Favored 18-electron Configuration Research Articles

Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron-Iron Multiple Bonding.

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (μ-RP)Fe2(μ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal Fe=Fe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration.

Low-energy high-spin binuclear hexamethylbenzene manganese carbonyl structures with partially bonded benzene rings

The geometries and energetics of binuclear hexamethylbenzene manganese carbonyls (Me6C6)2Mn2(CO)n (n = 4, 3, 2, 1), including the experimentally known tetracarbonyl, have been examined using density functional theory. Relatively low- energy structures include trans- and cis-(η6-Me6C6)2Mn2(CO)2(µ-CO)2 doubly bridged tetracarbonyl structures and a triplet (η6-Me6C6)2Mn2(µ-CO)3 triply bridged structure analogous to experimentally known isoelectronic binuclear cyclopentadienyliron carbonyls. However, in addition, a series of low-energy quintet spin state structures (η10–2n-Me6C6)Mn(CO)n–Mn(η6-C6Me6) (n = 4, 3, 2) is found. The tetracarbonyls and tricarbonyl of this series have partially bonded hexamethylbenzene rings suggesting a certain fragility of the hexamethylbenzene-manganese linkage not found in the isoelectronic cyclopentadienyliron compounds. For the dicarbonyl and monocarbonyl (Me6C6)2Mn2(CO)n (n = 2, 1) systems, singlet structures having the triple or quadruple manganese-manganese bonds necessary to give each manganese atom the favored 18-electron configuration are very high-energy structures relative to isomeric triplet and quintet spin state structures.

Carbon-hydrogen bond activation in bridging cyclobutadiene ligands in unsaturated binuclear vanadium carbonyl derivatives.

The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory (DFT). The lowest energy (C4H4)2V2(CO)8 structure consists of two C4H4V(CO)4 units linked by a V-V single bond of length 3.4Å. The two lowest energy (C4H4)2V2(CO)7 structures also have formal V-V single bonds. The "extra" two electrons to give each vanadium atom in these heptacarbonyls the favored 18-electron configuration can come from either an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings or from a four-electron donor bridging η2-µ-CO group with a short V-O distance. The lowest energy (C4H4)2V2(CO)6 structure has a formal V≡V triple bond of length 2.52Å similar to the V≡V triple bond of length 2.46Å found in the experimentally known cyclopentadienyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structures for the more highly unsaturated (C4H4)2V2(CO)n (n = 5, 4, 3, 2) have at least two four-electron donor bridging η2-µ-CO groups and a vanadium-vanadium bond order sufficient to give each vanadium atom at least a 16-electron configuration. The structures and energetics of the binuclear cyclobutadiene vanadium carbonyls (C4H4)2V2(CO)n (n = 8, 7, 6, 5, 4, 3, 2) have been investigated by density functional theory. The two lowest energy (C4H4)2V2(CO)7 structures include one with an agostic C-H-V interaction activating a hydrogen atom from one of the cyclobutadiene rings and another with a four-electron donor bridging η2-µ-CO group with a short V-O bonding distance.

Binuclear 1,3-diphosphacyclobutadiene vanadium carbonyls: Bending of the P2C2 ring in an unsaturated system with a vanadium-vanadium multiple bond

The structures and energetics of the binuclear 1,3-diphosphacyclobutadiene vanadium carbonyls of the type (Me2C2P2)2V2(CO)n have been investigated by density functional theory. The lowest energy (Me2C2P2)2V2(CO)n (n = 8, 7, 6, 5) structures are singlet spin state structures with various combinations of single and double vanadium-vanadium bonds and six-electron donor bridging η1,η4-Me2C2P2 ligands to give each vanadium atom the favored 18-electron configuration. Higher energy singlet (Me2C2P2)2V2(CO)6 structures have terminal η4-Me2C2P2 ligands and formal VV triple bonds similar to that found in the experimentally known binuclear cyclopentadienylvanadium carbonyl derivative (η5-C5H5)2V2(CO)5. The lowest energy structure of the highly unsaturated (Me2C2P2)2V2(CO)4 by a significant margin has an unprecedented type of eight-electron donor bent tetrahedrane-like Me2C2P2 ligand not found in the low energy structures of the later first row transition metals. The lowest energy structure of the even more highly unsaturated (Me2C2P2)2V2(CO)3 has a vanadium (η4-Me2C2P2)2V sandwich functioning as a bidentate chelating ligand to a V(CO)3 unit supplemented by a VV triple bond.

