Electron Deficient Molecules Ans 3C-2E Bonds

Examples of molecules with three centre two electron bonds

and their representation within the covalent bond classification When he was an undergraduate (18 years old) in Balliol he published a paper with his Tutor Mr Bell FRS in Nature in 1045.1945. The title was...

The Representation of Covalent Molecules • Most organic compounds can be represented by using a black line to represent 2c-2e bonds. • In contrast, many inorganic compounds cannot be represented so simply because the bonding must often be described in terms of dative interactions or 3c-2e interactions. • In many situations, black lines are often just used to indicate connectivity and molecular shape, rather than the nature of the bonding and its electronic structure. • Electronic structure can, however, be indicated by the appropriate use of arrows and formal charges. Such structures are described as structure-bonding (SB) representations and are very useful for identifying the MLXZ class and correct electron count of a molecule and are the basis of the Covalent Bond Classification method.

Cl Cl

H3N NH3 H3N NH3 Co Co

H3N Cl H3N Cl Cl Cl

H H H H H H B B B B H H H H H H Connectivity Structure-Bonding Representation Representations of B2H6 H H H H H H B B B B Orbital H H H H H H

H H H H H H B B B B Historic H H H H H H The use of the half-arrow distinguishes this as a 3c-2e bond H H H Preferred H B B H H HALF-ARROW

Common examples of µ-X bonds in coordination compounds H H M C M M

agostic bridging hydride

For electron counting purposes the half arrow represents donation of two electrons to a central atom. The two electrons of the 3c-2e bond are seen by both boron atoms. Hence each boron has an electron count of 8. An alternative method of counting electrons for metal dimers and cluster with bridging hydrides or alkyls

The “half-electron” method apportions the one electron originating from a bridging hydride or alkyl ligand equally to both metals. The metal-metal bond order is predicted using the formula, with a view to the metals obeying the 18 electron rule.

18# − ! !"#$%& '( ) − ) $'+,- = 2

Where m is the number of metal atoms and N is the total number of valence electrons

The problem with this method is that the application of the formula requires no knowledge of the structure and, for example, does not distinguish between terminal and bridging hydrogen ligands.

Also the predictions contradict the results of modern theoretical methods. Occurrence and Representation of µ–X Bonds Determination of correct bond order in transition metal compounds with bridging hydrogen ligands.

• Bond order ambiguities are common in the literature. • Os3(CO)10H2 has been represented in the literature with a variety of bonding descriptions, varying from a Os=Os double bond to no bond.

Os(CO)4 Os(CO)4 Os(CO)4 H H H (OC)3Os Os(CO)3 (OC)3Os Os(CO)3 (OC)3Os Os(CO)3 H H H

Os has 18 electron class of ML4X2 and NO Os–Os Bond MO theory always agrees with the predictions of bond order using the half- arrow method. X

Occurrence of Closed µc–Z Bonds in Coordination Chemistry • Compounds with closed µc–Z bonds are most commonly encountered in transition metal dihydrogen compounds, which were first discovered by Kubas in 1984.

PCy3 CO CO CO H H OC W OC Cr H H OC OC

PCy3 CO

Kubas et al Upmacis et al

JACS 1984, 106, 451 JCS Chem. Comm. 1985, 27 • Dihydrogen compounds can be considered to possess closed µc–Z interactions because all three orbitals overlap.

M 0e Z H H 1e 1e X X

Examples of µ-Zc (closed) bonds in transition metal compounds

M M H H C C Dihydrogen Olefin Compounds compounds

Rh C C

Has a sugma C-C bond A.B.Dhaokin, A.S. Weller JOMC, 730 (2013) 90 - 94 [Rh(PR3)(BinoreS)]√æ complexes

N N R R C

B Pt

Si Si Me Ph 2 Me2Ph Pt[NHC(Dip)2]-(SiMe2Ph)2 (NHC = ; Dip = 2,6-diiso-propylphenyl)

Berthon(Gelloz,-G.;-de-Bruin,-B.;-Tinant,-B.;-Marko,ÃÅ I.-E .-Angew.-Chem.,-Int.-Ed.-2009,-48,-3161.

