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 LnCuKjt-X^Cu'L* (n * 1, 2). It is found that iodide shows a shorter Cu/Cu separation (2.89 A) than does chloride (3.07 A) in [Cu(PCy3)X]2 species. An extended Huckel analysis of the bonding in these molecules indicates the Cu* 1 11s 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 a and tt Cu/Cu bonding interactions. Such bonding interaction is diminished when the terminal phosphine ligand is replaced by a T-donor (halide), in agreement with literature data for CU2X42" species.Analogous weakening is effected by addition of another terminal ligand to each copper (i.e., I^Cu^-X^Cu'I^).The geometry within the Cu(m-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 Cu2X42species in solids containing various cations and in L2Cu(m-X)2CuL2. Crystallographic data: for Cu(PCy3)2l (at -174 °C), a = 9.634 (2) A, b = 22.975 (5) A, c = 9.058 (2) A, a = 97.38 (1)°, 0 = 114.49 (1)°, and y * 93.40 (1) °with Z = 2 in space group Pi;for [Cu(PCy3)I]2 (at -172 °C), a = 9.757 (2) A, b = 12.780 (3)_A, c -8.808