Binuclear cyclopentadienyliridium hydride chemistry: Terminal versus bridging hydride and cyclopentadienyl ligands

The cyclopentadienyliridium hydrides Cp2Ir2Hn (Cp=η5-C5H5; n=6, 4, 2, 0), and CpIrHn (n=4, 2) related to the experimentally known pentamethyl cyclopentadienyliridium hydrides Cp∗2Ir2Hn (Cp∗=η5-Me5C5; n=6, 2) and Cp∗IrH4 have been investigated by density functional theory. The lowest energy Cp2Ir2Hn (n=6, 4, 2) structures are predicted to have terminal Cp rings with the central Ir2 unit bridged by two hydrogen atoms. For the hexahydride Cp2Ir2H6, such doubly bridged Cp2Ir2(μ-H)2H4 structures are bent, leading to trans and cis structures of similar energies with an unbridged Cp2Ir2H6 isomer lying only ∼4kcal/mol above the bridged structures. This suggests fluxional behavior consistent with experimental data on the temperature dependence of the proton NMR spectrum of the closely related Cp∗2Ir2H6. The tetrahydride Cp2Ir2H4 is predicted to undergo slightly exothermic disproportionation into Cp2Ir2H6+Cp2Ir2H2 and thus not be a viable species. This is consistent with the failure to find any Cp∗2Ir2H4 in the Cp∗2Ir2Hn systems. The doubly bridged dihydride Cp2Ir2(μ-H)2 is a particularly favorable species since it lies more than 18kcal/mol in energy below any other isomer. Higher energy Cp2Ir2Hn (n=4, 2) structures have one or two bridging Cp rings and exclusively terminal hydrogen atoms. Related structures are the lowest energy structures for the hydride-free Cp2Ir2. A higher energy Cp2Ir2 structure consists of two CpIr units linked solely by an Ir–Ir bond. Analysis of the frontier molecular orbitals indicates this Ir–Ir bond to be the quadruple bond required to give each iridium atom the favored 18-electron configuration. However, this quadruple bond is a 2σ+2π bond with no δ components and thus differs from the σ+2π+δ quadruple bond found in the long-known Re2Cl82−.

Nickelacyclopentadienylchromium tricarbonyl unit as a bulky pseudohalogen in cyclopentadienylchromium complexes leading to low-energy high-spin structures.

Recent studies, particularly from the laboratory of Buchalski and co-workers, have resulted in the syntheses of nickelacyclopentadienyl and nickelafluorenyl metallacycles that can function as pentahapto ligands in transition-metal complexes, similar to the ubiquitous cyclopentadienyl ligand. The structures and energetics of the neutral binuclear chromium carbonyls (CpNiC4H4)2Cr2(CO)n (n = 6, 5, 4, 3; Cp = η(5)-C5H5) containing the unsubstituted nickelacyclopentadienyl ligand have been investigated by density functional theory. The lowest energy (CpNiC4H4)2Cr2(CO)n (n = 6, 4) structures are similar to the corresponding experimentally characterized Cp2Cr2(CO)n structures with predicted Cr-Cr distances of ∼3.22 and ∼2.27 Å corresponding to the formal single and triple bonds, respectively. This gives the chromium atoms, as well as the nickel atoms, the favored 18-electron configuration. These species appear to be promising synthetic targets. However, the lowest energy (CpNiC4H4)2Cr2(CO)n (n = 5, 3) structures, as well as two (CpNiC4H4)2Cr2(CO)4 structures ∼10 to 12 kcal/mol in energy above the global minimum, can be dissected into a discrete pseudohalogen (CpNiC4H4)Cr(CO)3 unit and a (CpNiC4H4)Cr(CO)n-3 unit linked by a Cr-Cr bond flanked by one to three generally weakly semibridging CO groups. In most cases, the chromium atoms in the (CpNiC4H4)Cr(CO)n-3 units of these structures have 14-16-electron configurations rather than the favored 18-electron configuration. This leads to triplet and even quintet spin states in the lowest energy structures.