N.Takagi and S. Sakaki, JACS, 2012, 134, 11749. Representation of Open µο–Z 3c-2e Bond

Z • • • •

X X Bent µο–Z bond

• The • dot-line representation is used to indicate that it is a 3-centre-2-electron bond and not two 2-centre-2-electron bonds.

2 sp BX3

X X

•

B • B

• • X X X X

almost sideway view sideway view

• Since Z does not contribute any electroms to the 3c-2e bond, the two X groups serve as an L donor to Z. CBC mrepresentation of B4H9 o The µ−Z (open) bond in B5H9 Z B

X X B B General figure for The µ−Zo (open) o µ−Z (open) bond bond in B5H9

o The µ−Z (open) bond in B5H9. All 4 basel B have tetrahedral BLX3 class. The µ−Zo (open) bond acts as a L'-ligand to the central boron, so that B also has the class BLX3

Examples of Open µο–Z 3c-2e Bonds

• ο • Open µ –Z bonds have been recognized only recently. ο A clear example of a compound that features a µ –Z bond is B5H9.

H H

B H H • B •B H H H B

H H

• ο • The notation proposed for the µ –Z bond is the dot-line. The presence of the dot signifies that this is a 3c-2e bond rather then two 2c-2e X–X bonds.

Other Examples of Open µο–Z 3c-2e Bonds •

The axia B–H moiety of B5H9 may be replaced by metal fragments to give ο [M](B4H8) derivatives that feature µ –Z bonds. H

H B H B H B• • H H HB B H H

CO OC CO

H H Fe Co H H B H B H B• • B• • H H HB H H HB B B H H H H Grimes Greenwood JACS 1977, 99, 5646 JCS Chem. Comm. 1974, 718

ML4X2 ML3X3 Class of Cobalt Class of iron CoL3X3 = 18 FeL4X/2 = 18 Bridging Boroles: Another Example of a µΟ–Z Interaction

CO CO OC Mn• Class

B Ph ML5X 18 e Mn• CO OC CO

Herberich Angew. Chem. Int. Ed. 1983, 22, 996 Examples of Compounds with µ–L Bonds R R R P O O Rh Rh O X O X n X = CPh2; R = Me, Et, Bu 16–electron, ML2X3

3 µ–L Fe Fe OC C CO Cr OC O CO

18–electron, ML4X2

18–electron, ML6 Symmetrically Bridging Carbonyl Compounds • Symmetrically bridging carbonyl compounds are invariably represented as "ketone" derivatives.

C M M

• Fe2(CO)9 was the first was polynuclear metal carbonyl to be characterized by X-ray diffraction, thereby confirming the presence of bent carbonyl ligands.

OC C CO OC CO Fe Fe

OC C C CO

O O

•Fe2(CO)9 is invariably drawn with an Fe–Fe bond to achieve an 18-electron configuration, and Braterman stated that “This bond is real, not formal” Bonding in Fe2(CO)9 O OC C CO OC CO Fe ? Fe OC C C CO O O • However, in the 1970s, Hoffmann pointed out that the Fe•••Fe interaction in Fe2(CO)9 is actually antibonding and repulsive.

• Numerous subsequent calculations support Hoffmann's analysis.

• Heijeser: “… along the Fe-Fe bond axis there is a negative density difference”; • Bauschlicher: “No evidence is found for a direct Fe-Fe bond. The Fe's are held together by three center Fe-CO-Fe bonds.” • Rosa: “… the two Fe(CO)3 fragments are not kept together by direct Fe-Fe bonding but by the presence of the bridging carbonyls.” • Ponec: “… the bonding interactions between the metal atoms do not have the character of a direct Fe-Fe bond anticipated on the basis of 18-electron rule.”

