Search

While the 1 : 1 reaction of 3 with an N-heterocyclic carbene ({(Me)CN(i-Pr)}2C:) in THF resulted in ligand-substituted product 4, the corresponding 1 : 2 reaction (in the presence of H2O) gives the first structurally characterized germanium tris(dithiolene)dianion 5 as the major product and the "naked" dithiolene radical 6˙ as a minor by-product. The structure and bonding of 4 and 5 were probed by experimental and theoretical methods. Our study suggests that carbene-mediated partial hydrolysis may represent a new method to access tris(dithiolene) complexes of main-group elements.

The 1-norbornyl (nor) ligand is known experimentally to form stable transition metal alkyl derivatives through direct metal–carbon bond formation. This appears to be related to its steric bulk and inaccessibility towards β-hydrogen elimination, as exemplified by the tetraalkyls (nor)4M, some of which are very stable. In this connection we have used density functional theory and the DLPNO-CCSD(T) method to investigate the 1-norbornylcobalt carbonyl derivatives (nor)Co(CO)n (n = 4, 3, 2, 1) and (nor)2Co2(CO)n (n = 7, 6, 5). Low-energy structures of the unsaturated systems (nor)Co(CO)n (n = 3, 2) and (nor)2Co2(CO)n (n = 6, 5) are found to have agostic hydrogen atoms from a CH2 group adjacent to the Co–C bond. Such agostic hydrogen atoms form a C–H–Co bridge with a bonding Co–H distance less than ∼2 A. In such structures unsaturation is relieved by donation of an additional two electrons from the C–H bond of this norbornyl CH2 group. In addition, structures in which carbonyl migration from cobalt to carbon has occurred to form acyl norCO ligands are among the lowest energy structures. The resulting acyl carbonyl groups of the norCO ligands serve as spacers between the bulky 1-norbornyl ligand and the cobalt carbonyl moiety. Furthermore, such neutral norCO acyl ligands can either be one-electron donors to a cobalt atom, bonding solely through the carbonyl carbon, or three-electron donor η2-μ-norCO groups bridging a central Co2 unit through both the acyl carbon and oxygen atoms. The strengths of the agostic C–H–Co interactions have been characterized by their reduced density gradient (RDG) values.

Density functional theory studies show that the lowest energy C4F8Fe(CO)4 structure is not the very stable experimentally known ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4 obtained from Fe(CO)12 and tetrafluoroethylene. Instead isomeric (perfluoroolefin)Fe(CO)4 structures derived from perfluoro-2-butene, perfluoro-1-butene, and perfluoro-2-methylpropene are significantly lower energy structures by up to ∼17 kcal mol−1. However, the activation energies for the required fluorine shifts from one carbon to an adjacent carbon atom to form these (perfluoroolefin)Fe(CO)4 complexes from tetrafluoroethylene are very high (e.g., ∼70 kcal mol−1). Therefore the ferracyclopentane isomer (CF2CF2CF2CF2)Fe(CO)4, which does not require a fluorine shift to form from Fe3(CO)12 and tetrafluoroethylene, is the kinetically favored product. The lowest energy structures of the binuclear (C4F8)2Fe2(CO)n (n = 7, 6) derivatives have bridging perfluorocarbene ligands and terminal perfluoroolefin ligands.

Quantitative conformational analysis (Eliel, Stereochemistry of Organic Compounds, Wiley-Interscience, 1994) has been with us at least as long as Pitzer's landmark 1937 paper (J. Am. Chem. Soc., 1937, 59, 276) on ethane. Cyclohexanes have played a critical role in the quest for understanding. Notably, 1,2,3,4,5,6 hexachlorocyclohexane (C6H6Cl6) was apparently synthesized for the first time by Michael Faraday in 1825 (Philos. Trans. R. Soc., B, 1825, 115, 440). The γ-1,2,3,4,5,6 hexachlorocyclohexane molecule subsequently acquired the common name lindane. Although banned or limited by many countries in 2006, nearly one billion tons of lindane has been manufactured and employed, mostly in agriculture, but also for treatment of human diseases such as lice. Although not as well characterized as lindane, other chlorocyclohexanes have been made and to some degree characterized. The pioneering experimental conformational studies by LeFevre and coworkers (J. Chem. Soc. B, 1970, 1608) of 1,2 dichlorocyclohexane, 1,1,2 trichlorocyclohexane, and 1,2,3,4,5,6 hexachlorocyclohexane are particularly noteworthy. The chlorocyclohexanes have also played a role in the development of molecular mechanics methods by Allinger and coworkers (J. Am. Chem. Soc., 1983, 105, 1716 and 1723). In the present research, we report the first systematic studies of all the chlorocyclohexanes, excluding those with two chlorines attached to a single carbon atom. We make careful comparisons with previous experimental and computational studies. A simple system is established to estimate the relative energies of the different isomers of a particular molecular species. Predicted dipole moments range from identically zero to 5.7 Debye.

