Tetraacetanilidodichromium and -dimolybdenum: Another Supershort Chromium-Chromium Bond and an Unexpected Structural Difference

The title compounds, Cr2[PhNC(0)CH3]4,1, and Mo2[PhNC(0)CH3]4, 2, the first tetraamidodimetal compounds with quadruple M-M bonds, are described. They are prepared by reaction of the M2(O2CCH3)4 compounds with Li[PhNC(O)CH3]. 1 was obtained as the unsolvated compound and the structure determined by X-ray crystallography. The crystals belong to space group 14 with a = 12.739 (6) Å, c = 9.251 (3) Å, and Z = 2. Each molecule has 4(S4) crystallographic symmetry and the Cr-Cr distance is 1.873 (4) Å. The molybdenum compound, 2, was also obtained in unsolvated form but crystals large enough for X-ray work could not be obtained. The bis(tetrahydrofuranate) Mo2[PhNC(0)CH3]-2THF did give suitable crystals, in space group P2}/c with a = 9.849 (2) Å, b = 11.622 (2) Å, c = 17.957 (2) Å, 0 = 95.32 (1)°, and Z = 2, in which each molecule resides on a crystallographic inversion center and has an Mo-Mo distance of 2.086 (2) Å. The ligand arrangement is different from that in 1; the MoN2O2 groups have a cis configuration and the overall symmetry of the MO2(NCO)4 grouping is C2k. This is the first time homologous Cr24+ and Mo24+ complexes have been found to differ structurally in a qualitative way.