• So why do organometallic chemists and text books continue to represent the molecule with an Fe–Fe bond? Bonding in Bridging Carbonyls O OC C CO OC CO Fe ? Fe OC C C CO O O • An indisputable feature of many calculations is that the bonding of a symmetrically bridging carbonyl ligand can be expressed in terms of two 3-center molecular orbitals.

O O

C C

σ–donation π–backdonation (λ+) (λ–)

• Representing a bridging carbonyl as a "ketone" derivative, i.e. two 2c-2e bonds, requires both of these molecular orbitals to be occupied. Bonding in Bridging Carbonyls • If there is no backbonding, the bonding must be represented as a 3c-2e interaction.

O O+ double bond single bond Tripledouble bondbond C C– M M M M σ–donation & π– σ–donation only is backdonation is equivalent equivalent to a single 3c–2e to two 2c–2e bonds bond • The three bridging CO ligands participate in three donor interactions, but only two backbonding interactions.

• Therefore, one of the CO ligands can only be represented as a 3c-2e bond. O O O+ C C C– Fe Fe Fe Fe Fe Fe C– C C C– C C O+ O O O+ O O O OC CO C OC CO Fe Fe

OC C C– CO O O+

18–electron with no Fe–Fe bond Bonding in Bridging Carbonyls

• 3c-2e "non-ketonic" bonding also rationalizes the absence of M–M bonds in other carbonyl compounds.

+ O + CO OC O CO O – C C C– Co Co Fe Fe C OC CO O OC CO OC

18–electron 18–electron

M. Benard, J.Am.Chem.Soc., 1978,, 100, 7740 . E.D. Jdemmis, A.R. Pinhas and R. Hoffmann, J.Am.Chem.Soc., 19801, 102, 2576,

A.A. Low, K.L. Kunze, P.J. MacDougall and M.B. Hall, Inorg, Chem 1991, 30 , 1079–1086.

