Calculated Hydride Donor Abilities of Five-Coordinate Transition Metal Hydrides [HM(diphosphine)2]+ (M=Ni, Pd, Pt) as a Function of the Bite Angle and Twist Angle of Diphosphine Ligands

Density functional theory (BLYP and B3LYP) and the polarized continuum model (PCM-UAO) for solvation have been used to investigate the effect of bite angle (P-M-P) of diphosphine ligands and the dihedral or twist angle between diphosphine ligands on the hydride donor abilities of Ni, Pd, and Pt [HM(diphosphine)₂^]+complexes. It is found that an increased bite angle for a given transition metal atom results in poorer hydride donor abilities. However, hydride donor abilities for these complexes also decrease as the size of the alkyl side groups on the phosphorus atom increase (Et > Me > H) and with the length of the metal phosphorus bond (Ni > Pd ≃ Pt). These trends correlate with an increase in the twist angle between the two diphosphine ligands, which increases from 0° for a square-planar configuration to 90° for a tetrahedral geometry. Shorter M - P bonds, larger substituents on the diphosphine ligands, and larger bite angles all result in increased steric interactions between diphosphine ligands and larger dihedral or twist angles between the diphosphine ligands. The twist angle correlates much more strongly with hydride donor abilities than do bite angles alone. As the twist angle increases, the hydride donor ability decreases in a linear fashion. A frontier orbital analysis has been carried out, and it is shown that the hydride donor ability of [HM(diphosphine)₂]⁺ complexes is largely determined by the energy of the lowest unoccupied molecular orbital of the corresponding [M(diphosphine)₂]²⁺ complex.