Four-electron Donor Bridging Carbonyl Group Research Articles

The role of the phosphorus lone pair in the low-energy binuclear phospholyl vanadium carbonyl structures: comparison with cyclopentadienyl analogues

The structures and energetics of the binuclear phospholyl vanadium carbonyls (C4H4P)2V2(CO)n (n = 7, 6, 5, 4, 3, 2, 1) have been investigated using density functional theory. The lowest energy heptacarbonyl (C4H4P)2V2(CO)7 structures resemble those of the dimanganese pentacarbonyl (C4H4P)2Mn2(CO)5 with one seven-electron donor bridging η5,η1-C4H4P ring and no direct vanadium–vanadium bond. Similarly, the lowest energy hexacarbonyl (C4H4P)2V2(CO)6 structure resembles that of the dimanganese tetracarbonyl (C4H4P)2Mn2(CO)4 with two seven-electron donor bridging η5,η1-C4H4P rings and no direct vanadium–vanadium bond. The lowest energy pentacarbonyl (C4H4P)2V2(CO)5 structure has terminal η5-C4H4P phospholyl rings and a formal V≡V triple bond of length ~ 2.45 A similar to the experimentally known and structurally characterized cyclopentadienyl analogue (η5-C5H5)2V2(CO)5. Various combinations of V=V double bonds or V≡V triple bonds, four-electron donor bridging carbonyl groups and seven-electron donor bridging η5,η1-C4H4P rings are found in the more highly unsaturated derivatives (C4H4P)2V2(CO)n (n = 4, 3, 2) to give 17- and 18-electron vanadium configurations for triplet- and singlet-state structures, respectively. Formal V≣V quadruple bonds are found only in the highly unsaturated lowest energy singlet (C4H4P)2V2(CO)n (n = 2, 1) structures, which, however, lie at somewhat higher energies than isomeric triplet structures.

Extreme Metal Carbonyl Back Bonding in Cyclopentadienylthorium Carbonyls Generates Bridging C2O2 Ligands by Carbonyl Coupling

Laboratory studies of the interaction of carbon monoxide with organoactinides result in the formation of isolable complexes such as Cp3UCO derivatives (Cp = cyclopentadienyl) as well as coupling reactions to give derivatives of the oligomeric anions C(n)O(n)(2-) (n = 2, 3, 4). To gain some insight into actinide carbonyl chemistry, binuclear cyclopentadienylthorium carbonyls Cp2Th2(CO)n (n = 2 to 5) as model compounds have been investigated using density functional theory. The most favorable such structures in terms of energy and thermochemistry are the tricarbonyl Cp2Th2(η(2)-μ-CO)3 having three four-electron donor bridging carbonyl groups and the tetracarbonyl Cp2Th2(η(4)-μ-C2O2)(η(2)-μ-CO)2 having not only two four-electron donor bridging carbonyl groups but also a bridging ethynediolate ligand formed by coupling two CO groups through C-C bond formation. The bridging infrared ν(CO) frequencies ranging from 1140 to 1560 cm(-1) in these Cp2Th2(CO)n (n = 3, 4) derivatives indicate extremely strong Th→CO back bonding in these structures, corresponding to formally dianionic CO(2-) and C2O2(2-) ligands and the favorable +4 thorium oxidation state. A characteristic of the Cp2Th2(η(2)-μ-CO)3 and Cp2Th2(η(4)-μ-C2O2)(η(2)-μ-CO)2 structures is their ability to add terminal CO groups, preferably to the thorium atom bonded to the fewest oxygen atoms. These terminal CO groups exhibit ν(CO) frequencies in a similar range as terminal CO groups in d-block metal carbonyls. However, these terminal CO groups are relatively weakly bonded to the thorium atoms as indicated by predicted CO dissociation energies of 14 kcal/mol for Cp2Th2(CO)5. Two low energy structures for the dicarbonyl Cp2Th2(CO)2 are found with two separate four-electron donor bridging CO groups and relatively short Th-Th distances of 3.3 to 3.4 Å suggesting formal single bonds and +3 thorium formal oxidation states. However, a QTAIM analysis of this formal Th-Th bond does not reveal a bond critical point thus suggesting a multicenter bonding model involving the bridging CO groups.

