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

Abstract

Unlike the coordinatively unsaturated NiS₄ core of bis(diethyl-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_z²)] 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 increase 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' resulting adduct. A comparative
Extended Huckel Molecular Orbital (EHMO) study of
[Ni{S₂P(OC₂H₅)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
atom pairs as well as by fragment molecular orbital (FMO)
analysis. The EHMO results, thus obtained, have been correlated
to the relevant force constants (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.