The rule breaking Cr2(CO)10. A 17 electron Cr system or a CrCr double bond?

Abstract

Density functional theory (DFT) has been used to investigate the conformations and thermochemistry on the singlet and triplet potential energy surfaces (PES) of Cr₂(CO)₁₀. The global minimum energy structure for the lowest singlet state of C_2h symmetry is consistent with a model of two interacting Cr(CO)₅ fragments in which one carbonyl in each fragment acts as an asymmetric four-electron donor bridging carbonyl, with chromium-chromium distances of 2.93 Å (B3LYP) or 2.83 Å (BP86). Avoiding a Cr⋯Cr bond by incorporating four-electron donor CO groups in this way allows each chromium atom in singlet Cr₂(CO)₁₀ to attain the favored 18-electron configuration by using, in a simple picture of the bonding, only the six octahedral sp³d² hybrids. The dissociation energy to two Cr(CO)₅ fragments or to Cr(CO)₆ + Cr(CO)₄ fragments is predicted to be 10 kcal mol⁻¹. The lowest triplet state of Cr₂(CO)₁₀ is predicted to lie ∼10 kcal mol⁻¹ above the singlet global minimum. In the case of triplet Cr₂(CO)₁₀ the lowest energy minima were found to be of C₂ and C_2h symmetry, with similar energies. The chromium-chromium distances in triplet Cr₂(CO)₁₀ were found to be shorter than those in the corresponding singlet structures, namely 2.81 (B3LYP) or 2.68 Å (BP86) suggesting a σ + 2(1/2) π CrCr double bond similar to the OO bond in O₂ or the FeFe bond in the experimentally observed triplet state (Me₅C₅)₂Fe₂(μ-CO)₃.