Characterization of Steric and Electronic Properties of NiN₂S₂ Complexes as S-Donor Metallodithiolate Ligands

The physical properties and structures of a series of six complexes of the type (NiN₂S₂)W(CO)₄ have been used to establish electronic and steric parameters for square planar NiN₂S₂ complexes as bidentate, S-donor ligands. According to the ν(CO) stretching frequencies and associated computed Cotton−Kraihanzel force constants of the tungsten carbonyl adducts, there is little difference in donor abilities of the five neutral NiN₂S₂ metallodithiolate ligands in the series. The dianionic Ni(ema)^2- (ema = N,N‘-ethylenebis(2-mercaptoacetamide)) complex transfers more electron density onto the W(CO)₄ moiety. A ranking of donor abilities and a comparison with classical bidentate ligands is as follows:  Ni(ema)⁼ > {[NiN₂S₂]⁰} > bipy ≈ phen > Ph₂PCH₂CH₂PPh₂ > Ph₂PCH₂PPh₂. Electrochemical data from cyclic voltammetry find that the reduction event in the (NiN₂S₂)W(CO)₄ derivatives is shifted to more positive potentials by ca. 0.5 V compared to the ca. −2 V Ni^II/I redox event in the free NiN₂S₂ ligand, consistent with the electron drain from the nickel−dithiolate ligands by the W(CO)₄ acceptor. Differences in Ni^II/I ΔE_1/2 values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the (μ-SR)₂ bridges. Thus the angle formed by the intersection of NiN₂S₂/WS₂C₂ planes has been established by X-ray diffraction analyses as a unique orientational feature of the nickel−dithiolate ligands in contrast to classical diphosphine or diimine ligands and ranges in value from 136 to 107°. Variable-temperature ¹³C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)₄ moiety to temperatures of 100 °C.