Metal-metal interactions in binuclear cyclopentadienylmetal carbonyls: Extending insight from experimental work through computational studies

Abstract

Abstract Binuclear cyclopentadienylmetal carbonyls of the type Cp2M2(CO)n (Cp = η5-C5H5) provide examples of stable compounds containing formal metal-metal double and triple bonds. In order to probe the extent of this rich area of organometallic chemistry, systematic theoretical studies on such binuclear derivatives of all of the first row transition metals, selected second and third row transition metals (Nb, Mo, Re, Os), lanthanides (La, Lu), and actinides (Th, U) have been undertaken in recent years using well-established density functional methods. In order to focus on the trends in the preferred Cp2M2(CO)n structures, the lowest energy such structures found in these studies are examined in this review. In most cases, except for titanium and the f-block metals, the favored structures have 18-electron configurations for both metal atoms. Structures having metal-metal triple bonds are favored over structures having metal-metal double bonds and metal-metal quadruple bonds. In many cases the doubly bonded structures are energetically disfavored with respect to disproportionation into singly and triply bonded structures as exemplified by the experimentally observed disproportionation of Cp2Cr2(CO)5 into Cp2Cr2(CO)6 + Cp2Cr2(CO)4. Quadruply bonded singlet Cp2M2(CO)n structures having 18-electron metal configurations lie at higher energies than isomeric triply bonded structures having 17-electron metal configurations for the first row transition metals ranging from vanadium to iron. This can relate to the weakness of the orbital overlap in the δ component of the quadruple bond. Four-electron donor bridging η2-μ-CO groups bonded to a central M2 unit through both their carbon and oxygen atoms are found in low-energy Cp2M2(CO)n structures of the oxophilic early transition metals titanium, vanadium, and niobium. Such bridging η2-μ-CO groups dominate in the predicted low-energy structures for the experimentally still unknown Cp2M2(CO)n (n = 4, 3, 2) derivatives of the f-block metals. However, for the lanthanides, as exemplified by lanthanum and lutetium, such η2-μ-CO groups couple through C C bond formation in the lowest energy structures to form bridging C2O2 and C3O3 units. This can be related to the reductive coupling of carbon monoxide to CnOn2 − (n = 2, 3, 4) dianions in cyclopentadienyl-cyclooctatetraene uranium(III) systems observed experimentally by Cloke and coworkers.