R. Ponec, G. Lendvay and J. Chaves, J.Computational Chem., 2008, 29, 1387 – 1398.

. . Organometallics Article

8 † complex supported by a stabilizing macrocyclic ligand, thus the bis(oxo)-bridged dimer [(η -Pn )Ti(μ-O)]2 (2), in which 24 illustrating the importance of ligand architecture in reactivity. the Ti−TiOrganometallics bond has been cleaved, and the dicarbonyl complex Article 5 5 † Recent studies by Cloke and co-workers with the mixed- Ti2(μ:η ,η -Pn )2(CO)2 (3), which was independently synthe- Me4R 55 sandwich U(III) complexes (COT′)UCp {THF}x (COT′ = sized from 1 with 2 equiv of CO. Herein we explore the scope 8 i t η -1,4-{SiMe3}2C8H6; R = Me, Et, Pr, Bu) have shown that the of this reductive transformation of CO2, investigating its steric environment around the metal center plays a key role in mechanistic pathway by the identification of key intermediates guiding the possible reductive transformation pathways of CO2, using spectroscopic and computational techniques. Further- 25 i.e., reductive coupling, disproportionation, or deoxygenation. more, by the use of isoelectronic CS2 and unsymmetrical COS Nonetheless, better understanding of the mechanisms of these as model molecules, we report the isolation and structural reductive transformations is required, with a view to tuning the characterizationOrganometallics of stable analogues for the proposed reaction Article structure of the active complex or the reaction conditions to intermediates, supporting the theoretical predictions. In the give more useful product outcomes and assessing the potential interests of brevity, an in-depth description of the bonding in for catalytic turnover.26 these molecules and other related compounds is provided in Divalent titanium sandwich complexes are an under- the companion paper.56 † Figure 7. X-band EPR spectrum of polycrystalline [Ti2Pn 2][B- developed series of compounds in this regard, despite the (C6F5)4] at room temperature (black line) and corresponding rich and varied reaction chemistry that “titanocenes” have ■ RESULTSsimulation (red line). 27,28 29 † shown with a variety of small molecules. Alt et al. Reactionbridgehead of C CO−C2 bonds),with and Ti are2Pn consistent2. The with reaction a singly ofFigure1 8.withTi−Ti bonding 1 orbital of Ti2Pn2CO2 (19a1), the LUMO of occupied MO (SOMO)12 16a1 (Figure13 2). bent CO2, and the bonding orbital (18a1) resulting from nucleophilic reported the synthesis of the titanium CO adduct Cp Ti- 5 5 5 5 † 2 2 equiv ofTi either2(μ:η ,η -Pn)2COCO2. The2 COor2 adductCO Ti2(μ:2η ,η was-Pn )2CO carried2 attack of out Ti2Pn2 on in CO2. has beenExample spectroscopically of CO characterizedacting in solutionas a µ atZ low ligand (CO2)(PMe3) via simple ligand substitution of Cp2Ti(PMe3)2, methylcyclohexane-d (MeCy-2 d )17 at −78 °C (Scheme 1). temperature but is too14 unstable to be isolated.14 Optimizing the Table 3. Energies (in eV) of Orbital Interactions Divided and a number of acetylene complexes of Cp2Ti have shown geometry of Ti2Pn2CO2 from various starting geometries led to According to Their Symmetries; The Various Molecules with Organometallics i Article Bis(pentalenea) minimum-energydititanium double structure-sandwich with C2v symmetrycompound (3). Ti Selected2Pn†i 2 (Pn†C 2=v Symmetry1,4-{Si Pr Are3}2C Divided8H4) into Ti2Pn2 and Ligand CO2 insertion into the Ti−C bond of the titanacyclopropene Scheme 1.geometric Reactivity parameters are of given 1 with in Table CO 1. 