The Remarkable Nb2(CO)12 with Seven-Coordinate Niobium: Decarbonylation to Nb2(CO)11 and Nb2(CO)10.

The dissociation of Nb2(CO)12 into two Nb(CO)6 units is predicted to require ∼13 kcal/mol so that Nb2(CO)12 rather than Nb(CO)6 is the anticipated initial oxidation product of the known Nb(CO)6(-) anion. This differs from the corresponding vanadium carbonyl chemistry where V(CO)6 rather than V2(CO)12 is found experimentally to be the oxidation product of V(CO)6(-). The lowest energy Nb2(CO)12 structure consists of two Nb(CO)6 fragments joined by a Nb-Nb bond of ∼3.4 Å length so that each niobium atom is heptacoordinate, counting the metal-metal bond. These niobium coordination polyhedra can be approximated by capped octahedra. Among unsaturated binuclear niobium carbonyls the lowest energy Nb2(CO)11 structure has a formal four-electron donor bridging η(2)-μ-CO group and a formal Nb-Nb single bond rather than only two-electron donor carbonyl groups and a formal Nb═Nb double bond. The Nb2(CO)11 structures with formal Nb═Nb double bonds and exclusively two-electron donor carbonyl groups lie more than 13 kcal/mol above this low-energy Nb2(CO)10(η(2)-μ-CO) structure. However, Nb2(CO)11 is predicted to be thermodynamically disfavored, owing to disproportionation into Nb2(CO)12 + Nb2(CO)10, a slightly exothermic process by ∼4 kcal/mol. The Nb2(CO)10 structures with formal Nb≡Nb triple bonds and all two-electron donor carbonyl groups appear to be particularly favorable, as suggested by high CO dissociation energies and viability toward disproportionation. Such structures are isolobal with Cp2Mo2(CO)4, which was the first stable metal carbonyl to be discovered with a short metal-metal distance, corresponding to a formal triple bond. Considerably higher energy Nb2(CO)10 structures (by more than 20 kcal/mol) have two four-electron donor bridging carbonyl groups and long niobium-niobium distances. Such structures can be considered to consist of a bidentate Nb(CO)6 "ligand" coordinating to a Nb(CO)4 unit through the two η(2)-μ-CO groups.

Unsaturation in binuclear cyclopentadienylrhodium carbonyls: Comparison with their cobalt analogs

The Cp 2Rh 2(CO) n ( n = 4, 3, 2, 1) derivatives have been examined by density functional theory using the BP86 and MPW1PW91 functionals. The known tricarbonyl Cp 2Rh 2(CO) 3 is predicted to have a singly bridged structure with a predicted Rh–Rh single bond distance of ∼2.70 Å in close agreement with the experimental value of 2.68 Å, determined by X-ray crystallography. In contrast to the cobalt analog, no evidence for a triply bridged Cp 2Rh 2(μ-CO) 3 structure was found. The known dicarbonyl Cp 2Rh 2(CO) 2 is predicted to have a doubly bridged structure with a predicted Rh Rh double bond distance of 2.58 Å in close agreement with the experimental Rh Rh double bond distance of 2.564 Å, found by X-ray crystallography for the permethylated derivative (η 5-Me 5C 5) 2Rh 2(μ-CO) 2. The monocarbonyl Cp 2Rh 2(CO) is predicted to have a four-electron donor bridging carbonyl group with a Rh–O distance of ∼2.5 Å and a Rh Rh double bond distance of ∼2.54 Å. This differs from Cp 2Co 2(CO) which was previously predicted to have only a two-electron donor bridging carbonyl group with a long Co⋯O distance and a short Co Co distance of ∼2.0 Å suggesting a formal triple bond. For Cp 2Rh 2(CO) 4 doubly bridged trans and cis isomers were found within ∼1.0 kcal/mol in energy with non-bonding Rh⋯Rh distances of ∼3.2 Å. However, these Cp 2Rh 2(CO) 4 isomers are predicted to be unstable with respect both to CO loss to give Cp 2Rh 2(CO) 3 and to fragmentation into two CpRh(CO) 2 units.