Infrared photodissociation spectroscopy of mass-selected heteronuclear iron-copper carbonyl cluster anions in the gas phase.

Mass-selected heteronuclear iron-copper carbonyl cluster anions CuFe(CO)n(-) (n = 4-7) are studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. The cluster anions are produced via a laser vaporization supersonic cluster ion source. Their geometric structures are determined by comparison of the experimental spectra with those calculated by density functional theory. The experimentally observed CuFe(CO)n(-) (n = 4-7) cluster anions are characterized to have (OC)4Fe-Cu(CO)n-4 structures, each involving a C3v symmetry Fe(CO)4(-) building block. Bonding analysis indicates that the Fe-Cu bond in the CuFe(CO)n(-) (n = 4-7) cluster anions is a σ type single bond with the iron center possessing the most favored 18-electron configuration. The results provide important new insight into the structure and bonding of hetronuclear transition metal carbonyl cluster anions.

The flexibility of the cycloheptatrienyl ring in cycloheptatrienylvanadium carbonyl derivatives

The cycloheptatrienylvanadium carbonyl (η7-C7H7)V(CO)3 is a stable species that can be synthesized by the reaction of V(CO)6 with cycloheptatriene. The related species of the types (C7H7)V(CO)n (n=5, 4, 3) and (C7H7)2V2(CO)n (n=5, 4, 3, 2, 1, 0), which are potentially accessible from (η7-C7H7)V(CO)3, have now been investigated by density functional theory. For the lowest energy singlet mononuclear (C7H7)V(CO)n (n=5, 4, 3) structures, the hapticity of the C7H7 ring is adjusted to give the vanadium atom the favored 18-electron configuration. Furthermore, for the lowest energy singlet binuclear (C7H7)2V2(CO)n (n=5, 4, 3) structures the hapticity of the C7H7 ring is adjusted to give a VV triple bond of ∼2.5Å length with an 18-electron configuration for each vanadium atom similar to the known binuclear cyclopentadienylvanadium carbonyl (η5-C5H5)2V2(CO)5. Evidence is presented for the presence of higher order multiple V–V bonds in the more highly unsaturated singlet (C7H7)2V2(CO)n (n=2, 1, 0) structures including a formal sextuple bond in the bent singlet (C7H7)2V2 structure. A lower energy coaxial triplet (C7H7)2V2 structure exhibiting the rare D7d point group symmetry is shown to have a σ+2π VV triple bond by analysis of its frontier molecular orbitals. The thermochemistry of the (C7H7)2V2(CO)n system suggests (η7-C7H7)2V2(μ-CO)3 to be a viable synthetic target analogous to the experimentally known and structurally characterized species (η6-C6H6)2Cr2(μ-CO)3 and (η5-C5H5)2Mn2(μ-CO)3.

Binuclear pentafluorocyclopentadienyl metal carbonyls of iron, cobalt, and nickel: Effect of fluorine substitution

Density functional theory studies on the binuclear pentafluorocyclopentadienyl metal carbonyls (C5F5)2M2(CO)n (M=Fe, n=4, 3, 2; M=Co, n=3, 2, 1; M=Ni, n=2, 1) using the B3LYP and BP86 methods find pentahapto η5-C5F5 ligands in all of the low energy structures. In general, the low energy structures of these η5-C5F5 derivatives parallel rather closely those of their hydrocarbon analogs with relatively little change upon complete substitution of the hydrogen atoms with fluorine atoms. The lowest energy (C5F5)2M2(CO)n (M=Fe, n=4; M=Co, n=3; M=Ni, n=2) structures are predicted to be closed-shell singlets with metal–metal distances of ∼2.6Å, ∼2.5Å, and ∼2.4Å, respectively, corresponding to the single bonds required to give each metal atoms the favored 18-electron configuration and similar to their experimentally known hydrocarbon analogs. For (C5F5)2Fe2(μ-CO)2(CO)2 the cis and trans isomers are predicted to have similar energies within 2kcal/mol similar to the well-known (C5H5)2Fe2(μ-CO)2(CO)2 except the relative energies of the cis and trans isomers are reversed. For the unsaturated derivatives (C5F5)2M2(CO)n (M=Fe, n=3, 2; M=Co, n=2, 1; M=Ni, n=1), the lowest energy structures have shorter metal–metal multiple bonds of orders 2 and 3 corresponding to the favored 18-electron configurations for the metal atoms.

