Binuclear chromium carbonyl complexes of the highly basic small bite bidentate diphosphine bis(dimethylphosphino)methane

Abstract

The structure and thermochemistry of the triads of binuclear chromium carbonyl complexes [(Me2P)2CH2]Cr2(CO)n (n=10, 9, 8), [(Me2P)2CH2]2Cr2(CO)n (n=8, 7, 6) and [(Me2P)2CH2]3Cr2(CO)n (n=6, 5, 4) have been investigated using density functional theory. The lowest-energy structures of the carbonyl-richest triad members [(Me2P)2CH2]Cr2(CO)10, [(Me2P)2CH2]2Cr2(CO)8, and [(Me2P)2CH2]3Cr2(CO)6 have long Cr⋯Cr distances indicating the absence of direct chromium bonds. The lowest energy structures of the intermediate triad members [(Me2P)2CH2]2Cr2(CO)7 and [(Me2P)2CH2]3Cr2(CO)5 systems have Cr–Cr single bonds of lengths of 3.13 and 3.39Å, respectively, and intact diphosphine ligands. However, in the lowest energy [(Me2P)2CH2]Cr2(CO)9 structure, one of the terminal CO groups is converted into a four-electron donor bridging η2-μ-CO group, retaining a long non-bonding Cr⋯Cr distance of 4.80Å. The lowest energy structure of the carbonyl-poorest [(Me2P)2CH2]Cr2(CO)8, [(Me2P)2CH2]2Cr2(CO)6, and [(Me2P)2CH2]3Cr2(CO)4 members of each triad show different features. In the lowest energy [(Me2P)2CH2]Cr2(CO)8 structure the ligand splits into separate Me2P and Me2PCH2 units, both of which bridge a Cr–Cr bond of length 3.06Å. The lowest energy [(Me2P)2CH2]2Cr2(CO)6 structure has a four-electron donor bridging η2-μ-CO group and a Cr–Cr single bond of length 2.91Å. The lowest energy [(Me2P)2CH2]3Cr2(CO)4 structure is a triplet structure with a Cr–Cr single bond of length 3.03Å.