The known sandwich compound [η5-(CH2)3N2(BPh)2CMe]2Fe in which adjacent C2 units are replaced by isoelectronic BN units can be considered as a boraza analogues of ferrocene similar to borazine, B3N3H6, considered as a boraza analogue of benzene. In this connection, the related bis(1,2,3,5-tetramethyl-1,2-diaza-3,5-diborolyl) derivatives (Me4B2N2CH)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) for all of the first row transition metals have been optimized using density functional theory for comparison with the isoelectronic tetramethylcyclopentadienyl derivatives (Me4C5H)2M. Low-energy sandwich structures having parallel B2N2C rings in a trans orientation are found for all seven metals. The 1,2-diaza-3,5-diborolyl ligand appears to be a weaker field ligand than the isoelectronic cyclopentadienyl ligand as indicated by higher spin ground states for some (η5-Me4B2N2CH)2M sandwich compounds relative to the corresponding metallocenes (η5-Me4C5H)2M. Thus (η5-Me4B2N2CH)2Cr has a quintet ground state in contrast to the triplet ground state of (η5-Me4C5H)2Cr. Similarly, the sextet ground state of (η5-Me4B2N2CH)2Mn lies ∼18 kcal mol−1 below the quartet state in contrast to the doublet ground state of the isoelectronic (Me4C5H)2Mn. These sandwich compounds are potentially accessible by reaction of 1,2-diaza-3,5-diborolide anions with metal halides analogous to the synthesis of [η5-(CH2)3N2(BPh)2CMe]2Fe.

The first magnesium-based dithiolene, 2, was prepared by reaction of the lithium dithiolene radical, 1˙, with 2-mesitylmagnesium bromide. Reaction of 2 with N-heterocyclic carbenes (in toluene) gave a carbene-stabilized magnesium monodithiolene complex, 3. Complex 3, in turn, is readily converted to a THF-solvated magnesium bis-dithiolene dianion, 4, via partial hydrolysis in polar solvents (i.e., THF/CH3CN). Compounds 2, 3 and 4 have been spectroscopically and structurally characterized and probed by DFT computations.

The unexpected 2004 discovery of decamethyldizincocene suggested some new possibilities for organometallic compounds in which an M2 unit is sandwiched between two planar carbocyclic rings. Density functional theory shows that the low-energy structures for the dizincocenes Zn2(C5X5)2 (X = H, F) are singlet coaxial structures having two (η5-C5X5)Zn units linked by a Zn–Zn single bond of length ∼2.3 A. However, the low-energy M2(C5H5)2 (M = Cu, Ni, Co, Fe) structures have perpendicular configurations with bridging C5H5 ligands. They exhibit increasingly higher spin states from singlet (Cu, Ni) to triplet (Ni,Co), quintet (Co,Fe), and septet (Fe). The low-energy structures of the perfluorinated systems M2(C5F5)2 (M = Co, Fe) have irregular geometries with one bent bridging C5F5 ring and one planar terminal pentahapto η5-C5F5 ring, with the metal atom bearing the terminal η5-C5F5 ligand has the favored 18-electron configuration while the other metal atom has only a lower electron configuration and vacant coordination sites.

The CO2-reduction activity of two Re(I)–NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H2 formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

Theoretical studies are reported on methylphosphepine, methylbenzophosphepine, their norcaradiene isomers, and their metal complexes with Fe(CO)3 and CpCo (Cp = η5-C5H5) units. Both methylphosphepine and methylphosphanorcaradiene are C6H6PCH3 species existing as anti/syn stereoisomer pairs with the methylphosphepine structures at slightly higher energies. The transition states for the interconversion of these isomers and their benzo derivatives lie ∼20 kcal mol−1 in energy above the methylphosphanorcaradiene isomers. Complexation of either C6H6PCH3 ligand with the transition metal units Fe(CO)3 and CoCp leads to energetically closely spaced η4-tetrahapto and η3-trihapto isomers of the methylphosphepine complexes and η4-tetrahapto isomers of the methylphosphanorcaradiene complexes. However, the bis(dihapto) (η2,2-C6H6PCH3)Fe(CO)3 and (η2,2-C6H6PCH3)CoCp complexes involving coordination of non-adjacent CC double bonds lie at significantly higher energies. Fusion of a benzene ring to the C6H6PCH3 rings in methylphosphepine and methylphosphanorcaradiene leads to significantly different structures of their lowest energy metal complexes. Thus the lowest energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures are η2,2 and η4 methylbenzophosphepine complexes, which avoid using any carbon atoms of the benzene ring in the ligand for metal complexation. Higher energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures have tetrahapto ligands with one or both CC double bonds of the benzene ring complexed with the metal atom.

Open metallocenes containing a metal sandwiched between two U-shaped pentadienyl or substituted pentadienyl ligands have been synthesized by Ernst and co-workers using reactions of pentadienyl or substituted pentadienyl anion reagents with metal halides. The complete series of such open metallocenes (2,4-Me2C5H5)2M of the first row transition metals (M = Ti to Ni) has now been examined using density functional theory. The experimentally known lowest energy structures of the early transition metals are open metallocene structures with two pentahapto U-shaped η5-2,4-Me2C5H5 ligands. Such structures can exist as syn-eclipsed, gauche-eclipsed, and/or anti-eclipsed conformations of similar energies. The theoretical predictions of the lowest energy conformations of the open metallocenes are in excellent agreement with available experimental data. Singlet and triplet spin state structures are preferred energetically for the Ti and Cr derivatives, respectively. For (η5-2,4-Me2C5H5)2V, doublet and quartet spin state structures are spaced in energy by 7.6 kcal mol−1. The (2,4-Me2C5H5)2Mn energy surface is complicated with doublet, quartet, and sextet spin state structures closely spaced in energy. In the sextet spin state of (2,4-Me2C5H5)2Mn only the end carbon atoms of the 2,4-Me2C5H5 ligand are within bonding distance of the manganese atom. This corresponds to tetrahedral C4Mn coordination similar to that found experimentally in the sextet ground state t-butyl derivative (2,4-tBu2C5H5)2Mn as well as the related Mn(CN)42−. The low energy structures for the “open ferrocene” (η5-2,4-Me2C5H5)2Fe are singlet spin state structures with two U-shaped pentahapto η5-2,4-Me2C5H5 ligands but otherwise analogous to ferrocene. The low energy (2,4-Me2C5H5)2M structures of the late transition metals Co and Ni have at least one trihapto η3-2,4-Me2C5H5 ligand. The lowest energy (2,4-Me2C5H5)2Ni structure is a singlet spin state structure with two trihapto η3-2,4-Me2C5H5 ligands and a 16-electron nickel environment analogous to the well-known bisallylnickel, (η3-C3H5)2Ni.