2 (R = Si Pr3)Fragments 30−33 reacts with CO2 forming a complex with the CO2bridging the two Ti atoms ring. In contrast, reaction of the permethyltitanocene 3 4 7 9 13 2 complex Cp*2Ti(η -Me3SiC2SiMe3) with CO2 yields, after Ti A1 Ti−116 −192 −81 −186 −80 A2 −2 −4 −15 0 −1 elimination of the silylalkyne, a mixture of the dimeric B1 −7 −8 −8 −136 −100 BC2 −15 −21 −24 −165 −125 carbonate (Cp*2Ti)2(CO3) and the carbonyl complex Cp*2Ti- + - 34,35 Organometallics O O Article (CO)2. Reductive disproportionation of CO2 has been from the HOMO of Ti2Pn2 is the predominant bonding interaction. The occupancies of the LUMO, HOMO, and achieved by Cp2Ti(CO)2 to generate CO and the carbonate- HOMO−1 of the Ti2Pn2 fragment in 3 are given in Table 4. 36 bridged tetranuclear complex [(Cp2Ti)2(CO3)]2. Titanium- The Ti−Ti distanceThe HOMO is short of (2.41 Ti2Pn† Å),2 indicating strong Table 4. Occupancies of the Fragment Orbitals of Ti2Pn2 in (III) complexes have also shown efficacy in reductive bonding between the Ti atoms. The pentalene rings are bent the Molecular Calculations for 2, 3, 4, 5, 6, 7, 8, 9, and 13 36 36−39 back slightly more than in Ti2Pn2. Examination of the MOs of 3 transformations of CO2 to yield oxo, carbonate, and (Figure 8) shows that the key bonding interaction is between 14b2 13b2 17a1 16a1 15a1 40 2 0 0 0 2.00 2.00 the LUMO of bent CO2 and primarily the HOMO of 2 (16a1+) oxalate products, and the relative cheapness and low toxicity 3 0.07 0.09 0.18 0.98 1.99 to form a stabilized orbital, 18a1, that is 2.4 eV more stable than 4 0.22 0.14 0.29 0.77 1.97 of this metal makes it attractive for fundamental reactivity Figurethe 7. Ti−X-bandTi bonding EPR spectrum orbital. In of localized polycrystalline bonding [Ti Pn terms,† ][B- the two donates to the empty2 2 5 0.13 0.12 0.23 0.89 1.98 (C6FM5)4−]M at bonds room temperature are replaced (black by one line) M and−M corresponding bond and a three- studies. 6 0.02 0.07 0.57 1.02 2.00 simulationcenter, (red two-electron line). LUMO (3c-2e) bondof bent linking CO the2 C of the CO2 to the M atoms. The two O atoms have a favorable but weak 7 0 0 0.39 1.31 1.97 Multiple bonds between metal atoms have fascinated 8 0.33 0.83 0.56 0.70 1.43 bridgeheadinteraction C−C with bonds), the Ti and atoms, are accounting consistent for with the a relatively singly longFigure 8. Ti−Ti bonding orbital of Ti2Pn2CO2 (19a1), the LUMO of 41 9 0.03 0.10 0.12 0.42 1.46 chemists for over 50 years, and M−M bonded complexes occupiedTi−O MO distance (SOMO) (2.27 16a Å).1 (Figure 2). bent CO2, and the bonding orbital (18a1) resulting from nucleophilic A. F. R.5 Kilpatrick,5 J. C. Green, and F. G. N. Cloke, Organometallics,5 5 † 2015, 150705080046009. Ti (μ:η ,η -Pn) CO . μ η η attack of Ti2Pn132 on CO20.11. 0.25 0.06 0.67 1.97 2 Further insight2 2 The into CO the2 adduct binding Ti2( of: CO, -Pn2 is)2 givenCO2 by a have also shown ability to facilitate the reductive activation of hasA. been F. R. Kilpatrick, spectroscopically J. C. Green, characterized and F. G. N. Cloke in, solutionOrganometallics, at low 2015, 150704080128001. fragment analysis. Upon bending of CO2, the LUMO is of a1 temperature but is too unstable to be isolated.17 Optimizing the Table 3. Energies (in eV) of Orbital Interactions Divided CO . Recent examples include systems featuring reduced Mg− symmetry and acts as an acceptor orbital. The CO2 HOMO Some remixing between the HOMO and LUMO does occur, 2 This resultedgeometry of Ti in2Pn a2CO color2 from various change starting geometriesfrom deep led to redAccording to dark to Their green, Symmetries; The Various Molecules with 42 43 44 and HOMO−1, located on the O atoms, are of a2 and b2 but on the whole the HOMO−1 of Ti2Pn2 retains its integrity a minimum-energy structure with C2v symmetry (3). Selected C2v Symmetry Are Divided into Ti2Pn2 and Ligand Mg, Cr−Cr, and Pd−Pd cores and a highly polar which accompaniedsymmetry.Figure 2. Thus,Frontier donation quantitative MOs from of these Ti2Pn into2 conversionwith the LUMOD2h symmetry of ofto form(11 ) andto the HOMOC an2v of the CO2 derivative, 19a1 (Figure 