ELECTRONIC-STRUCTURE OF OCTAHEDRAL COPPER(I) CLUSTER COMPLEXES - [H6CU6(PH3)6] AND [CCU6(PH3)6]2+

The structure and bonding in the octahedral cluster complexes [H6Cu6(PH3)6] and [CCu6(PH3)6]2+, and also in a number of related mononuclear and cluster phosphine complexes of copper, have been studied by the linear combination of Gaussian-type orbital local density functional (LCGTO-LDF) method. The metal-metal and metal-ligand bond lengths in [H6Cu6(PH3)6] (Cu-Cu = 2.50; Cu-P = 2.12; Cu-H = 1.76 angstrom) agree closely with those of a recently reported X-ray crystallographic structure determination for the corresponding PPh3 complex [H6Cu6(PPh3)6]. A value of 990 cm-1 is calculated for the as yet unmeasured totally symmetrical Cu-H stretching frequency in this complex. The corresponding frequency for [H6Cu6], 1360 cm-1, lies close to the value 1400 cm-1, which has been tentatively assigned to the symmetrical vibrational mode of H atoms chemisorbed on triply bridging sites on copper metal. [CCu6(PH3)6]2+ has a minimum energy configuration (Cu-Cu = 2.63; Cu-P = 2.10; Cu-C = 1.86 angstrom) which is more stable than that of (CCu4(PH3)4] + 2[Cu(PH3)]+ by 835 kJ mol-1. A comparison with the calculated binding energy of [CAu6(PH3)6]2+ shows that the copper analogue should be almost as stable as the known cluster ion [CAu6(PPh3)6]2+. Electronic structure calculations were also carried out on the copper(I) complexes [Cu(PH3)n]+ and on the copper(0) complexes [Cu(PH3)n]0, n = 1-4, in the course of this study. Comparison of the results of the copper(I) complexes with those recently reported for the analogous NH3 complexes reveals a greater stability for the three- and four-coordinate species with the phosphine ligand. The results for the copper(0) complexes show some unusual features which can be associated with a change in the nature of the HOMO from n = 1 to 2, and this behavior parallels that observed experimentally for tbc analogous ethylene complexes by matrix isolation EPR spectroscopy.