Togni's reagents have become very popular trifluoromethylating reagents in organic synthesis. The existing form of Togni's reagent I is a hypervalent iodine compound which lies much higher in energy than its ether isomer. The high-energy hypervalent iodine form makes Togni's reagent I very effective and versatile. The energy differences between the two forms correlate with the trans influence of the substituents. The five-membered ring in the benziodoxole-based scaffold is an important reason for its existence in the higher-energy form. The relation to Buchwald's 2014 research is discussed.

Alkali metal salts of the 2-phosphaethynolate anion PCO− synthesized from reactions of CO with NaPH2 or K3P7 have recently become available in quantities for the synthesis of transition metal complexes of the potentially ambidentate PCO ligand (Angew. Chem., Int. Ed., 2013, 38, 10064). This is exemplified by the recently reported rhenium carbonyl complex (triphos)Re(CO)2(PCO) (triphos = MeP(CH2PPh2)3). Density functional theory studies on the related manganese carbonyl complexes Mn(CO)n(PCO) (n = 5, 4, 3) and Mn2(CO)n(PCO)2 (n = 8, 7, 6, 5) are now reported. For the binuclear systems the low-energy Mn2(CO)8(PCO)2 structures are singlet spin state structures having two bridging P-bonded phosphaketenyl μ-PCO ligands without a direct Mn–Mn bond. Carbonyl loss from Mn2(CO)8(μ-PCO)2 is predicted to lead to migration of CO groups from phosphorus to manganese resulting in Mn2(CO)n+2(μ-P2) structures with bridging diphosphido groups as the lowest energy Mn2(CO)n(PCO)2 isomers (n = 7, 6, 5). Isomeric Mn2(CO)6(PCO)2 structures with dihapto bridging η2-μ-PCO ligands at ∼30 kcal mol−1 above the global minimum are also found representing intermediates in the migration of CO groups from phosphorus to manganese. For the mononuclear systems the P-bonded Mn(CO)n(PCO) (n = 5, 4) phosphaketenyl structures are found to lie 20 to 28 kcal mol−1 in energy below the isomeric O-bonded Mn(CO)n(PCO) phosphaethynoxy isomers consistent with previously reported results by Grutzmacher and coworkers on R3E(PCO)–R3E(OCP) systems (R = iPr, Ph; E = Si, Sn, Ge, Pb). The lowest energy structure for the tricarbonyl Mn(CO)3(PCO) is a singlet structure with an unusual trihapto η3-PCO ligand. However, higher energy isomeric Mn(CO)3(PCO) structures with P-bonded phosphaketenyl or O-bonded phosphaethynoxy ligands and tetrahedral Mn coordination are also found.

The synthesis, electrochemical activity, and relative photodecomposition rate is reported for four new Mn(i) N-heterocyclic carbene complexes: [MnX(N-ethyl-N'-2-pyridylimidazol-2-ylidine)(CO)3] (X = Br, NCS, CN) and [MnCN(N-ethyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3]. All compounds display an electrocatalytic current enhancement under CO2 at the potential of the first reduction, which ranges from -1.53 V to -1.96 V versus the saturated calomel electrode. Catalytic CO production is observed for all species during four-hour preparative-scale electrolysis, but substantial H2 is detected in compounds where X is not Br. All species eventually decompose under both 350 nm and 420 nm light, but cyanide substituted complexes (X = CN) last significantly longer (up to 5×) under 420 nm light as a result of a blue-shifted MLCT band.

The cyclopentadienyliron trifluorophosphine hydride CpFe(PF3)2H, in contrast to CpFe(CO)2H, is a stable compound that can be synthesized by reacting Fe(PF3)5 with cyclopentadiene. Theoretical studies on the binuclear Cp2Fe2(PF3)n (n = 5, 4, 3, 2) derivatives derived from CpFe(PF3)2H indicate the absence of viable structures having PF3 ligands bridging Fe–Fe bonds solely through the phosphorus atom. This contrasts with the analogous Cp2Fe2(CO)n systems for which the lowest energy structures have two (for n = 4 and 2) or three (for n = 3) CO groups bridging an iron–iron bond. Higher energy singlet Cp2Fe2(PF3)3 structures have a novel four-electron donor bridging η2-μ-PF3 ligand bonded to one iron atom through its phosphorus atom and to the other iron atom through a fluorine atom. Other higher energy triplet and singlet Cp2Fe2(PF3)2 structures are of the Cp2Fe2F2(μ-PF2)2 type having terminal fluorine atoms and bridging μ-PF2 ligands. The lowest energy Cp2Fe2(PF3)5 structure is actually Cp2Fe2(PF3)3(PF4)(μ-PF2) with a bridging PF2 group and a terminal PF4 group. Such structures are derived from a Cp2Fe2(PF3)4(μ-PF3) precursor by migration of a fluorine atom from the bridging PF3 group to a terminal PF3 group with a low activation energy barrier.