8). Figure 7. X-band EPR spectrum ofgeometric polycrystalline parameters [Ti Pn are† ][B- given in Table 1. Fragments Figure 4. MOs of 1 and 2 derived from π5 and π4 of pentalene. Tisymmetry2Pn2, which is (2 2 of). a21 symmetry, is forbidden. Fragment analysis Thus, CO2 may be regarded as acting as a μ-Z ligand. heterobimetallic Zr−Co complex recently reported by(C ThomasF ) ] at room temperature (black line) and corresponding 5 5 † 6 5 4 intermediateenables species, the energies4 of, the that bonding was interactions observed of the Tiby2Pn low-temperature2 Ti23(μ:η ,η -Pn) 42CS2. The 7 adduct of9 CO2 to 13 Ti2Pn 2 has not 45 simulation (red line). fragment with the CO2 fragment to be separated according to A1 been−116 structurally−192 characterized,−81 but− the186 product− of80 CS2 addition and co-workers. spectroscopic measurements but had not been previously17 † n symmetry. The energies attributable to the various interactions A2 has. −Geometry2 − optimization4 −15CV of Ti2 ofPn 0 2CS Ti2 Pnled− to1 structurein THF/0.14, M [ Bu N][PF ] revealed two fi 1 2 2 4 6 Our interest in M−M interactions is focused on thebridgehead aromatic C−C bonds), and are consistentare given with in Table a singly 3. The energy values con rm that donation B1 analogous−7 to 3. Key−8 structural−8 parameters−136 are− given100 in Table 1, detected in a room-temperatureFigure 8. Ti−Ti reaction. bonding orbital of The Ti2Pn2COH2 (19a NMR1), the LUMO of major redox processes within the electrochemical window, as occupied MO (SOMO) 16a1 (Figure 2). bent CO2, and the bonding orbital (18aB2 1) resulting−15 from− nucleophilic21 −24 −165 −125 ligand pentalene (Pn, C8H6), which is related to Cp by5 the5 spectrum of 4 5 at5 −† 30 °C showed four sharp doubletsE in the shown inDOI: Figure 10.1021/acs.organomet.5b00363 5; the data are summarized in Table 2. Process Ti2(μ:η ,η -Pn)2CO2. The CO2 adduct Ti2(μ:η ,η -Pn )2CO2 attack of Ti2Pn2 on CO2. Organometallics XXXX, XXX, XXX−XXX +/0 edge-sharing ring fusion of two cyclopentadienyl fragmentshas been and spectroscopically characterized in solution at low from the HOMO of Ti2Pn2 is theI, centeredpredominant at bondingE1 = −2.48 V vs FeCp , is assigned to a aromatic region,17 consistent with a diamagneticinteraction. complex The occupancies that of the LUMO, HOMO, and/2 2 temperature but is too unstable to be isolated. Optimizing the Table 3. Energies (in eV) of Orbital Interactions Divided † − shows a unique variety of coordination modes in its HOMO−1 of the Ti2Pn2 fragment inreduction3 are given in to Table the 4. monoanion [Ti Pn ] . Repetitive potential geometry of Ti2Pn2CO2 fromexhibits various startingC2 molecular geometries ledsymmetry to According on to the Their NMR Symmetries; time The scale, Various Molecules while with 2 2 46 13 1 C Symmetry Are Divided into Ti Pn and Ligand organometallic complexes. In particular, the abilitya minimum-energy of Pn to structurethe with CC{2vThesymmetryH} Ti−Ti spectrum ( distance3). Selected is short showed (2.412v Å), a indicating singlet strong at 219Table ppm 4.2 Occupancies2 with of no the Fragment Orbitals of Ti Pn in Fragments 2 2 bind two metal centers with η5 hapticity has provengeometric ideal parameters for arefurther given in Tablebonding labeled 1. between13 theC Ti signals. atoms. The In pentalene situ rings IR are studies bent the of Molecular4 at − Calculations65 °C for 2, 3, 4, 5, 6, 7, 8, 9, and 13 back slightly more than in Ti2Pn2. Examination of the3 MOs of 3 4 7 9 13 the synthesis of bimetallic analogues of the ubiquitious (Figure 8) shows that the key bonding interaction is between −1 14b2 13b2 17a1 16a1 15a1 showed distinct bands at 1678A1 and− 1236116 cm−192 , which2−81 shifted0−186 0 to −80 0 2.00 2.00 the LUMO of bent CO−2 1and primarily the HOMO of 2 (16a1) A2 −2 −4 3−150.07 0 0.09−1 0.18 0.98 1.99 bis(Cp)metallocenes, so-called “double-sandwich” com- 1637 andto form 1217 a stabilized cm orbital,, 18a respectively,1, that is 2.4 eV more in stable the than isotopically labeled 47−53 13 the Ti−Ti bonding orbital. In localizedB bonding1 terms,−7 the two −8 4 −8 0.22−136 0.14−100 0.29 0.77 1.97 5 0.13 0.12 0.23 0.89 1.98 plexes. We have recently reported the synthesis and [ C]4M.