A monotonic increase of formal metal–metal bond orders from one to five upon loss of carbonyl groups from binuclear benzene chromium carbonyls

Benzene forms a binuclear chromium carbonyl derivative (η6-C6H6)2Cr2(μ-CO)3, shown by X-ray crystallography to have a very short CrCr distance, suggesting the formal triple bond required to give each chromium atom the favored 18-electron configuration. We now describe theoretical studies on the entire series of binuclear benzene chromium carbonyls (C6H6)2Cr2(CO)n (n=5,4,3,2,1). The predicted Cr–Cr distances in the lowest energy singlet structures determined by the BP86 method decrease monotonically as carbonyl groups are lost starting from 2.95Å in (C6H6)2Cr2(CO)5 to 1.95Å in (C6H6)2Cr2(CO) corresponding to a steady increase in the formal bond order from one to five. This increase in formal Cr–Cr bond order is also supported by a monotonic increase in the Wiberg bond indices ranging from 0.29 for the single bond in (C6H6)2Cr2(CO)5 to ∼2 for the formal quintuple bond in (C6H6)2Cr2(CO).

Carbonyl versus butadiene dissociation in binuclear butadiene cobalt carbonyls

Dicobalt octacarbonyl is known to react with diolefins to give substitution products of the types (diene)Co2(CO)6 and (diene)2Co2(CO)4. The butadiene derivatives (C4H6)Co2(CO)n (n=6, 5, 4, 3, 2) have been investigated by density functional theory using the B3LYP and BP86 methods. The lowest energy (C4H6)Co2(CO)n (n=6, 5, 4) structures have bridging CO groups and terminal butadiene ligands. For the (C4H6)Co2(CO)6 and (C4H6)Co2(CO)5 structures the CoCo distances of ∼2.5Å suggest formal single bonds. However, for the lowest energy (C4H6)Co2(CO)4 structure the significantly shorter CoCo distance of ∼2.3Å suggests the formal triple bond required to give the cobalt atom the favored 18-electron configuration. Bridging butadiene ligands are also found in (C4H6)Co2(CO)n structures including all of the lowest energy (C4H6)Co2(CO)3 and (C4H6)Co2(CO)2 structures. Both the B3LYP and BP86 methods predict butadiene dissociation from (C4H6)2Co2(CO)4 to be energetically favored over CO dissociation by ∼8kcal/mol. For (C4H6)2Co2(CO)n (n=3, 2) the BP86 method predicts CO dissociation to be favored energetically over butadiene dissociation by ∼8kcal/mol. However, the B3LYP method predicts essentially equal CO and butadiene dissociation energies within ∼1kcal/mol from (C4H6)2Co2(CO)n (n=3, 2).

Open chains versus closed rings: Comparison of binuclear butadiene cobalt carbonyls with cyclic hydrocarbon analogs