Thiophene is known experimentally to be desulfurized by Fe3(CO)12 under mild conditions to give the tricarbonyl ferrole (η4,η2-C4H4)Fe2(CO)6. A similar reaction of benzothiophene (thianaphthene) with Fe3(CO)12 gives a (C8H6S)Fe2(CO)6 complex in which an iron carbonyl moiety has inserted into the thiophene ring to give a thiaferranaphthalene ligand. Density functional theory shows this experimental structure to be the lowest energy structure. Furthermore, the lowest energy structures of the diiron pentacarbonyl (C8H6S)Fe2(CO)5 are simply derived from this (C8H6S)Fe2(CO)6 by loss of a CO group retaining the thiaferranaphthalene ligand. However, a higher energy isomeric (η6,η2-C8H6S)Fe2(CO)5 structure retains the original benzothiophene ligand with the C6 ring bonded to an Fe(CO)2 moiety as a hexahapto ligand and the CC double bond of the C4S ring bonded to an Fe(CO)3 moiety as a dihapto ligand with an Fe→Fe dative bond between the iron atoms. Similar insertion of an iron atom into a thiophene ring to give a thiaferrabenzene ring is predicted to occur in the lowest energy (C6H4S2)Fe2(CO)6 structure derived from either the anti or syn isomers of thienothiophene. However, the bonding of the exocyclic iron atom to the resulting thiaferrabenzothiophene ligand involves atoms in both rings in contrast to the (C8H6S)Fe2(CO)6 complex where the benzene ring is not involved in the ligand–iron bonding.

The chemistry of trinuclear osmium carbonyl hydrides is a rich area with the three H2Os3(CO)n derivatives (n = 12, 11, 10) all being known stable compounds ultimately obtained from Os3(CO)12 and hydrogen under various conditions. Density functional theory studies on the H2Os3(CO)n systems (n = 12, 11, 10, 9, 8) correctly predict the structures previously reported experimentally for n = 12, 11, and 10. These include a linear structure for H2Os3(CO)12 and triangular structures for H2Os3(CO)11 and H2Os3(CO)10. However, the H2Os3(CO)11 system is predicted to be a fluxional system with the four low energy isomers lying within 2 kcal mol−1 of energy. Three of these H2Os3(CO)11 isomers, all with one terminal hydrogen and one bridging hydrogen, have been observed experimentally by NMR. In addition, the lowest energy isomer has been isolated and structurally characterized by X-ray crystallography. In contrast to H2Os3(CO)11, the lowest energy H2Os3(CO)10 structure, namely the known structure with an OsOs edge bridged by both hydrogen atoms and all terminal CO groups, lies ∼10 kcal mol−1 below the next lowest energy isomer. The predicted CO dissociation energies of the H2Os3(CO)n derivatives (n = 12, 11, 10) suggest this H2Os3(CO)10 structure to be the “thermodynamic sink” in the H2Os3(CO)n systems, consistent with its synthesis from Os3(CO)12 and H2 at 120 °C and atmospheric pressure. The lowest energy structures of the more highly unsaturated H2Os3(CO)n (n = 9, 8) can be derived from this (μ-H)2Os3(CO)10 structure by removal of CO groups from the osmium atom remote to the doubly bridged OsOs edge of the Os3 triangle, with relatively little change in the central (μ-H)2Os3 triangle geometry.

Oxidative degradation of the octahedral dianion [Fe6C(CO)16]2− with an interstitial carbon atom leads eventually to the neutral Fe4C(CO)13 cluster with a butterfly-shaped central Fe4C unit. The complete series of related Fe4C(CO)n (n = 16, 15, 14, 13, 12, 11) derivatives have now been investigated using density functional theory. For the lowest energy Fe4C(CO)n (n = 16, 15, 14, 13) structures the geometries obey the n + f = 18 rule where f is the number of Fe–Fe bonds. This leads to a spiropentane geometry with two Fe–Fe bonds for Fe4C(CO)16, a central bent Fe–Fe–Fe–Fe chain for Fe4C(CO)15, a distorted trigonal pyramidal structure with four Fe–Fe bonds for Fe4C(CO)14, and the experimentally observed butterfly structure with five Fe–Fe bonds for Fe4C(CO)13. A symmetrical higher energy centered tetrahedral structure for Fe4C(CO)12 with six Fe–Fe bonds also follows the n + f = 18 rule. However, the lowest energy Fe4C(CO)n (n = 12, 11) structures are derived from the lowest energy Fe4C(CO)13 structure by removal of CO groups with retention of the central Fe4C butterfly unit.

