AB-INITIO STUDY OF THE COORDINATION MODES OF THE TETRAHYDROBORATO LIGAND - STRUCTURE OF THE [CU(BN4)(PH3)(N)] (N=1, 2, 3) COMPLEXES

All-electron ab initio calculations (MP4/Mp2 level) have been performe d on [Cu(BH4)(PH3)(n)] (n = 1, 2, 3) complexes. Full-geometry optimiza tions were carried out in each case, and the stationary points were ch aracterized by the diagonalization of the analytically calculated Hess ian matrix. The eta(2) coordination mode, with a tetrahedral arrangeme nt around the copper atom, is the most stable structure for n = 2, whi le for n = 3 a strongly nonlinear eta(1) coordination mode is preferre d. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental d ata, the eta(3) structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (eta (1) --> eta(2) for n = 3 and eta(3) --> eta(2) for n = 1) are small. F inally, a mechanism for the exchange between terminal and bridging hyd rogen atoms is proposed for each complex under study.