Journal ~)/" Cluster Science, Vol. 6, No. 3, 1995 Multicentered Bonding and Quasi-Aromaticity in Metal-Chalcogenide Cluster Chemistry 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 com- pounds 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 com- pounds 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 com- pounds 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 agree- ment 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 com- pounds with Mo atoms in the IV oxitation state forming a discrete equi- lateral 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 con- tinuous 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.