The interaction of carbon monoxide with organoactinides has recently been shown experimentally, particularly by Cloke and co-workers, to result in coupling to give the oligomeric anions CnOn2− (n = 2, 3, 4). In order to model possible intermediates in reactions of this type, we have used density functional theory to explore the systems (C8H8)Th(CO)n (n = 1 to 5) and (C8H8)2Th2(CO)n (n = 2 to 7) related to the known “thorocene”, (η8-C8H8)2Th. Thorium was chosen as the actinide for this work since its chemistry almost entirely involves the single diamagnetic +4 oxidation state. All of the binuclear (C8H8)2Th2(CO)n structures found in this work have long Th⋯Th distances ranging from 4.4 to 5.0 A suggesting the absence of direct Th–Th bonds. Two (C8H8)2Th2(CO)2 isomers of similar energies in which the two CO groups have coupled to form trans and cis isomers of a bridging η4-μ-C2O2 ligand are low energy structures. These bridging η4-μ-C2O2 ligands exhibit ultralow ν(CO) frequencies around 1000 cm−1 indicating strong back donation of thorium d and f electrons into C–O antibonding orbitals. Most of the carbonyl richer (C8H8)2Th2(CO)n (n = 3 to 7) structures are derived from one of these basic (C8H8)2Th2(CO)2 structures by addition of terminal CO groups. An exception is the lowest energy (C8H8)2Th2(CO)4 structure which has C4v symmetry with four equivalent separate η2-μ-CO groups bridging the thorium atoms. The thermochemistry of these systems suggest (C8H8)Th(CO)4 and (C8H8)2Th2(CO)n (n = 2, 4) to be the most promising synthetic objectives, which are potentially obtainable by reductive carbonylation of the known (C8H8)ThCl2.

Density functional theory (DFT) studies on Mn2(CS)2(CO)8 using the B3LYP and BP86 methods show that no less than eight different unbridged structures are of significantly lower energies than the lowest energy doubly bridged structure. The Mn–Mn single bonds in these Mn2(CS)2(CO)8 structures range from 2.99 ± 0.02 A for the four structures with staggered equatorial CO/CS groups to 3.12 ± 0.04 A for the four structures with eclipsed equatorial CO/CS groups. The six lowest energy Mn2(CS)2(CO)7 structures all have four-electron donor bridging η2-μ-CE groups (E = S, O) and formal Mn–Mn single bonds of lengths 2.95 ± 0.01 A, rather than only two-electron donor CO and CS groups and formal MnMn double bonds. The Mn2(CS)2(CO)7 structures with an η2-μ-CS group are of lower energy than those with an η2-μ-CO group. These Mn2(CS)2(CO)7 structures are similar to the lowest energy structure for Mn2(CO)9 predicted previously as well as (Ph2PCH2PPh2)2Mn2(CO)4(η2-μ-CO), which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy Mn2(CS)2(CO)6 structures are predicted have a single four-electron donor bridging η2-μ-CS group and a formal MnMn double bond. However, at only slightly higher energies, Mn2(CS)2(CO)6 structures are found with two η2-μ-CS groups and a formal Mn–Mn single bond. A formal MnMn triple bond of length 2.36 ± 0.03 A is found in an even higher energy unbridged Mn2(CS)2(CO)6 structure, similar to the lowest energy Mn2(CO)8 structure found in a previous theoretical study. The lowest energy structures for Mn2(CS)2(CO)5 have two η2-μ-CS groups and a formal MnMn double bond of length 2.57 ± 0.03 A.

The tetranuclear iron carbonyls Fe4(CO)n (n = 16, 15, 14) have been investigated using density functional theory (DFT). Low-energy structures are predicted for Fe4(CO)16, Fe4(CO)15, and Fe4(CO)14 in which the Fe4 units are rhombi, planar butterflies, and tetrahedra with four, five, and six Fe–Fe single bonds, respectively, to give all four iron atoms the favored 18-electron configuration. However, for Fe4(CO)14 an alternative low energy structure is predicted with a planar Fe4 butterfly and one of the five iron–iron distances short enough to correspond to a formal FeFe double bond, thereby also leading to the favored 18-electron configuration. The dissociation energy of Fe4(CO)16 into Fe(CO)5 + Fe3(CO)11 or Fe3(CO)12 + Fe(CO)4 is predicted to be very low (

We have investigated the reaction of Re(dmb)(CO)(3)COOH with CO(2) using density functional theory, and propose a mechanism for the production of CO. This mechanism supports the role of Re(dmb)(CO)(3)COOH as a key intermediate in the formation of CO. Our new experimental work supports the proposed scheme.

The synthesis and structure of a Zn-Zn-bonded compound supported by a doubly reduced alpha-diimine ligand, [Na(THF)2]2 x [LZn-ZnL] (L = [(2,6-(i)Pr2C6H3)N(Me)C]2(2-)) are reported, with a Zn-Zn bond length of 2.399(1) angstroms.

Analysis of model compounds such as Fe2(CO)6, C2 and HBBH shows that pi-bonds left to themselves are shorter than sigma-bonds; in many ways sigma-bonds prevent pi-bonds from adopting their optimal shorter distances.

The metal carbonyl complexes [η4-(CH2)3C]Fe(CO)3 and [η4-(CH2)3C]Cr(CO)4 have been synthesized containing the umbrella-shaped trimethylenemethane ligand. The prospect of using this ligand in the metal sandwich complexes [(CH2)3C]2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) has now been investigated by density functional theory. The lowest energy structures of such complexes have the metal atom sandwiched between two tetrahapto trimethylenemethane ligands. Singlet spin state structures are strongly preferred for the titanium and nickel derivatives and doublet spin state structures for the vanadium and cobalt derivatives. Similarly, the triplet spin state is preferred for the iron derivative by more than 11 kcal mol−1. The preferred spin states for the chromium and manganese derivatives depend on the functional used for the structure optimization. Thus the B3LYP and B3LYP* methods predict the higher spin states, namely triplet for chromium and quartet for manganese. However, the BP86 method predicts the lower spin states of singlet for chromium and doublet for manganese. The higher spin state structures for the late transition metal derivatives, namely quintet [(CH2)3C]2Fe, quartet [(CH2)3C]2Co, and triplet [(CH2)3C]2Ni, have trihapto rather than tetrahapto trimethylenemethane ligands. The frontier molecular orbitals in the singlet [(CH2)3C]2M derivatives (M = Ti, Ni) suggest strong metal–ligand σ and π bonding but insignificant metal–ligand δ bonding.

