule (II), the interaction between the bridging atoms and the metal atoms is the edge-localized 2c-2e bonding, forming 3 (Co-S) a bondings with each S atom. The molecule (III) adopts face-localized 4c-2e bondings, tbrming one (C»-+ {Co3}) cr bonding with each bridgmg CO group. On the other hand, as the carbonyl CO group in the molecule (HI) is a stronger )z-acid ligand, they are able to accept a back-donation from the lone electron pairs on the Co atom. Thus in the molecule (III), except the (C»--+ {Co3}) a bondings, there are also two types of back donation bondings between the Co atoms and the bridging C» atom. They are the (Co--+{C»,}) er bonding and the (Co--+ {Ch4}) ~t bonding (cf Fig. 9). However, no back-donation bonding between the Co atom and the bridging S atom occurs in the case of the molecule (II). It is these multicentered back-donation bondings and the face-localized bridging bondings that lead to the distinct short Co-Co distance compared with those in the molecule (II), 2.50 A for the former and 2.868 A for the latter. Insofar as the interaction between the terminal ligands and the metal atoms is concerned, in the molecule (IlI), the quite strong back-donation (Co ~ C,) 7r bonding from the Co atom to the terminal CO ligand shortens the distance between the Co atom and the terminal CO ligand. In contrast, there is merely little back-donation between Co-P in the molecule (II), with population 3.77% from the Co atom to the terminal PPh 3 ligand. Indeed, experinaents have indicated a shorter Co-CO bond length of 1.70 Ä in the molecule (III) than the 2.235 A of the Co-P distance in the molecule (II). It should be pointed out that the pair of cluster valence electrons forming the a{Co(,} bond in the molecule (Il) is delocalized over the whole cluster core, orient radially to the center of this core; while in the case of molecule (lll), each pair of the valence electrons is locally delocalized on a delta-face of the cluster octahedron. In connection with the lone electron pairs on the Co atoms, for the molecule (III), as there exists a certain the significant amount of back-donation bondings fiTom the Co atoms to the bridging CO groups, the lone electron pair, in fact, does not exist alone. However, in the case of the molecule (II), each Co atom has three highly localized lone electron pairs. In summary, for the hexanuclear cobalt compounds studied with the octahedral cluster core, the bridging atoms play an important role in stabilizing cluster molecules. Thus as the different bridging atoms give rise MetaI-Chalcogenide Cluster Chemistry 373 to strikingly different structures of cluster valence electrons, they form new types of delocalization schemes, differing rernarkably flora that in the case of [Co6(//3 -- C0)8(C0)614

Multi-centered bonding in quasi-aromatic ligand [1] A series of reactions of the S-bridged nido cubane-type [Mo3S4] 4~- cluster ion with a metal atom M or metal cation M q+ to give a cubane- type [Mo3S4 .ML,,] ¢4+q}+ (M=Mo, W, Fe, Ni, Cu, Sn, Sb; L=ligand), or a sandwich-type [Mo3S 4 - M. S4Mo3] 8+ (M= No, an, Hg) and doubly bridged double-cubane-type [Mo3S4.MM.S4M03] s+ (M=Cu, Co) cluster compounds have been recently reported~26 3°L These reactions may be formally regarded as the following "building-block" reactions for rational syntheses of sandwich and half-sandwich mixed metal complexes: [3] + [I]--, [4] (1) [3] +[l] +[3] ---, [7] (2) [3] + [1 ] + [1] + [3] ~ [8] (3) where [3] and [1] denote [Mo384] 4+ and M (or M «+) respectively. It is interesting that the reactions (1) and (2) are rather similar to the syntheses of the 7r-bis-arene-metal and arene-metal-carbonyl complexes. Thus structurally these products may also be regarded as sandwich and half-sandwich metal complexes, i.e., quasi-arene-metal