Structural and Bonding Investigation of the Donor-Acceptor Complex 1-Sn(Phenanthroline)-2,3-[Si(CH3)(3)](2)-2,3-C2B4H4: A Comparative Study of the Group 14 Metallacarborane-Lewis Base Complexes

Closo-1-Sn-2,3-[Si(CH3)(3)](2)-2,3-C2B4H4 (1) reacts with 1,10-phenanthroline to form the complex 1-(C12H8N2)Sn-2,3-[Si(CH3)(3)](2)-2,3-C2B4H4. When crystals of this complex were grown slowly from benzene, a crystalline modification (2) was obtained in which the unit cell consisted of four identical molecules of the base-stannacarborane and six benzene molecules of salvation. When the complex was sublimed, a second crystalline modification (3) was obtained whose unit cell consisted of two crystalIographically different molecules (3-1 and 3-2). In all three forms the tin was not symmetrically bonded to the carborane but was slipped toward the boron side of the C2B3 face, and the phenanthroline molecules were oriented opposite to the cage carbons so that the plane of the base and the carborane faces gave acute dihedral angles. However, the three forms showed a great deal of variation in the extent of slippage and the magnitudes of the base-carborane dihedral angles. Since these distortions are common structural features of base-group 14-carborane and cyclopentadienyl systems, MNDO-SCF molecular orbital calculations were carried out on the model compounds 1-(C12H8N2)Sn-2,3-C2B4H6, 1-(C10H8N2)Sn-2,3-C2B9H11, and [(C10HBN2)SnC5(CH3)(5)](+) to determine what factors dictate the structures of these complexes. The results showed that competing bonding interactions give rise to a very broad minimum in energy as a function of slippage and base orientation. Small energy variations, such as those produced by crystal packing forces, could produce large structural changes. Complex 2 crystallizes in the monoclinic space group P2(1)/n with a = 11.096 (9) angstrom, b = 26.51(2) angstrom, c = 11.729 (7) angstrom, beta = 107.43 (6)degrees U = 3291 (4) angstrom(3), and Z = 4. Full-matrix least-squares analysis converged at R = 0.044 and R-omega = 0.055. Complex 3 crystallizes in the triclinic space group P (1) over bar with a = 10.251 (3) angstrom, b = 13.845(9) angstrom; c = 19.168(9) angstrom, alpha = 71.12 (5)degrees, beta = 89.29 (3)degrees, gamma = 84.62 (4) degrees, boolean OR = U = 2562 (2) angstrom(3), and Z = 4. The structure refined at R = 0.079 and R-omega = 0.125.