The reaction of Co2(CO)8 with butadiene has long been known to give the binuclear (C4H6)2Co2(CO)2(μ-CO)2. In addition, other (diene)2Co2(CO)2(μ-CO)2 derivatives have been synthesized. In this connection the binuclear butadiene cobalt carbonyls (C4H6)2Co2(CO)n (n=4, 3, 2, 1) have now been investigated by density functional theory. For (C4H6)2Co2(CO)4 several doubly carbonyl bridged stereoisomers were found differing only in the arrangements of the butadiene ligands and with predicted Co–Co single bond distances in the range 2.53–2.61Å. A similar multiplicity of stereoisomers is not possible for the corresponding cyclobutadiene derivative (C4H4)2Co2(CO)4. The lone unbridged (C4H6)2Co2(CO)4 structure lies ∼13kcal/mol above the lowest energy doubly bridged isomer with a significantly longer Co–Co distance of 2.74Å. For (C4H6)2Co2(CO)3 the lowest energy structures are doubly carbonyl bridged structures with CoCo double bond distances in the range 2.45–2.51Å. However, triply carbonyl bridged (C4H6)2Co2(μ-CO)3 structures with significantly shorter CoCo distances of 2.24–2.30Å are also found. For (C4H6)2Co2(CO)2 several structures with bridging butadiene ligands and terminal carbonyl groups are energetically competitive with those with bridging carbonyl groups and terminal butadiene ligands, unlike the corresponding cyclobutadiene complexes. The lowest energy singlet (C4H6)2Co2(CO) has a two-electron donor bridging carbonyl group and an extremely short Co–Co distance of 2.13Å suggesting the formal quadruple bond required to give each cobalt atom the favored 18-electron configuration. This differs from the cyclobutadiene analog (C4H4)2Co2(CO) in which the bridging carbonyl group is a four-electron donor η2-μ-CO group and the cobalt-cobalt distance is much longer at ∼2.5Å.

Triple-Decker-Sandwich versus Rice-Ball Structures for Tris(benzene)dimetal Derivatives of the First-Row Transition Metals

Compounds of the type M(2)Bz(3) (Bz = benzene, C(6)H(6)) have been of interest since the related triple-decker mesitylenechromium sandwich (1,3,5-Me(3)C(6)H(3))(3)Cr(2) has been synthesized and characterized structurally by X-ray crystallography. Theoretical studies predict the lowest-energy M(2)Bz(3) structures of the early transition metals Ti, V, and Cr to be the triple-decker sandwiches trans-Bz(2)M(2)(η(6),η(6)-μ-C(6)H(6)) having quintet, triplet, and singlet spin states, respectively. In these structures, the central benzene ring functions as a hexahapto ligand to each metal atom. The singlet rice-ball cis-Bz(2)M(2)(μ-C(6)H(6)) structures with a 2.64-Å Mn═Mn double bond or a 2.81-Å Fe-Fe single bond are preferred for the central transition metals Mn and Fe. Singlet triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) return as the lowest-energy structures for the late transition metals Co and Ni but with the central benzene ring only partially bonded to each metal atom. Thus, the lowest-energy cobalt derivative has a trans-Bz(2)Co(2)(η(3),η(3)-μ-C(6)H(6)) structure in which the central benzene ring acts as a trihapto ligand to each metal atom. Similarly, the lowest-energy nickel derivative has a trans-Bz(2)Ni(2)(η(2),η(2)-μ-C(6)H(6)) structure in which the central benzene ring acts as a dihapto ligand to each metal atom, leaving an uncomplexed C═C double bond. The metal-metal bond orders in the singlet "rice-ball" structures cis-Bz(2)M(2)(μ-C(6)H(6)) (M = Mn, Fe) and the hapticities of the central benzene rings in the singlet late-transition-metal triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) (M = Co, Ni) are governed by the desirability for the metal atoms to attain the favored 18-electron configuration.

Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η 5-C 5H 5) 2Cr 2(CS) 2(CO) 3 in contrast to (η 5-C 5H 5) 2Cr 2(CO) 5