Binuclear organometallic systems incorporating free cyclopentadiene (C5H6) as a ligand rather than the ubiquitous cyclopentadienyl ligand (C5H5) have never been reported in the literature. However, the hexafluorocyclopentadiene ligand (C5F6) is known to form the stable binuclear carbonyl derivatives (η2,η2-C5F6)Fe2(CO)8 and trans-(η3,η1-C5F6)Co2(CO)7. The related hexafluorocyclopentadiene iron carbonyl systems (C5F6)Fe(CO)n (n = 4, 3, 2) and (C5F6)Fe2(CO)n (n = 8, 7, 6, 5) have now been explored using density functional theory. The hexafluorocyclopentadiene ligand is found to use one or both of its CC double bonds to bind either as a dihapto η2-C5F6 monoolefin or a tetrahapto η4-C5F6 diolefin in mononuclear (C5F6)Fe(CO)n structures. In binuclear iron carbonyl structures lacking direct iron–iron bonds, such as the lowest energy trans- and cis-(η2,η2-C5F6)Fe2(CO)8 structures as well as higher energy trans-(η2,η2-C5F6)Fe2(CO)n (n = 7, 6, 5) structures, the hexafluorocyclopentadiene ligand binds as a bis(dihapto) diolefin. However, the lowest energy (C5F6)Fe2(CO)n (n = 7, 6, 5) structures have cis stereochemistries with direct iron–iron bonds of orders ranging from one to three. In such lowest energy structures the hexafluorocyclopentadiene ligand is bonded to the Fe2 unit as a trihapto–monohapto η3,η1-C5F6 ligand.

Binuclear palladium complexes of planar hydrocarbon ligands are of particular interest since both the coaxial structure (η5-Me5C5)2Pd2(μ-CO)2 and the perpendicular structure (μ-C6H6)2Pd2(Al2Cl7)2 have been synthesized as stable compounds. Here we report theoretical studies on a family of related compounds. For the formal Pd(II) derivatives Cp2Pd2X2 (Cp = η5-C5H5; X = F, Cl, CN) the perpendicular structures with direct Pd–Pd bonds are predicted to lie in energy below the isomeric coaxial structures. These coaxial structures have long Pd⋯Pd distances indicating the absence of palladium–palladium bonds. The lowest energy perpendicular Cp2Pd2X2 structures (X = F, CN) and a higher energy similar Cp2Pd2Cl2 structure contain a substituted η4-C5H5X cyclopentadiene ligand obtained by the addition of X to one of the Cp ligands. For the formal Pd(I) complexes Cp2Pd2L2 (L = CO and CS) the coaxial structures lie in energy below the isomeric perpendicular structures with a particularly large energy separation of ∼19 kcal mol−1 for the thiocarbonyl derivatives. However, for Cp2Pd2(CNCH3)2 the coaxial and perpendicular isomers have essentially the same energies.

The readily available CH3N(PF2)2 ligand has been shown experimentally to form the binuclear iron carbonyl complexes CH3N(PF2)2Fe2(CO)n (n = 8, 7) and [CH3N(PF2)2]2Fe2(CO)n (n = 6, 5) in which the central Fe2 units are bridged by the CH3N(PF2)2 ligands. In the CH3N(PF2)2Fe2(CO)n series the lowest energy CH3N(PF2)2Fe2(CO)8 structure has Fe(CO)4 moieties bonded to each phosphorus atom in the CH3N(PF2)2 ligand with all terminal CO groups and no iron–iron bond. Loss of one CO group from this structure to give CH3N(PF2)2Fe2(CO)7 converts one of the remaining terminal CO groups into a bridging CO group and introduces a formal Fe–Fe single bond of length ∼2.7 A. Further loss of a CO group from this CH3N(PF2)2Fe2(CO)7 structure retains the Fe–Fe single bond to give a Fe2(CO)6(μ-CH3NPF2)(μ-PF2) structure with all terminal CO groups but separate bridging PF2 and CH3NPF2 groups arising from P–N bond cleavage of the CH3N(PF2)2 ligand. Still further decarbonylation of this Fe2(CO)6(μ-CH3NPF2)(μ-PF2) structure results in simple loss of one of the terminal CO groups while retaining the central Fe2(μ-CH3NPF2)(μ-PF2) unit including the formal Fe–Fe single bond of length ∼2.7 A. The lowest energy structures of the [CH3N(PF2)2]2Fe2(CO)n (n = 6, 5, 4, 3) series can be derived from the corresponding CH3N(PF2)2Fe2(CO)n+2 structures by replacing one terminal CO group on each iron atom by the second bridging CH3N(PF2)2 ligand.

