A Comparative Molecular Orbital Study and Vibrational Analysis of Bis(Diethyldithiophosphato) Nickel(II) and its Pyridine Adduct

摘要

Unlike the coordinatively unsaturated NiS_4 core of bis(dimethyl-dithiocaramato) nickel(II), bis(diethyldithiophosphato) nickel(II) readily forms bis-pyridine (Py) adduct with distortion of chelate rings but maintaining their coplanarity. The adduct formation is facilitated by donation of electrons from the highest occupied molecular orbital, HOMO (N(p_z)] of pyridine to the lowest unoccupied molecular orbital, LUMO (Ni(d_z~2)] of nickel(II) diethyldithiophosphate. As a result of adduct formation and concomitant coordination expansion, the Ni-S bond length and S-P-S bond angle are increased while the S-Ni-S bond angle is decreased. These changes are attributed to the decrease of net positive charge on the nickel atom with a simultaneous incrase of net negative charges on the sulfur atoms. The former effect increases the Ni-S bond length resulting in a decrease of S-Ni-S bond angle while the latter effect increases the S-P-S bond angle. These structural changes cause a considerable change in the properties of the reslting adduct. A comparative Extended Huckel Molecular Orbital (EHMO) study of [Ni{S_zP(OC_2H_5)_2}_2] and its pyridine adduct is, therefore, made to elucidate the effect of adduct formation on the bonding characteristics of the parent complex by calculating the net charges (Q) on individual atoms, reduced overlap populations (ROP) between different atoms pairs as well as by fragment molecular orbital (FMO) analysis. The EHMO results, thus obtained, have been correlated to the relevant force constant (f) and IR frequencies determined by normal coordinate analysis (NCA). There is good agreement between the calculated and the observed frequencies which are assigned by the potential energy distribution (PED) calculation.