| In recent years,transition metal carbonyl compounds have become a hot field and an important topic in metal-organic chemistry.In this paper,BP86 and M06-L,two commonly used and efficient methods of density functional theory(DFT),are used to study the binuclear cyclobutadiene chromium carbonyls(C4H4)2Cr2(CO)n(n=7,6,5,4,3)and the binuclear phospholylvanadium carbonyls(C4H4P)2V2(CO)n(n=7,6,5,4,3,2,1).The main results are as follows:First,the binuclear cyclobutadiene chromium carbonyls(C4H4)2Cr2(CO)n(n=7,6,5,4,3),potentially accessible from the experimentally known(?4-C4H4)Cr(CO)4,have been examined by DFT calculations.The low-energy structures for the heptacarbonyl(C4H4)2Cr2(CO)7 have the terminal(?4-C4H4)2Cr2(CO)7 ring with a central Cr–Cr single bond bridged by one or three CO groups and the remaining CO groups as terminal ligands evenly distributed among the chromium atoms.The lowest energy structure for the hexacarbonyl(C4H4)2Cr2(CO)6has a(?4-C4H4)2Cr(CO)2 complex functioning as a bidentate chelating ligand to a Cr(CO)4 fragment through the formation of two three-center C-H-Cr bonds with agostic hydrogen atoms,exhibiting the short Cr-H distances of?1.85?.The pentacarbonyl(?4-C4H4)2Cr2(CO)5 is similar to the experimentally known(?5-C5H5)2V2(CO)5,with terminal?4-C4H4 rings,two bridging CO groups and a central Cr?Cr triple bond.The three lowest energy structures for the tetracarbonyl(?4-C4H4)2Cr2(CO)4 have the spinal singlets with two four-electron donor bridging?2-?-CO groups,a terminal CO group on each chromium atom,and a central Cr=Cr double bond.The lowest energy structure for the more highly unsaturated tricarbonyl(?4-C4H4)2Cr2(CO)3 by a margin of more than 12 kcal/mol is a spinal triplet,with a terminal?4-C4H4 ring,a bridging?4,?1-C4H4 ring through an agostic C-H-Cr interaction,and a central Cr?Cr triple bond.Thermochemal analysis suggests that the lowest energy structures for all the(C4H4)2Cr2(CO)n(n=7,6,5,4)systems,especially the pentacarbonyl,are feasible towards carbonyl dissociation.Second,the structures and energetics of the binuclear phospholylvanadium carbonyls(C4H4P)2V2(CO)n(n=7,6,5,4,3,2,1)have been investigated using DFT calculations.The lowest energy structure of heptacarbonyl(C4H4P)2V2(CO)7 is close to that of the dimanganese pentacarbonyl(C4H4P)2Mn2(CO)5,with one seven-electron donor bridging?5,?1-C4H4P ring and no direct vanadium-vanadium bond.Similarly,the lowest energy structure of hexacarbonyl(C4H4P)2V2(CO)6 resembles that of the dimanganese tetracarbonyl(C4H4P)2Mn2(CO)4,with two seven-electron donor bridging?5,?1-C4H4P rings and no direct vanadium-vanadium bond.The lowest energy structure of pentacarbonyl(C4H4P)2V2(CO)5has terminal?5-C4H4P phospholyl rings and a formal V?V triple bond of length?2.45?similar to the experimentally known and structurally characterized cyclopentadienyl analogue(?5-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?5,?1-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 the structures of spinal triplet and singlet,respectively.Formal V==V quadruple bonds are found only in the highly unsaturated lowest energy spinal singlet of(C4H4P)2V2(CO)n(n=2,1),which,however,lie at somewhat higher energies than the isomers of spinal triplets. |
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