Ab initio Study of the Coordination Modes of the Tetrahydroborato Ligand: Structure of the [Cu(BH 4 )(PH 3 ) n ] ( n = 1, 2, 3) Complexes

All‐electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH 4 )(PH 3 ) n ] ( n = 1, 2, 3) complexes. Full‐geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η 2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η 1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η 3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η 1 → η 2 for n = 3 and η 3 → η 2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.