The cyclopentadienylchromium carbonyl thiocarbonyls Cp 2Cr 2(CS) 2(CO) n ( n = 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The lowest energy Cp 2Cr 2(CS) 2(CO) 4 structure can be derived from the experimentally characterized unbridged Cp 2Cr 2(CO) 6 structure by replacing the two terminal carbonyl groups furthest from the Cr–Cr bond with two terminal CS groups. The two lowest energy Cp 2Cr 2(CS) 2(CO) 3 structures have a single four-electron donor η 2-μ-CS group and a formal Cr–Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp 2Cr 2(CO) 5 these Cp 2Cr 2(CS) 2(CO) 3 structures are viable with respect to disproportionation into Cp 2Cr 2(CS) 2(CO) 4 and Cp 2Cr 2(CS) 2(CO) 2 and thus are promising synthetic targets. The lowest energy Cp 2Cr 2(CS) 2(CO) 2 structures have all two-electron donor CO and CS groups and short Cr Cr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp 2Cr 2(CS) 2(CO) 2 structures are closely related to the known structure for Cp 2Cr 2(CO) 4. In addition, several doubly bridged structures with four-electron donor η 2-μ-CS bridges are found for Cp 2Cr 2(CS) 2(CO) 2 at higher energies. The global minimum Cp 2Cr 2(CS) 2(CO) structure is a triply bridged triplet with a Cr Cr triple bond (2.299 Å by BP86). A higher energy singlet Cp 2Cr 2(CS) 2(CO) structure has a shorter Cr–Cr distance of 2.197 Å (BP86) suggesting the formal quadruple bond required to give each chromium atom the favored 18-electron configuration.

Mononuclear Homoleptic Allyl Complexes of the First Row Transition Metals: Species with Unusual Metal Electronic Configurations

A number of evanescent unsubstituted homoleptic allyl derivatives M(C(3)H(5))(n) of the first row transition metals have been reported in the literature. In addition, the much more thermally stable silylated derivatives M[C(3)H(3)(SiMe(3))(2)](2) (M = Cr, Fe, Co, Ni) are reported to survive vacuum sublimation without significant decomposition. In this connection, the complete series of homoleptic allyl derivatives M(C(3)H(5))(n) (n = 2, 3; M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni) have been studied theoretically using density functional theory. In most of the lowest energy predicted M(C(3)H(5))(n) structures all of the allyl groups are bonded as trihapto η(3)-C(3)H(5) ligands and the metals have considerably less than the normally favored 18-electron configuration. Such ligands can be considered formally as bidentate ligands with the metal atom connected to the centers of the two C-C bonds of the η(3)-C(3)H(5) group. The later transition metal diallyls M(C(3)H(5))(2) (M = Cr, Mn, Fe, Co, Ni) form two stereoisomers of similar relative energies, namely the C(2h) staggered isomer and the C(2v) eclipsed isomer with the orientation of the η(3)-C(3)H(5) groups corresponding to square planar metal coordination of the bidentate η(3)-C(3)H(5) ligands. The staggered and eclipsed Ni(C(3)H(5))(2) isomers have been observed experimentally by NMR. Less symmetrical M(C(3)H(5))(2) structures are found for the earlier transition metals Sc, Ti, and V in which the orientation of the allyl groups corresponds to tetrahedral metal coordination. The triallylmetal derivatives M(C(3)H(5))(3) are predicted to be thermodynamically viable with respect to allyl loss to give the corresponding diallylmetal derivatives, except for triallylnickel. The lowest energy Ni(C(3)H(5))(3) structure has two trihaptoallyl ligands and one monohaptoallyl ligand, whereas the lowest energy Mn(C(3)H(5))(3) structures have only one trihaptoallyl ligand and two monohaptoallyl ligands. Otherwise, the M(C(3)H(5))(3) complexes have structures with three trihaptoallyl ligands corresponding formally to octahedral metal coordination. The M(C(3)H(5))(3) complexes (M = Cr, Co) thus correspond to a well-known series of "classical" octahedral coordination complexes, namely, those of the d(3) Cr(III) and the d(6) Co(III), respectively.

Binuclear Cyclopentadienylmetal Cyclooctatetraene Derivatives of the First Row Transition Metals: Effects of Ring Conformation on the Bonding of an Eight-Membered Carbocyclic Ring to a Pair of Metal Atoms