Possibilities for Titanium−Titanium Multiple Bonding in Binuclear Cyclopentadienyltitanium Carbonyls: 16-Electron Metal Configurations and Four-Electron Donor Bridging Carbonyl Groups as Alternatives

The structures for the binuclear Cp(2)Ti(2)(CO)(n) derivatives (Cp = eta(5)-C(5)H(5); n = 8, 7, 6, 5, 4, 3, 2) have been optimized using density functional theory. Furthermore, the thermodynamics of CO dissociation, disproportionation into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1), and dissociation into mononuclear fragments of these Cp(2)Ti(2)(CO)(n) derivatives have been studied. An unbridged Cp(2)Ti(2)(CO)(8) structure with a long approximately 3.9 A Ti-Ti bond is found. As expected from the long Ti-Ti bond, the predicted dissociation energy of this dimer into CpTi(CO)(4) fragments is relatively low at 7 +/- 3 kcal/mol. The lowest energy Cp(2)Ti(2)(CO)(6) structure has two CpTi(CO)(3) units linked by a formal approximately 2.8 A Ti identical withTi triple bond and thus is the next member of the M identical withM triply bonded series Cp(2)V(2)(CO)(5), Cp(2)Cr(2)(CO)(4), Cp(2)Mn(2)(CO)(3), all three of which are stable compounds. The lowest energy structures of Cp(2)Ti(2)(CO)(7), Cp(2)Ti(2)(CO)(5), and Cp(2)Ti(2)(CO)(4) all contain one or two four-electron donor bridging eta(2)-mu-CO groups. However, they are not likely to be stable molecules since their disproportionation energies into Cp(2)Ti(2)(CO)(n+1) + Cp(2)Ti(2)(CO)(n-1) are either nearly thermoneutral (n = 5) or exothermic (n = 7 and 4). The lowest energy structure of Cp(2)Ti(2)(CO)(3), in which all three carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a approximately 3.05 A formal Ti-Ti single bond, is a promising synthetic target since it is thermodynamically stable with respect to both CO dissociation and disproportionation into Cp(2)Ti(2)(CO)(4) + Cp(2)Ti(2)(CO)(2). In the lowest energy Cp(2)Ti(2)(CO)(2) structure both carbonyl groups are four-electron donor eta(2)-mu-CO groups bridging a formal 2.74 A Ti[triple bond]Ti triple bond. These low energy Cp(2)Ti(2)(CO)(n) (n = 3, 2) structures have only a 16-electron titanium configuration rather than the usually favorable 18-electron configuration for metal carbonyl complexes.

Unsaturation and variable hapticity in binuclear azulene manganese carbonyl complexes

Azulene is reported to react with Mn(2)(CO)(10) to give trans-C(10)H(8)Mn(2)(CO)(6), which has been shown by X-ray crystallography to have a bis(pentahapto) structure with no metal-metal bond. This structure is found by density functional theory to be the lowest energy C(10)H(8)Mn(2)(CO)(6) structure. However, a corresponding bis(pentahapto) cis-C(10)H(8)Mn(2)(CO)(6) structure, also without an Mn···Mn bond, lies within ~1 kcal mol(-1) of this global minimum. The lowest energy C(10)H(8)Mn(2)(CO)(5) structure is singlet cis-η(5),η(5)-C(10)H(8)Mn(2)(CO)(5) with an Mn→Mn dative bond from the Mn(CO)(3) group to the Mn(CO)(2) group. However, a singlet cis-η(6),η(4)-C(10)H(8)Mn(2)(CO)(5) structure with a normal Mn-Mn single bond lies within ~6 kcal mol(-1) of this global minimum. The lowest energy structures of the more highly unsaturated C(10)H(8)Mn(2)(CO)(n) (n = 4, 3, 2) systems all have cis geometries and manganese-manganese bonds of various orders. The corresponding global minima are triplet cis-η(5),η(3)-C(10)H(8)Mn(2)(CO)(3)(η(2)-μ-CO) for the tetracarbonyl with a four-electron donor bridging carbonyl group, singlet cis-η(5),η(5)-C(10)H(8)Mn(2)(CO)(3) for the tricarbonyl, and triplet cis-η(6),η(4)-C(10)H(8)Mn(2)(CO)(η(2)-μ-CO) for the dicarbonyl.