complexes with the [Mo384] 4+ cluster ion looked upon as a quasi-arene type ligand. Upon further comparison of the LMOs tor the prototype arene-metal sandwich complex (CöH«,)2Cr and the half-sandwich complex C6H6Cr(CO)3 with the LMOs Ihr the sandwich-type [Mo3S 4 .M.S4M03] 8+ and the half-sandwich-type [ M03 $4 - ML,,] (4 +. q) - complexes, it is found that there are two types of chemical bonds between the metal M and the ligands L, one of which behaves as an L(Tr)--, M bonding, in which the vacant hybridized AOs of the "intruder" metal atom (either the six d257)3 octahedral AOs or the four sT)3 tetrahedral AOs) are oriented so as to point toward the octahedral or tetrahedral vertices, and the electrons of the three-centered two-electron n bondings in the arene ligand or the quasi- aromatic ligand [Mo384] 4+ are donated to these vacant hybridized AOs with appropriate symmetry in the bonding combination. In the other type of M-L bond, the lone electron pair, localized essentially on the d AOs of the "intruder" metal atom, are backward donated to these ligands to some extent, tbrming M(2) --, L bonding. For (C~,H(,)~ Cr, the six vacant d25p 3 hybridized AOs of the central Cr atom orient themselves respectively toward the centers of electron density 374 Chen of each of the six occupied three-centered two-electron 7r bondings in the two benzene ligands to form six arene --+ Cr four-centered bondings. On the other hand, the two occupied d type AOs (d~:. d~ ,) of the Cr atom inter- act with the vacant lr*-orbital set of the two benzene ligand molecules to back-donate electrons from the rnetal atom to the benzene ligands, forming an M -+ arene type of multicentered bonding. In the case of C«,H~»Cr{CO)~, the six vacant hybridized (12~,/~3 AOs of the Cr atom accept the six pairs of electrons flom the three-centered two-electron rr bondings in the benzene ligand and the three carbonyl ligands. It is interesting to note that there are Cr~ C,,H«, back-donation bondings in (C~,H«,)2 Cr, while there are only Cr~ CO in C«,H(,Cr(CO) 3. When a Mo atom is sandwiched between two [MoaS4] 4+ ligands, forming a {[Mo~S4] 2 .Mo} s+ cluster cation, the six vacant d2,~'p3 hybridized AOs of the central Mo atom match perfectly the six three-centered two-electron Mo S Mo rr bondings of the two [Mo384] 4+ ligands to form six L(~z) --+ M four-centered bondings, as in the case of (C«,H:,)~Cr. There is, on the other hand, some back-donation of electrons from the central Mo atom to the ligands, somewhat similar to the M-+ L back bonding. In the case of the half-sandwich complex { [Mo3S4]. NJ} 4~ , the °intruder" Ni atom provides three vacant .V)3 hybridized AOs to accept three pairs of electrons t~om the three Mo SMo 7r bonds of the ligand [MoaS4] 4+ lbrming three [Mo3S4] 4+ -+ M bondings. There are, however, five Ione pairs of electrons localized on the five 3d AOs of the Ni atom, two of which donate significantly backward to the [Mo384] 4+ ligand to tbrm an M--+ [Mo384] 4+ back bonding. Moreover, for the cluster ion {[Mo3Sa]-Cu} 5+, similar to the case of {[Mo~S4].Ni} 4~, three [ Mo~S4] 4+ --+ M bondings are formed with the Cu atom contributing three vacant sp 3 hybridized AOs, thus interacting with the three occupied three- centered two-electron 7r bondings in the ligand [Mo~S4] 4 ~. However, the live lone pairs of d-electrons are localized entirely in the five 3d AOs of the Cu atom without any back-donation of electrons to the ligand [MoaS4] 4' due to the stabilization of the completely filled 3d AOs of the Cu atom. Finally, in the case of { [ Mo:~ $4 ] - CuCu- [ $4 Mo 3] } s + two three-centered two-electron Cu-S--Cu er bondings and four (Cu S) cr bondings are formed, instead of the three [Mo3S4] 44 --+fu bondings in the case of {[Mo3S4].Cu} s'. The ligand [Mo3S4] 4+ loses in this case the initial 7r-type LMOs, different from the case of the halfsandwich type complex {[Mo3S4]Cu} s+ Meanwhile, the five lone pairs of d-electrons are still localized on the five 3d AOs of each Cu atom. On account of the formation of the L(r~)-+M bonding, the three three-centered two-electron ~z bondings in the ligand itself are somewhat weakened; both the C-C bondings in the arene ligand and the Mo-S MelaI-Chalcogenide Cluster Chemistry 375 bondings in the [Mo384] 4+ ligand are slightly longer than the corresponding bond lengths in the parent molecule and ion. The 1.398 Ä C-C bond distance in the parent molecule CöH(, is lengthened to 1.423 A in (C«,H«,)2-Cr, and to 1.423 Ä and 1.406 Ä in the case of Gart~er(CO) 3 corresponding respectively to ligands eclipsed and staggered with respect to the CO ligands~3~L It is interesting to note that the patent quasi-aromatic ligand [ Mo3S4] 4+ has shorter bond distances between the bridging S» and Mo atom than between the capping S, and Mo atom, and thus, as soon as the "intruder" metal atom is sandwiched between the [Mo384] 4~ ligands with formation of new L(Tr) -+ M bondings, the initial Mo S» bond distances are remarkably lengthened, closer to the Mo-S,. bond length, whereas the Mo-Mo bond lengths are also slightly increased. Shibahara ~:~'''~ has recently suggested that there are two types of driving forces for the formation of sandwich and half-sandwich type mixed- metal clusters from the nido-cubane-type ion [Mo3S~] 4+ and the metal atorn, namely the reducing ability of the metal and the affinity of the metal toward the bridging S atoms. It seems obvious that the two bonding factors rather resenable the two kinds of bondings between [ Mo3S414+-M, in which the L(~r) --+ M bonding behaves as the afl]nity of metal to S atom, while the back donation bonding M --+ L from the metal to the ligand plays a reductive role li)r the Mo atoms in the [Mo384] 4 ligand. Therefore a study of addition reactions of various metals to the quasi-aromatic ligand [M3(/«--X)(/I-- Y)3] 4+ (M= Mo, W; X, Y=O, S, Se, Te)taust be a very interesting subject.

CONCLUSIONS As mentioned above, there are a variety of cluster compounds with such a stoichiometry and a number of electrons available that they cannot be discribed in terms of a single Kekule structural fornlula alone. Multicen- tered bondings developed on the basis of LMO theory exist, in general, in these cluster molecules. Upon comparing the LMOs with the canonical molecular orbital (CMO) description, the multicentered bondings in the LMO theory give us at once a clear-cut intuitive bonding picture and a visual description for the delocalization of bonding electrons. Therefore, this extension of multicentered bondings in the classical valence theory enables us to account for unusual structural characteristics and chemical reactivities for these cluster compounds. It is worthy of note that there exist several types of multicentered bonding, for example, the three-centered (Mo-S-Cu) er bonding in [CI2FeS2MoS2Cu(PPh3)2] and the three-centered (Mo-S-Mo) bonding in the puckered six-membered ring {M03S3} of the 376 Chen

M03 S4(dtp)4 "H ~O. On the other hand, polyhedral cluster molecules might adopt usually two types of radial and tangential multicentered bondings, for instance, the radial {C06} a bonding in [C06(/~3-S)s (PPh3)«,] and the tangential (Co--+ {C»4}) 7r bonding in [Co6(/13 -X)s(CO)6] 4 . From the angle of bonding electrons there exist two types of covalent and donation multicentered bondings, including back-donation bonding. In summary, the existence of multicentered bondings in metal-chalcogenide cluster chemistry is bound to enrich the modern chemical valence theory to a significant extent.

