Homoleptic binuclear chromium carbonyls: why haven’t they been synthesized as stable molecules?☆

Abstract Density functional theory has been used to explore possible homoleptic binuclear Cr carbonyls Cr 2 (CO) n ( n  = 11, 10, 9, and 8) using the pure DFT method BP86 and the hybrid Hartree–Fock DFT method B3LYP. The binuclear Cr 2 (CO) 11 is computed to be thermodynamically unstable with respect to dissociation into mononuclear fragments in contrast to the experimentally known Cr 2 (CO) 10 (μ-H) − . This may account for the failure to synthesize Cr 2 (CO) 11 as a stable compound. Optimized structures for the formally unsaturated Cr 2 (CO) 10 are a singlet with two four-electron donor bridging CO groups and no formal metal–metal bonding and a triplet with no bridging CO groups and a Cr Cr double bond similar to the O O bond in O 2 . The more highly unsaturated homoleptic binuclear chromium carbonyls Cr 2 (CO) 9 and Cr 2 (CO) 8 are computed to be stable with respect to dissociation into mononuclear fragments in contrast to Cr 2 (CO) 11 and Cr 2 (CO) 10 . The optimized structure for Cr 2 (CO) 9 is a singlet Cr 2 (CO) 6 (μ-CO) 3 with a short metal–metal distance (∼2.3 A) consistent with the Cr≡Cr triple bond required for an 18-electron configuration for each Cr atom. The global minimum for Cr 2 (CO) 8 is a closely related Cr 2 (CO) 5 (μ-CO) 3 structure derived from the Cr 2 (CO) 6 (μ-CO) 3 global minimum by loss of one of the terminal CO groups with little change in the Cr≡Cr distance. Higher energy minima for Cr 2 (CO) 8 include two different Cr 2 (CO) 6 (μ-CO) 2 structures, one formulated with two four-electron donor μ-CO groups bridging two Cr(CO) 3 groups and the other with similar μ-CO groups bridging a Cr(CO) 4 and a Cr(CO) 2 group.