−M These bonds are IR replaced bands by one M are−BM2 bond assigned and−15 a three- to−21 asymmetric−24 −165 and −125 6 0.02 0.07 0.57 1.02 2.00 center, two-electron (3c-2e) bond linking the C of the CO2 to characterization of the titanium double-sandwich complex symmetric ν(OCO) stretches, respectively, of the7 CO02 ligand 0 0.39 1.31 1.97 the M atoms. The two O atoms havefrom a the favorable HOMO but weak of Ti2Pn2 is the predominant bonding 5 5 † 8 0.33 0.83 0.56 0.70 1.43 Ti2(μ:η ,η -Pn )2 (1) using the silylated pentalene ligand 1,4- by analogyinteraction with with the Ti atoms, the accounting monomericinteraction. for the relatively The complex occupancies long of Cp the2 LUMO,Ti(CO HOMO,2)- and i Ti−29O distance (2.27 Å). 9 0.03 0.1057 0.12 0.42 1.46 † HOMO−1 of the Ti2Pn2 fragment13 in 3 are0.11 given in0.25 Table 4. 0.06 0.67 1.97 {Si Pr3}2C8H4 (Pn ). In addition to providing a rare example of (PMe3),Furtherwhich insight into was the bindingstudied of CO in-depth2 is given by by a Mascetti et al. fragment analysis. Upon bending of CO2, the LUMO is of a1 a Ti−Ti multiple bond, 1 exhibits a solid-state structureThe unique Ti−Ti distance isand short shows (2.41symmetry Å), a indicating C-coordinatedand acts as strong an acceptor orbital.Table bonding 4. The Occupancies CO mode.HOMO of Compound theSome Fragment remixing Orbitals4 betweencould of theTi2Pn HOMO2 in and LUMO does occur, bonding between the Ti atoms. The pentalene rings are bent the Molecular2 Calculations for 2, 3, 4, 5, 6, 7, 8, 9, and 13 and HOMO−1, located on the O atoms, are of a2 and b2 but on the whole the HOMO−1 of Ti2Pn2 retains its integrity among the M2Pn2 complexes in which the pentalene ligandsback slightly are more than innot Ti Pn be. Examination isolatedFigure of the 3.because MOsDerivation of 3 of of its the susceptibility frontier orbitals to of Ti further2Pn2 from reaction. those of 54 2 2 symmetry. Thus, donation from these into the14b LUMO of13b to form17a the HOMO16a of the15a CO2 derivative, 19a1 (Figure 8). two metallocenes. 2 2 1 1 1 nonparallel and tilted around the Ti core. DFT studies(Figure 8) shows on that the keyWhen bondingTi2Pn interaction42, whichwas is of is allowed a1 betweensymmetry, is to forbidden. warm Fragment from analysis−78 Thus,°C CO to2 may ambient be regarded as acting as a μ-Z ligand. 2 2 0 0 05 5 2.00 2.00 † the LUMO of bent CO2 and primarilyenables the HOMO the energies of 2 of(16a the1 bonding) interactions of the Ti2Pn2 Ti2(μ:η ,η -Pn)2CS2. The adduct of CO2 to Ti2Pn 2 has not 3 0.07 0.09 0.18 0.98 1.99 the model system Ti2Pn2 calculated an M−M bond orderto form a ofstabilized 2 orbital,temperature 18a1, thatfragment is 2.4 eV with more under the stable CO2 fragment than dynamic to be separated vacuum, according the to redbeen structurally mono(oxo) characterized, but the product of CS2 addition 4 0.22 0.1417 0.29 0.77 1.97 the Ti−Ti bonding orbital. In localizedsymmetry. bonding The terms, energies the attributable two to the various interactions has. Geometry optimization of Ti2Pn2CS2 led to structure 4, and an optimized geometry with C molecular symmetry, complex 5 was isolated (Schemefi 1). 