MULTIPLE STRUCTURAL VARIANTS OF LNCUI(MU-X)2CUILN (N = 1, 2) - INFLUENCE OF HALIDE ON A SOFT POTENTIAL-ENERGY SURFACE

The crystal structures Of Cu(PCy3)2I and [Cu(PCy3)I]2 are reported in order to furnish data to help understand the variation of Cu/Cu distances with changing halide in planar structures of the type LnCuI(mu-X)2CuILn (n = 1, 2). It is found that iodide shows a shorter Cu/Cu separation (2.89 angstrom) than does chloride (3.07 angstrom) in [Cu(PCy3)X]2 species. An extended Huckel analysis of the bonding in these molecules indicates the Cu(I) s and p orbitals to be most important in bonding and shows larger Cu/Cu overlap populations for the stronger donor (iodide over chloride). This is traced to weak sigma and pi Cu/Cu bonding interactions. Such bonding interaction is diminished when the terminal phosphine ligand is replaced by a pi-donor (halide), in agreement with literature data for Cu2X4(2-) species. Analogous weakening is effected by addition of another terminal ligand to each copper (i.e., L2CuI(mu-X)2CuIL2). The geometry within the Cu(mu-X)2Cu rhombus is shown to exist in a broad potential energy well, and diminished Cu/Cu interactions (i.e., longer Cu/Cu distances) are compensated by improved interaction of halide orbitals with in-plane out-of-phase Cu/Cu orbitals. This explains the surprisingly large in-plane distortions observed for Cu2X4(2-) species in solids containing various cations and in L2Cu(mu-X)2CuL2. Crystallographic data: for Cu(PCy3)2I (at -174-degrees-C), a = 9.634 (2) angstrom, b = 22.975 (5) angstrom, c = 9.058 (2) angstrom, alpha = 97.38 (1)-degrees, beta = 114.49 (1)-degrees, and gamma = 93.40 (1)-degrees with Z = 2 in space group P1BAR; for [Cu(PCy3)I]2 (at -172-degrees-C), a = 9.757 (2) angstrom, b = 12.780 (3) angstrom, c = 8.808 (2) angstrom, alpha = 94.26 (1)-degrees, beta = 116.74 (1)-degrees, and gamma = 96.37 (1)-degrees with Z = 1 (dimer) in space group P1BAR.