ACKNOWLEDGMENTS This review of our previous work is dedicated to Professor Jiaxi Lu as he completes 80 years. With my wannest regards and best wishes for Professor Lu, and may there be many more good years to come. The author gratefully acknowledges the supports of this research by National Natural Science Foundation of China, the State Key Laboratory of Struc- turm Chemistry, and the State Key Laboratory of Rare Earth Materials Chemistry and Applications. The author is also grateful to Professors Q.-E. Zhang, C.-W. Liu, J.-Q. Huang tbr their helpful discussions,

REFERENCES

1. J.-X. Lu and Z.-D Chen (1994). hzt. Rer. Phys. Chem. 13, 85. 2. J.-X. Lu, An hwited paper presented at a plenac~ session of the China-USA hilateral con- j«ren«e on nitrogen fi.xation (Madison, Wisconsin, 1982). 3. T.-L. Sheng, S.-W. Du, and X.-T. Wu (1993). J. Cluster Sei. 4~ 89. 4. T.-L. Sheng, S.-W. Du, and X.-T. Wu (1993). PoO:hedron 12, 111. 5. X.-L. Kong, Z.-D. Chen, T.-L. Sheng, and X. T. Wu (1994). Chhwse J. Stru«t. (7wm. 13, 139. 6. (a) A. Bino, F. A. Cotton and Z. Dori (1979). J. Am. Chem. Soc. 101, 3842: (1979). htorg. (Twm. Acta. 33, L133: (b) F. A. Cotton, T. R. Felthouse, and D. G. Lay (1980). J. Am. CJwm. So«. 102, 1431; (c) X.-T. Lin, Y.-H. Lim J.-L. Huang and J.-Q. Huang (1987). Kexue Tongbao 32, 810. 7. J.-Q. ttuang, J.-L. Huang, M.-Y. Shang, S.-F. Lu, X.-T. Lin, Y.-H. Lin, M.-D. Huang, H.-H Zhuang, and J.-X. Lu (1988). Pure Appl. (Twm 60, 1185; (1987). (Ttinese J. Stru«t. (Jwm. 6, 219 (in Chinese). 8. T. Shibahara, H. Akashi, S. Nagahata, H. Hanor, and H. Kuroya (1989). htorg. Chem. 28, 362, and reference therein. 9. T. Shibahara and M. Yamasak (1991). htorg. Chem. 30 1687, and reference therein. 10, P. Kathirgamanathan, M. Martinez, and A. G. Sykes (1985) J. Chem, So«. CTwm. Com- muh. 1437; M. Martinez~ B-L Ooi and A. G. Sykes (1987). J. Am. Chem. Soc. 109, 4615. 11. T. Shibahara, T. Yamada, and H. Kuroya (1986), ]llorg. Chim. A«ta. 113, LI9. 12. T. Shibahara, H. Hattori, and H. Kuroya (1984). J. Am. CVwm. So«. 106, 2710. 13. T. Shibahara, H. Miyake, K Obayashi, and H. Kuroya (1986). ('hein. Lett. 139. MetaI-Chalcogenide Cluster Chemistry 377

