CAPPING CONSIDERATIONS IN MAIN-GROUP TRANSITION-METAL CLUSTERS - SYNTHETIC, STRUCTURAL, AND THEORETICAL DISCUSSIONS OF [SB2CO4(CO)10(MU-CO)]-/2-, [BI2CO4(CO)10(MU-CO)]-/2-

Unstable SbCo3(CO)12 (1a) is generated from the reaction of SbCl3 and Na[Co(CO)4] in acidified aqueous solution. The compound decomposes under a variety of conditions to give varying mixtures of [Sb2Co4(CO)10(mu-CO)]- ([2a]-), [Sb2Co4(CO)10(mu-CO)]2-([2a]2-), and [Co(CO)4]-. The two antimony-cobalt cluster compounds have been isolated as [Et4N]+ or [PPN]+ salts. X-ray analyses show them to be isostructural with a tetrahedral Co2Sb2 core capped on the two CoSb2 faces with Co(CO)3 groups. [PPN][2a]: orthorhombic space group Pbca with a = 18.408 (3) angstrom, b = 33.038 (7) angstrom, c = 16.301 (3) angstrom, V = 9914 (3) angstrom-3, Z = 8, R = 0.053, R(w) = 0.063. [PPN][2a]: triclinic space group PBAR1 with a = 15.618 (3) angstrom, b = 19.333 (4) angstrom, c = 15.124 (3) angstrom, alpha = 93.44 (2)-degrees, beta = 113.66 (2)-degrees, gamma = 69.60 (1)-degrees, Z = 2, V = 3903 (2) angstrom-3, R = 0.045, R(w) = 0.057. Cluster anions with the same formulation but with bismuth replacing antimony, [2b]- and [2b]2-, are not generated in the same fashion but can be accessed by the reduction of BiCo3(CO)9 with cobaltocene. [Cp2Co][2b] has been characterized by single-crystal X-ray analysis: orthorhombic space group Pbcn with a = 14.973 (4) angstrom, b = 12.662 (4) angstrom, c = 14.671 (4) angstrom, Z = 4, V = 2782 (2) angstrom-3, R = 0.051, R(w) = 0.066. Both [2a]- and [2b]- are paramagnetic with ESR signals found at g = 2.032 and 2.107, respectively. The mono-and dianions are related to each other by chemically and electrochemically reversible le redox cycles: E1/2 = 0.54 V (Sb), -0.61 V (Bi). Considering the E-E interactions, the three compounds in this study are electron rich and violate normal electron counting rules. Molecular orbital calculations at the extended Huckel level were carried out on these molecules, as well as related derivatives. It is shown that the molecules can be regarded as either pentagonal bipyramids or bicapped tetrahedra. In both derivations the extra electrons enter into skeletal nonbonding or weakly antibonding molecular orbitals. The favored electron counts for other skeletal isomers and derivatives are determined and compared to experiment.