Binuclear chromium carbonyl complexes of methylaminobis(difluorophosphine): metal–metal bonds versus four-electron donor bridging carbonyl groups

Abstract

Binuclear chromium carbonyl complexes of the general type [MeN(PF₂)₂]_mCr₂(CO)_n, including the experimentally known [MeN(PF₂)₂]₃Cr₂(CO)_n (n = 6, 5) species, have been studied by density functional theory (DFT) methods. The lowest energy structures for the three series of [MeN(PF₂)₂]_mCr₂(CO)_n (m = 1, 2, 3) structures can be grouped into three triads, namely [MeN(PF₂)₂]Cr₂(CO)_n (n = 10, 9, 8), [MeN(PF₂)₂]₂Cr₂(CO)_n (n = 8, 7, 6), and [MeN(PF₂)₂]₃Cr₂(CO)_n (n = 6, 5, 4). The carbonyl richest structures of each triad, namely [MeN(PF₂)₂]Cr₂(CO)₁₀, [MeN(PF₂)₂]₂Cr₂(CO)₈, and [MeN(PF₂)₂]₃Cr₂(CO)₆ have all terminal carbonyl groups, no chromium–chromium bond, and the MeN(PF₂)₂ ligands bridging the pair of chromium atoms. However, for [MeN(PF₂)₂]₃Cr₂(CO)₆ a structure with two of the three MeN(PF₂)₂ ligands chelating to single chromium atoms are energetically competitive. Low-energy singlet spin state structures for the intermediate and carbonyl poorest members of each triad can incorporate a variety of features such as chromium–chromium single and double bonds, MeN(PF₂)₂ ligands split into bridging MeNPF₂ + PF₂ groups, and four-electron donor bridging η²-μ-CO groups as required to give each chromium atom the favored 18-electron configuration. Such four-electron donor bridging η²-μ-CO groups are not found in low-energy structures of related binuclear carbonyl complexes [MeN(PF₂)₂]_mM₂(CO)_n (M = Fe, Ni; m = 1, 2) of the later transition metals iron and nickel.