Poly(tertiary phosphines and arsines). 18. Preparation and structure of bis{.mu.-[(methylamino)bis(dimethoxyphosphine)]}-bis(dicarbonylcobalt), a binuclear complex with approximate square-pyramidal and trigonal-bipyramidal coordination of cobalt atoms in the same molecule

Abstract

The preparation and the X-ray crystal-structure analysis of bis)..mu..-((methylamino)bis-(dimethoxyphosphine)))-bis(dicabonylcobalt), a binuclear cobalt carbonyl complex containing the ligand CH/sub 3/N(P(OCH/sub 3/)/sub 2/)/sub 2/ is described. A single-crystal X-ray diffraction study of this complex shows a structure with a cobalt-cobalt bond 2.498 angstrom long bridged by two CH/sub 3/N(P(OCH/sub 3/)/sub 2/)/sub 2/ ligands. The two cobalt atoms are both five-coordinate and have identical sets of ligands, each of the two being bonded to the other cobalt atom, to two carbonyl groups, and to two trivalent phosphorus donors. However, the two cobalt atoms are not equivalent; one has trigonal-bipyramidal coordination and the other has square-pyramidal coordination. The trigonal-bipyramidal cobalt atom has the other cobalt atom and a carbonyl group in the axial position. The square-pyramidal cobalt atom has a carbonyl group in the apical position. The difference in coordination of the two cobalt atoms probably results from the packing of a fluxional molecule which has a symmetrical average structure in solution. Although the v(CO) frequencies in the infrared spectrum in solution all appear in the terminal region, the complex in the crystal contains a carbonyl group in a borderline semibridging position with Co-C distances of 1.756 and 2.812 angstrom, probably as the resultmore » of the crystal packing. The atoms of th Co/sub 2/(CO)/sub 4/ unit in the complex are essentially coplanar, and the least-squares best plane through these atoms is an approximate mirror plane for the molecule.« less