Reactions of activated quadruply bonded molybdenum dimers. Formation of new rectangular clusters containing the tetrametal analog of cyclobutadiyne

A new quadruply bonded molybdenum dimer, Mo/sub 2/Cl/sub 4/(P(C/sub 6/H/sub 5/)/sub 3/)/sub 2/(CH/sub 3/OH)/sub 2/, has been prepared, characterized by structure analysis, and shown to exhibit unusual reactivity. In benzene solution the new dimer undergoes self-addition across the quadruple bond resulting in formation of a novel tetrameric cluster unit. Structure determination of the triethylphosphine substituted tetramer, Mo/sub 4/Cl/sub 8/(PEt/sub 3/)/sub 4/, revealed the molecular, rectangular cluster unit with two short Mo--Mo bonds, 2.211(3) A, and two long bonds, 2.901(2) A. The short Mo--Mo bonds are unsupported by bridging ligands but the long bonds are each bridged by two Cl atoms, and the coordination sphere of each Mo is completed by one Cl and one PEt/sub 3/ terminal ligand. The Mo--Mo bond distances, diamagnetism, and number of electrons available for bonding are consistent with assessment of bonding in the rectangle as two triple and two single bonds, the tetrametal analogue of cyclobutadiyne. 2 figures.