The recent (2010) synthesis of the boronylplatinum halide (R3P)2Pt(BO)Br by Braunschweig and coworkers makes interesting the chemistry of metal carbonyl boronyls isoelectronic with the binary metal carbonyls Mx(CO)y. In this connection, theoretical studies on Fe2(BO)2(CO)8 isoelectronic with the known Mn2(CO)10 predict symmetrical unbridged structures consisting of Fe(BO)(CO)4 units linked by an iron–iron single bond of ∼2.9 A length. These Fe2(BO)2(CO)8 structures with a staggered arrangement of the BO and CO ligands are similar to the experimental structure of Mn2(CO)10. Three such structures are found within ∼3 kcal mol−1 of energy differing only in the locations of the terminal BO groups. Related higher energy unbridged eclipsed Fe2(BO)2(CO)8 structures are found with small imaginary vibrational frequencies of around 30i cm−1. Following the corresponding normal modes leads to the corresponding staggered structures. Even higher energy symmetrical Fe2(μ-CO)2(BO)2(CO)8 structures are found with two bridging carbonyl groups lying 15 to 21 kcal mol−1 above the global minimum, again similar to the isoelectronic Mn2(CO)10 system. In addition to the above structures higher energy unsymmetrical Fe2(BO)2(CO)8 structures are found, which have no analogues in Mn2(CO)10 chemistry. These include structures in which an Fe(CO)4 unit is connected to an Fe(BO)(CO)4 unit through a bridging BO group without an iron–iron bond as well as a structure in which an Fe(CO)5 unit is connected to an Fe(BO)2(CO)3 unit by an Fe–Fe dative bond. The lowest energy Fe2(BO)2(CO)8 structure is viable towards symmetric dissociation into two Fe(BO)(CO)4 radicals as well as unsymmetrical dissociation into Fe(CO)5 + Fe(BO)2(CO)3 with dissociation energies in excess of 15 kcal mol−1.

Ragogna and co-workers recently (2009 and thereafter) reported the syntheses and structures of a family of remarkably stable tridentate dications (Ch = S, Se and Te). We present a theoretical study of these fascinating compounds. For a better understanding of the extraordinary robustness of these dications, the analogous monocations and neutral species are also explored. The dications with C2v symmetries are genuine minima; for the monocations the equilibrium structures have Cs symmetry, with three distinct Ch–N distances; the neutral molecules have C1 symmetry, with one Ch–N bond effectively broken. Our theoretical results, including the natural charges, Wiberg bond indices (WBIs), and HOMO–LUMO gaps, provide reasonable explanations for their remarkable stability. The global minima of the monocations (with Cs symmetry) lie lower in energy than the corresponding C2v structures by 3.4, 1.1, and 0.3 kcal mol−1, respectively, for S, Se, and Te. The global minima of the neutrals (with C1 symmetry) lie lower in energy than their C2v structures by 23.7, 14.9, and 6.9 kcal mol−1 for the S, Se, and Te compounds, respectively. From our theoretical investigations, the viabilities for the dications, monocations, and the neutrals of these chalcogen(II) compounds fall in the order of S > Se > Te.

Although benzene and naphthalene do not have electron affinities in the conventional sense, perfluorobenzene and perfluoronaphthalene have nonzero electron affinities. Theoretical methods extensively calibrated with experiment for the prediction of electron affinities (EAs) predict the EAs of perfluorobenzene, perfluoronaphthalene and perfluoroanthracene as 0.69, 1.02 and 1.84 eV, respectively. A rough estimate of 2.39 eV is made for the electron affinity of perfluorotetracene. Thus the perfluoro polycyclic aromatic hydrocarbons (PAHs) are predicted to be effective electron acceptors.

Reaction of carbene-stabilized diphosphorus, L:P-P:L (L: = :C{N(2,6-Pr(i)(2)C(6)H(3))CH}(2), 1), with excess BH(3)·THF affords the boronium salt [L:P(μ-BH(2))P:L](+)[B(2)H(7)](-), 2, which contains a three-membered P(2)B ring. When 2 is dissolved in THF, compounds 1 and 2 exist in a dynamic solution equilibrium.

The known butadiene-iron carbonyl derivatives (η4-C4H6)Fe(CO)3, (η2-C4H6)Fe(CO)4, and (η2,2-C4H6)[Fe(CO)4]2 are potential precursors to binuclear derivatives of the type (C4H6)2Fe2(CO)n related to the known cyclobutadiene derivative (η4-C4H4)2Fe2(μ-CO)3. In this, density functional theory predicts viable structures with tetrahapto C4H6 ligands as well as one, two, and three bridging carbonyl groups and formal Fe–Fe single bonds, FeFe double bonds, and FeFe triple bonds, respectively, for (C4H6)2Fe2(CO)5, (C4H6)2Fe2(CO)4, and (C4H6)2Fe2(CO)3. The tetracarbonyl (C4H6)2Fe2(CO)4 is predicted to be unfavorable with respect to disproportionation into (C4H6)2Fe2(CO)5 + (C4H6)2Fe2(CO)3. The carbonyl-richer binuclear derivatives (C4H6)2Fe2(CO)6 and (C4H6)2Fe2(CO)7 are predicted to have structures having one or two dihapto η2-C4H6 ligands, respectively, in addition to formal Fe–Fe single bonds. However, these carbonyl-rich structures are predicted to dissociate into mononuclear (η2-C4H6)Fe(CO)4 and (η4-C4H6)Fe(CO)3 fragments. Natural bond order (NBO) analysis is used to explore qualitative electronic structure considerations.