Binuclear Cp(2)M(2)(μ-C(8)H(8)) derivatives have been synthesized for M = V, Cr, Co, and Ni and have now been studied theoretically for the entire first row of transition metals from Ti to Ni. The early transition metal derivatives Cp(2)M(2)(μ-C(8)H(8)) (M = Ti, V, Cr. Mn) are predicted to form low-energy cis-Cp(2)M(2)(μ-C(8)H(8)) structures with a folded C(8)H(8) ring (dihedral angle ∼130°) and short metal-metal distances suggesting multiple bonding. These predicted structures are close to the experimental structures for M = V, Cr with V≡V and Cr≡Cr bond lengths of ∼2.48 and ∼2.36 Å, respectively. The middle to late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Mn, Fe, Co, Ni) with a twisted μ-C(8)H(8) ring and no metal-metal bonding. The hapticity of the central μ-C(8)H(8) ring in such structures ranges from five for Mn and Fe to four for Co and three for Ni and thus depend on the electronic requirements of the central metal atom. This leads to the favored 18-electron configuration for both metal atoms in the singlet Fe, Co, and Ni structures but only 17-electron metal configurations in the triplet Mn structure. In addition, the late transition metals form trans-Cp(2)M(2)(μ-C(8)H(8)) structures (M = Fe, Co, Ni), with the tub conformation of the μ-C(8)H(8) ring functioning as a tetrahapto (M = Fe, Co) or trihapto (M = Ni) ligand to each CpM group. A μ-C(8)H(8) ring in the tub conformation also bonds to two CpFe units as a bis(tetrahapto) ligand in both singlet and triplet cis-Cp(2)Fe(2)(μ-C(8)H(8)) structures.

Analogies between binuclear phospholyl and cyclopentadienyl manganese carbonyl complexes: seven-electron donor bridging phospholyl rings

Phosphacymantrene, (η5-C4H4P)Mn(CO)3, is a known stable compound and a potential precursor to binuclear (C4H4P)2Mn2(CO)n derivatives analogous to isovalent (C5H5)2Mn2(CO)n derivatives. In this connection the structures and energetics of binuclear derivatives have been investigated by density functional theory. The lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures have seven-electron donor η5,η1-C4H4P ligands bridging the two metal atoms, which are too far apart for a direct Mn–Mn bond. However, the lowest energy (C4H4P)2Mn2(CO)3 structure is predicted to be a triply carbonyl bridged structure with a formal MnMn triple bond of length 2.17 A analogous to (η5-C5H5)2Mn2(μ-CO)3, which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy (C4H4P)2Mn2(CO)2 structures have one terminal η5-C4H4P ring and one bridging seven-electron donor η5,η1-C4H4P ring. However, only slightly higher energy (C4H4P)2Mn2(CO)2 structures are found with two η5-C4H4P rings and thus analogous to previously found (η5-C5H5)2Mn2(CO)2 structures. Such a triplet (η5-C4H4P)2Mn2(CO)2 structure is predicted to have a formal MnMn triple bond of length 2.20 A. The two analogous singlet (η5-C4H4P)2Mn2(CO)2 structures have shorter MnMn bonds of length 2.07 A, which are interpreted as the formal quadruple bonds required to give each Mn atom the favored 18-electron configuration.

Variable hapticity of the cyclooctatetraene ring in sandwich compounds of the first row transition metals

Density functional theory methods (B3LYP and BP86) indicate that the preferred structures for such early transition metal derivatives are (η 8-C 8H 8)M(η 4-C 8H 8) (M = Ti, V, Cr) with one octahapto η 8-C 8H 8 ring and one tetrahapto η 4-C 8H 8 ring. In such structures only 12 of the 16 carbon atoms of the two C 8H 8 rings are bonded to the metal, leading to 16-, 17-, and 18-electron complexes, respectively, in accord with the experimentally known structures for the Ti and V derivatives. The preferred structures for the Mn and Fe derivatives are (η 6-C 8H 8)M(η 4-C 8H 8) (M = Mn, Fe) with one hexahapto and one tetrahapto C 8H 8 ring and thus having 17- and 18-electron configurations, respectively, in accord with experimental data on the iron complex. The lowest energy structure for the cobalt complex is (η 4-C 8H 8)Co(η 2,2-C 8H 8) with two different types of tetrahapto C 8H 8 rings and a 17-electron metal configuration. The nickel complex (C 8H 8) 2Ni appears to prefer a structure with a 16-electron configuration and two trihapto C 8H 8 rings, similar to the known (η 3-C 3H 5) 2Ni rather than a bis(tetrahapto) structure with the favored 18-electron configuration. These theoretical studies indicate that in (C 8H 8) 2M derivatives of the first row transition metals, the number of carbon atoms in the pair of C 8H 8 rings involved in the bonding to the central metal atom gives the metal atoms 16-, 17-, or 18-electron configurations.

