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 Å) 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 Å) 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 the analogous ethylene complexes by matrix isolation EPR spectroscopy.