Neutral bis(pentalene)metal sandwich compounds have been synthesized for the first row transition metals titanium and iron. In this connection, the complete series of first row transition metal bis(pentalene)metal complexes (C8H6)2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) has now been investigated by density functional theory in order to evaluate the effect of the metal electronic requirements on the hapticity of the pentalene ligands. The lowest energy structure for the titanium complex is the 18-electron complex (η8-C8H6)(η6-C8H6)Ti with one octahapto and one hexahapto pentalene ligand rather than the previously suggested 20-electron complex (η8-C8H6)2Ti with two octahapto pentalene ligands. The experimental NMR observations on (C8H6)2Ti can then be reinterpreted as interchange of octahapto and hexahapto bonding of the pentalene units through a low energy bis(octahapto) (η8-C8H6)2Ti transition state. The lowest energy structure for (C8H6)2V has two fulvene-like hexahapto pentalene ligands and a local vanadium environment similar to the well-known dibenzenevanadium. The lowest energy structures of the later first row transition metals have two pentahapto pentalene ligands with local metal environments similar to those in the corresponding metallocenes. In the manganese and iron structures of this type, the remaining unpaired electrons on one of the uncomplexed carbon atoms in each η5-C8H6 system form a C–C single bond to couple the two pentalene ligands. This coupling of the two pentalene systems through a C–C single bond is similar to that found experimentally by X-ray crystallography in bis(pentalene)iron, (η5-C8H6)2Fe.

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).

The report (2009) of an unusual and unexpected Fe3(μ3-BF)2(μ-BF)(CO)9 structure, an isoelectronic fluoroborylene analogue of the long known Fe3(CO)12, makes of interest the structures of the unsaturated Fe3(BF)3(CO)8 isoelectronic with Fe3(CO)11. In this connection, seven singlet and eight triplet structures are found for Fe3(BF)3(CO)8 within 30 kcal/mol in energy of the global minimum. Natural bond orbital (NBO) analysis aids in the understanding of the electronic structure of these systems. None of the predicted structures has even a single terminal BF group. In the three lowest energy singlet Fe3(BF)3(CO)8 structures two of the BF groups are face-semibridging ligands whereas the third BF group is a simple edge-bridging ligand. The face-semibridging BF groups in these structures are joined to the Fe3 triangle with two short Fe–B bonds of ∼2.0 A and one long Fe–B bond of 2.4 A. A fourth singlet Fe3(B3F3)(CO)7(η2-μ-CO) structure, lying only ∼7 kcal/mol above the lowest energy singlet structure, has a central Fe3B3 capped tetrahedron analogous to the central Os6 bicapped tetrahedron in the long-known isoelectronic Os6(CO)18. This last cluster suggests the potential accessibility of fluoroborane metal carbonyl clusters having deltahedral structures related to those found in the deltahedral boranes.

Reaction of [(2,4,6-Pri3C6H2)2C 6H3]GaCl2 with P(SiMe3)3 affords the unusual organometallic compound [(2,4,6-Pri3C6H2)2C 6H3]Ga{H2PGa(H)PH2}G a[C6H3(C6H2- Pri3-2,4,6)2] 1 characterized by multinuclear NMR, complete (C, H, Ga and P) elemental analyses, IR spectroscopy and single crystal X-ray diffraction; 1 is significant as it contains a rare Ga–Ga–Ga [2.5145(13) and 2.7778(14) A] linkage even as its formation is facilitated by the unusual stripping of the sterically demanding m-terphenyl ligand from the central gallium atom.

The cobalt carbonyl nitrosyls Co(NO)(CO)n (n = 4, 3, 2, 1) and Co2(NO)2(CO)n (n = 5, 4, 3, 2) have been studied by density functional theory. The lowest energy structures for the mononuclear derivatives Co(NO)(CO)n (n = 3, 2, 1) parallel those of the corresponding isoelectronic Ni(CO)n+1 derivatives. In addition, a thermodynamically unstable Co(NO)(CO)4 structure is predicted with a bent nitrosyl group. The potential energy surfaces of the binuclear cobalt carbonyl nitrosyl derivatives Co2(NO)2(CO)n (n = 5, 4, 3, 2) are complicated by the similar energies of analogous structures with bridging carbonyl or bridging nitrosyl groups. However, these structures are analogous to those previously predicted for the isoelectronic binuclear nickel carbonyls Ni2(CO)n+2. Thus the lowest energy structures of Co2(NO)2(CO)5, Co2(NO)2(CO)4, and Co2(NO)2(CO)3 have one, two, and three bridging groups, respectively, just like the isoelectronic Ni2(CO)n+2. For Co2(NO)2(CO)5 and Co2(NO)2(CO)4 the otherwise analogous structures with bridging carbonyl and/or nitrosyl groups are of very similar energies. However, for Co2(NO)2(CO)3 there appears to be a particularly stable triply carbonyl bridged structure with an unusually short CoCo distance consistent with the formal triple bond required to give the cobalt atoms the favored 18-electron configuration.

Ab initio coupled cluster methods indicate that the isomerization of cyclopropane to propene (Ea = 64–66kcal mol–1 experimentally) might involve two differentmechanisms: the barrier for the newly proposed pathway through propylidene is only slightly higher (ΔE‡ = 66.6kcal mol–1) than that of the traditional trimethylene route(ΔE‡ = 64.2 kcal mol–1).

High level ab initio structures for 3-ethynylcyclopropene contain a typical cyclopropene CC double bond of 1.296 A, in sharp contrast to a recent X-ray structure in which the CC double bond (1.255(2) A) is purported to be the shortest yet observed among all hydrocarbons.

The exceptional 0.19 A lengthening (to 1.720 vs. 1.53 A in ethane) of the Csp3–Csp3 bond in tetraphenyldihydrocyclobutaarenes is attributed to a combination of cyclobutene ring strain (0.04 A), through-bond coupling (0.08 A) and steric repulsion (0.07 A) by comparison with model systems.