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 Å) 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) 2group.