Co 2(CO) 4μ-(CH 2O) 2PN(Et)P(OCH 2) 22]: A molecule with a symmetrical formula but an unsymmetrical structure

Abstract

The diphosphazane ligand (CH 2O) 2PN(Et)P(OCH 2) 2 reacts readily with [Co 2(CO) 8] to form the red-black complex [Co 2(CO) 4μ-(CH 2O) 2PN(Et)P(OCH 2) 2 2]. A single crystal X-ray diffraction study of this complex shows the two cobalt atoms linked by a metalmetal bond of length 2.635(2) Å, and by two bridging diphosphazane ligands. Both cobalt atoms are five-coordinate with an identical set of ligands, comprising the other cobalt atom, two carbonyl groups and two phosphorus donor atoms. However, one cobalt atom has near trigonal-bipyramidal coordination while the other has highly distorted square-pyramidal coordination, the apical positions in both cases being occupied by phosphorus atoms. Significantly, the different coordination at each cobalt atom is achieved by the twisting of each diphosphazane ligand about the CoCo bond; the CoPPCo torsion angles are 29 and 40°. Indeed, in solution, rapid twisting of the diphosphazane ligands renders the phosphorus atoms equivalent on the NMR time scale, as evidenced by a single peak in the 31P 1H NMR spectrum of the molecule. The solution and solid state structures of the title compound are compared with those previously reported for teh closely-related complex [Co 2(CO) 4μ-(MeO) 2PN(Et)P(OMe) 2 2].