Terminal versus bridging cyclobutadiene rings in binuclear nickel carbonyl derivatives: A cube-antiprism twist of the cyclobutadiene rings in the perpendicular structures

Cyclobutadiene–nickel derivatives are accessible from reactions of dichlorocyclobutenes with nickel carbonyl. In this connection the binuclear cyclobutadiene–nickel carbonyls (C4H4)2Ni2(CO)n (n = 3, 2, 1) have been examined theoretically as possible sources of new dimetallocene structures and new structures with metal–metal multiple bonds. The lowest energy (C4H4)2Ni2(CO) isomer is predicted to have a perpendicular structure with two bridging C4H4 rings forming an elongated square antiprism enclosing the pair of nickel atoms. The NiNi distance of ∼2.25 A corresponds to the formal triple bond required to give both nickel atoms the favored 18-electron configuration, with all eight carbon atoms of the two C4H4 rings involved in M–C bonding. A related low energy structure is found for (C4H4)2Ni2(CO)2 except that the two bridging C4H4 rings form an elongated cube enclosing a pair of nickel atoms at a somewhat longer bonding distance (∼2.33 A). In this case only a NiNi double bond is required to give both nickel atoms the 18-electron configuration. Other low energy (C4H4)2Ni2(CO)n (n = 1, 2, 3) structures include a monocarbonyl (η4-C4H4)2Ni2(η2-μ-CO) with a four-electron donor bridging carbonyl group, a coaxial doubly bridged dicarbonyl (η4-C4H4)2Ni2(μ-CO)2, and cis and trans isomers of a bent singly bridged tricarbonyl (η4-C4H4)2Ni2(CO)2(μ-CO).

Formation of a four-electron donor carbonyl group in the decarbonylation of the unsaturated H 2C 2Fe 2(CO) 6 tetrahedrane as an alternative to an iron–iron triple bond

The unsaturated Fe 2C 2 tetrahedrane derivatives R 2C 2Fe 2(CO) 6 (R = Ph, t Bu) are among the many products obtained from reactions of the alkynes RC CR with iron carbonyls. In this connection theoretical studies have been performed on the simplest such compounds H 2C 2Fe 2(CO) n ( n = 6, 5) for comparison with the experimentally known structure of the t-butyl derivative t-Bu 2C 2Fe 2(CO) 6 and in order to predict the decarbonylation pathways for such (alkyne)Fe 2(CO) 6 derivatives. These theoretical studies predict an Fe 2C 2 tetrahedrane structure for H 2C 2Fe 2(CO) 6 with a formal Fe Fe double bond very similar to the experimental structure for t-Bu 2C 2Fe 2(CO) 6. Decarbonylation of H 2C 2Fe 2(CO) 6 is predicted to give an H 2C 2Fe 2(CO) 5 isomer retaining the Fe 2C 2 tetrahedrane structure, with an Fe Fe double bond but with the unprecedented feature of a four-electron donor bridging carbonyl group in an M 2C 2 tetrahedrane structure. The formation of formal Fe Fe triple bonds appears to be avoided in even the higher energy H 2C 2Fe 2(CO) 5 structures. These include three triplet Fe 2C 2 tetrahedrane structures with formal Fe Fe double bonds as well as a coordinately unsaturated singlet structure, still with an Fe Fe double bond.

Dimetallocene Carbonyls of the Third-Row Transition Metals: The Quest for High-Order Metal−Metal Multiple Bonds

