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

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 3.39 Å, 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 4.80 Å. 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 3.06 Å. 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 2.91 Å. The lowest energy [(Me₂P)₂CH₂]₃Cr₂(CO)₄ structure is a triplet structure with a Cr–Cr single bond of length 3.03 Å.