PHPME(2) LIGAUD SUBSTITUTION IN THE TETRACOBALT CLUSTER CO-4(CO)(10)(MU(4)-PPH)(2) - X-RAY-DIFFRACTION STRUCTURE OF CO-4(CO)(8)(PPHME(2))(2)(MU(4)-PPH)(2)

The thermal substitution chemistry of the tetracobalt cluster CO4(CO)( 10)(mu(4)-PPh)(2) with the phosphine ligand PhPMe(2) (2.5 equiv) has b een explored and found to afford the bis(phosphine)-substituted cluste r Co-4(CO)(8)(PPhMe(2))(2)(mu(4)-PPh)(2) as the major reaction product . The regiochemistry and stereoselectivity exhibited by the two phosph ine ligands in Co-4(CO)(8)(PPh-Me(2))(2)(mu(4)-PPh)(2) have been unamb iguously established by X-diffraction analysis as having a 1,3-cis ori entation. Co-4(CO)(8)(PPhMe(2))(2)(mu(4)-PPh)(2) crystallizes in the m onoclinic space group P2(1)/n, a = 10.314(1) Angstrom, b = 18.051(3) A ngstrom, c = 21.313(2) Angstrom, beta = 90.10(1)degrees, V = 3968.0(8) Angstrom(3), Z = 4, d(calc) = 1.590 g cm(-3); R = 0.051, R(w) = 0.042 for 4987 observed reflections with I > 3 sigma(I). Generalizations co ncerning the stereochemical disposition of two P-ligands about the Co- 4(CO)(8)P-2(mu(4)-PPh)(2) (where P = phosphine or phosphite) polyhedro n are discussed with respect to the cone angle of the P-ligand and its steric interactions with the capping phenylphosphinidene group.