Theoretical studies of the third-row transition-metal derivatives Cp(2)M(2)(CO) (Cp = eta(5)-C(5)H(5); M = Os, Re, W, Ta) indicate that the lowest-energy structures have lower spin states and similar or higher metal-metal bond multiplicities than the corresponding first-row transtion-metal derivatives. Therefore, Cp(2)Os(2)(CO) is predicted to be a singlet with an Os-Os formal quadruple bond, whereas Cp(2)Fe(2)(CO) is a triplet. Similarly, Cp(2)Re(2)(CO) is predicted to be a singlet with a very short rhenium-rhenium distance, which is consistent with the formal quintuple bond required to give both rhenium-rhenium atoms the favored 18-electron configuration. This contrasts with the manganese analogue Cp(2)Mn(2)(CO) for which the lowest-energy structure is a septet with a formal Mn-Mn single bond. The tungsten derivative Cp(2)W(2)(CO) is predicted to be triplet with a four-electron donor bridging carbonyl group. This contrasts with Cp(2)Cr(2)(CO) predicted to be a septet (S = 3) with a two-electron donor carbonyl group. For Cp(2)Ta(2)(CO), the lowest-energy structure is predicted to be a triplet with a formal Ta identical withTa triple bond and a four-electron donor carbonyl group. However, Cp(2)V(2)(CO) is predicted to be a quintet with a formal V horizontal lineV double bond. In addition to these Cp(2)M(2)(CO) structures with one Cp ring bonded to each metal atom, higher-energy Cp(2)M-MCO structures are found with both Cp rings bonded to the same metal atom. The lowest-energy Cp(2)M-MCO structures are triplets (M = Os, W) or quintets (M = Re) with agostic hydrogen atoms for M = Os and Re. In these structures, the spin density is concentrated on the metal atom of the MCO group. These results suggest that lower spin states clearly become more viable for highly unsaturated metal complexes upon descending the periodic table.

Dimetallocene carbonyls: The limits of the 18-electron rule and metal–metal multiple bonding in highly unsaturated molecules of the early transition metals

The structures and energetics of Cp 2M 2(CO) (M = Mn, Cr, and Ti) have been investigated by density functional theory (DFT) for comparison with previously reported studies on Cp 2V 2(CO). For all four metals energetically competitive structures are found with singlet, triplet, quintet, and septet spin multiplicities. However, significant spin contamination was found for the triplet Cp 2Mn 2(CO) and Cp 2Cr 2(CO) structures. The lowest energy structures for Mn and Cr are both septet states, namely Cp 2Mn 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and an Mn Mn distance of 2.523 Å and Cp 2Cr 2(μ-CO) with a two-electron donor bridging carbonyl group and a Cr Cr distance of 2.436 Å. However, for the Ti analogue the lowest energy structure is singlet Cp 2Ti 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and a Ti Ti distance of 2.364 Å. The higher energy singlet structures of Cp 2M 2(CO) have the very short metal–metal distances of 1.879 Å (M = Mn) and 1.729 Å (M = Cr) suggesting very high formal bond orders of five and six, respectively.

Interplay between Two-Electron and Four-Electron Donor Carbonyl Groups in Oxophilic Metal Systems: Highly Unsaturated Divanadocene Carbonyls

The divanadocene carbonyls Cp2V2(CO)n (n = 5, 4, 3, 2, 1; Cp = eta5-C5H5) have been studied by density functional theory using the B3LYP and BP86 functionals. The global minimum for Cp2V2(CO)5 with a V[triple bond]V distance of 2.452 A (BP86) is essentially the same as the structure of the known Cp2V2(CO)5 determined by X-ray diffraction. The global minimum of Cp2V2(CO)4 is a triplet electronic state with a V[triple bond]V distance of 2.444 A (BP86). However, slightly higher energy singlet Cp2V2(CO)4 structures are found either with a V[triple bond]V distance of 2.547 A (BP86) and one four-electron donor bridging CO group or with a V[quadruple bond]V distance of 2.313 A (BP86) and all two-electron donor bridging CO groups. Comparison is made between Cp2V2(CO)3 and the recently synthesized quintuply bonded RCrCrR (R = bulky aryl group) complexes of Power and co-workers. Four-electron donor bridging carbonyl groups become more prevalent upon further decarbonylation, leading ultimately to three singlet Cp2V2(eta2-mu-CO)2 isomers as well as triplet, quintet, and septet structures of Cp2V2(CO) with extremely low nu(CO) frequencies around 1400 cm(-1). Our most remarkable structural finding is the extremely short vanadium-vanadium distance (1.80 A, BP86) predicted for the singlet structure of Cp2V2(CO).