Heptahapticity in Binuclear (Cycloheptatrienyl)molybdenum Carbonyl Derivatives: The Interplay between Ring Hapticity/Planarity and Metal–Metal Multiple Bonding

Theoretical studies on the binuclear (cycloheptatrienyl)molybdenum carbonyl complexes [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5, 4, 3, 2, 1, 0) indicate structures with fully bonded heptahapto eta(7)-C(7)H(7) rings and four or fewer carbonyl groups to be energetically competitive. This is in striking contrast to the corresponding chromium derivatives. The lowest energy of such a structure for [(eta(7)-C(7)H(7))(2)Mo(2)(CO)(4)] is a singlet unbridged structure with a formal Mo--Mo single bond with a length of approximately 3.2 A. A higher-energy pentahapto structure [(eta(5)-C(7)H(7))(2)Mo(2)(CO)(4)] is also predicted with a formal Mo [triple bond]Mo triple bond with a length of approximately 2.56 A. Low-energy structures are predicted for [(C(7)H(7))(2)Mo(2)(CO)(3)] with two heptahapto eta(7)-C(7)H(7) rings, either one or two bridging carbonyl groups, and formal Mo=Mo double bonds with a length of approximately 2.8 A. However, the global minimum for [(C(7)H(7))(2)Mo(2)(CO)(3)] is a [(eta(7)-C(7)H(7))(eta(5)-C(7)H(7))Mo(2)(CO)(3)] structure with a formal Mo[triple bond]Mo triple bond with a length of approximately 2.53 A. The lowest-energy structures for [(C(7)H(7))(2)Mo(2)(CO)(2)] and [(C(7)H(7))(2)Mo(2)(CO)] have heptahapto eta(7)-C(7)H(7) rings and predicted metal-metal bond lengths of approximately 2.54 and 2.31 A, respectively, consistent with the formal triple and quadruple bonds, respectively, needed to give both metal atoms the favored 18-electron configuration. The lowest-energy structures for the carbonyl-richer systems [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5) contain one trihapto eta(3)-C(7)H(7) ring and one pentahapto eta(5)-C(7)H(7) ring.

Metal−Metal Bonding in Binuclear Metal Carbonyls with Three Bridging Methylaminobis(difluorophosphine) Ligands: Iron, Cobalt, and Nickel Derivatives

The [CH3N(PF2)2]3M2(CO)n (M = Fe, Co, Ni; n = 2, 1, 0) derivatives, in which a metal-metal bond is bridged by three CH3N(PF2)2 groups similar to the known very stable [CH3N(PF2)2]3Co2(CO)2, have been investigated by density functional theory. The lowest energy structures for the dicarbonyls [CH3N(PF2)2]3M2(CO)2 (M = Ni, Co, Fe) are predicted to be closed-shell singlets with metal-metal distances of approximately 3.94 A, approximately 2.80 A, and approximately 2.63 A, respectively, corresponding to the metal-metal bonds of orders 0, 1, and 2, required to give both metal atoms the favored 18-electron configurations. For the monocarbonyls [CH3N(PF2)2]3M2(CO) (M = Ni, Co, Fe), unsymmetrical structures with a carbonyl group terminally bonded to one metal are greatly preferred energetically over structures in which the carbonyl group bridges the pair of metal atoms. The lowest energy structures for [CH3N(PF2)2]3M2(CO) (M = Ni, Co, Fe) are singlet, triplet, and singlet states, respectively, with metal-metal distances of approximately 2.77 A, approximately 2.38 A, and approximately 2.13 A, respectively. These distances correspond to the metal-metal dative bonds of orders 1, 2, and 3, required to give both metal atoms the favored 18-electron configuration. Among the carbonyl-free species the iron compound [CH3N(PF2)2]3Fe2 is of particular interest since its very short Fe[double double bonds]Fe distance of approximately 2.02 A suggests the formal quadruple bond required to give both iron atoms the favored 18-electron configuration.