5 could also be obtained 2v M−M bonds are replaced by oneare M− givenMbecomes bond in Table and 3. athe The three- energy 12b2 valuesorbital con5 rm in that0.13C2 donationv symmetry, 0.12analogous 0.23and to its3. Key 0.89 energy structural 1.98 parameters are given in Table 1, 6 0.02 0.07ff 0.57 1.02 2.00 providing a relatively open structure in which thecenter, frontier two-electron (3c-2e)by bond the linking slowdecreases. the addition C of the COThis2 ofto situation 1 equiv is reminiscent of N2O to of1E thevia e Toeplerect of bending line. DOI: 10.1021/acs.organomet.5b00363 the M atoms. The two O atoms1 have ain favorable parallel but metallocenes. weak 257 0 0 0.39 1.31 1.97 Organometallics XXXX, XXX, XXX−XXX orbitals are sterically accessible. interaction with the Ti atoms,The accountingH NMRfor the relatively spectrum long of8 5 is0.33 again 0.83 consistent 0.56 with 0.70 a 1.43 Electrochemical Studies.9 0.03Cyclic voltammetry0.10 0.12 (CV) 0.42 of 1.46 The combination of a low-valent group IV metalTi−O and distance a (2.27 Å). diamagnetic complex with C2 symmetry, and elemental analysis Ti Pn† was carried out13 to assess0.11 the 0.25 stability 0.06 of the 0.67 mixed- 1.97 metal−metal double-bonded system led us to exploreFurther the insight into theand binding mass of CO spectrometry22 is2 given by a measurements support the proposed fragment analysis. Upon bending of CO2,valence the LUMO form is of a of1 the bimetallic complex and to choose an reactivity of 1 with small molecules. We recently communicatedsymmetry and acts as anformulation. acceptor orbital.appropriate The The CO2 HOMO molecular chemicalSome redox structure remixing agent between determined for the its HOMO preparation and by LUMO single- on does a occur, † and HOMO−1, located on the O atoms, are of a and b but on the whole the HOMO−1 of Ti Pn retains its integrity Figure 5. Overlaid CV scans (three cycles) for Ti2Pn 2 in THF/0.1 M ff 2 2 “ 2 2 ” n −1 the room-temperature reaction of 1 with 1 equiv of COsymmetry.2 to give Thus, donationcrystal from these X-raysynthetic into the di LUMOraction scale. of (XRD)to form the reveals HOMO of a thedimetallaepoxide CO2 derivative, 19a1 (Figure 8). [ Bu4N][PF6 ] at a scan rate of 100 mV s . Ti2Pn2, which is of a1 symmetry, is forbidden. Fragment analysis Thus, CO2 may be regarded as acting as a μ-Z ligand. 5 5 † C DOI: 10.1021/acs.organomet.5b00363 enables the energies of the bonding interactions of the Ti2Pn2 Ti2(μ:η ,η -Pn)DOI:2CS2 10.1021/acs.organomet.5b00315. The adduct of CO2 to Ti2Pn 2 has not B Organometallics XXXX, XXX, XXX−XXX fragment with the CO2 fragment to be separated according to been structurallyOrganometallics characterized, butXXXX, the product XXX, XXX of CSXXX2 addition 17 − symmetry. The energies attributable to the various interactions has. Geometry optimization of Ti2Pn2CS2 led to structure 4, are given in Table 3. The energy values confirm that donation analogous to 3. Key structural parameters are given in Table 1,

E DOI: 10.1021/acs.organomet.5b00363 Organometallics XXXX, XXX, XXX−XXX Summary

1.!There are four different types of 3c-2e bonds: µo-Z, µc-Z, µ-X and µ-L.

2.!Structure-bonding (S-B) representations of a molecule that utilize the half- arrow or dot-line representation of the molecules which contain 3c-2e bonds indicate electron counts and bond orders that are much more in accord with theory than those predicted by other representations that focus on oxidation states and other methods of counting electrons.

3.!SB representations that use the half-arrow and dot-line formalisms provide a simple and direct method for counting electrons and also determining the MLX class of any atom in in a molecule.

4. Useful information pertaining to the CBC method may be found at:

“Application of the Covalent Bond Classification Method for the Teaching of Inorganic Chemistry.” Malcolm L. H. Green and Gerard Parkin J. Chem. Educ. 2014, 91, 807. http://www.covalentbondclass.org