14. S.-F. Lu, M.-Y. Shang, J.-Q. Huang, J.-L. Huang, and J.-X. Lu (1988). Sei. Shl. B31, 147. 15. A. Bino, F. A. Cotton, and Z. Dori (1978). J. Am. Chem. Soc. 100, 5252. 16. X.-T. Lin, Y.-H. Lin, J.-L. Huang, and J.-Q. Huang (1987). K«xue TmNbao 32, 810. 17. H. Kobayashi, T. Shibahara, and N. Uryu (1990). Bull. (Tzem. Soc. Japan 63, 799. 18. E. L. Muetterties, Boron l~vdrid« Chemistry (Academic Press, New York, 1975). 19. K. Wade (1976). A(h. Inorg. Chem. Radiochem. 18, 1. 20. R. B. King and D. H. Rouvray (1977). J. Am. Chem. Soc. 99, 7834. 21. P. Chini (1967). Chem. (~)mmun. 440: V. Albano, P. Chini and V. Scatturin (1968). Chem. Commun. 163; P. Chini and V. Albano (1968). J. Organomet. Chem. 15, 433: V. Albano, P. Chini, and V. Scatturin (1968). J. Organomet. Ch«m. 15, 423; P. Chini, V. Albano and S. Martinengo (1969). J. Orf,,anomet. Chem. 16, 471: P. Chini. S. Martinengo, and V. Albano, Pro«. brt. 5),mp. Metal Carbonyls and Derivatives, Venice Sept. 24 1968, A-3, htorg. Chem A«ta, V. AIbano, G. Ciani and P. Chini (1974). J. Chem Soc. Dalton 7)'ans. 4 432. 22. Z.-D. Chen, C.-W. Liu, and J.-X. Lu (1994). (Ttinese J. Stru«t. Chem. 13, 69. 23. (a) D. Fenske et al. (1988). Angew. Chem. bit. Ed. Engl. 27, 1277; (b) F. Cecconi et al. (1986). Polyhedron 5, 2021; (c) A. Bencini, et al. (1992). J. Am. Chem. Soc. 114 989; (d) V. Albano et al. (1969). ,L Orgalmmet. Chem. 16, 461. (e) M. C. Hong et al. (1989). hlor~. Chem. 159, 1: (i) M. C. Hong et al. (1990). Ch#wse J. Struct. Chem. 9, 47. 24. (a) B. K. Teo (1984). htorg. Chem. 23, 1251: (b) B. K. Teo, G. Longoni, and F. R. K. Chung (1984). htorg. Chem. 23, 1257; (c) B. K. Teo (1983). J. Chem. Soc. (Twm. Commun. 1362. (d) D. M. P. Mingos (1985). J. Chem. So«. Chem. Commun. 1352; (e) R. L. Johnston and D. M. P. Mingos (1986). hu»JN. (Twm. 25, 1661; (i) D. M. P. Mingos (1986). Chem. So«. Re~:. 15, 31: (g) D. M. P. Mingos and R. L. Johnson (1987). Struct. Bonding (Berlin) 68, 29. 25. Z.-D. Chen, C.-W. Litt, and J.-X. Lu (1994). Chinese J. Slruct. Chem. 13, 146. 26. (a) T. Shibaharai et al. (1991). hmr¢. ('hein. 30, 2693: (b) (1986). J. Am. Chem. Soc. 108, 1342: (c) (1988). Coord. (17wm. 18, 233: (d) (1988). J. Am. Chem. So« 110, 3313; (e) (1987). J. Am. Chem. So«. 109, 3495: (f) (1989). bmrg. Chem. 28, 2906; (g) (1986). hmr;, Chim. Acta 116, L25; (h)(1991 ). Chem. Lett. 689. 27. S.-F. Lu, N.-Y. Zhu, X.-T. Wu, and J.-X. Lu (1989). J. Mol. Stru«t. 197, 15; M. Nasreldin, Yue-Jin Li, F. E. Mabbs, and A. G. Sykes (1994). htorg, Chem. 33, 4283. 28. S.-F. Lu, J.-Q. Huang, Y.-H. Lin, and J.-L. Huang (1987). A«ta ('hhn. Shti«a 3, 199. 29. B. U Ooi and A. G. Sykes (1989). hmrg. Chem. 28, 3799. 30. S.-F. Lu, J.-Q. Huang, Y.-H. Lin, and J.-L Huang (1987). Huaxue Xuebao 45, 666. 31. E. U Muetterties, J. R. Bleeke, E. J. Wucherer, and T. Albright (1982). Owm. Rer. 82, 499.