Characterization of Steric and Electronic Properties of NiN_2S_2 Complexes as S-Donor Metallodithiolate Ligands

摘要

The physical properties and structures of a series of six complexes of the type(NiN_2S_2)W(CO)_4 have been used to establish electronic and steric parameters for square planar NiN_2S_2 complexes as bidentate,S-donor ligands.According to the v(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 NiN_2S_2 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)~= > {[NiN_2S_2]~0} > bipy approx= phen > Ph_2PCH_2CH_2PPh_2 > Ph_2PCH-2PPh_2.Electrochemical data from cyclic voltammetry find that the reduction event in the(NiN_2S_2)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 NiN_2S_2 ligand,consistent with the electron drain from the nickel-dithiolate ligands by the W(CO)_4 acceptor.Differences in Ni~(II/I)DELTAE_(1/2)values appear to have a ligand dependence which is related to a structural feature of the hinge angle imposed by the(mu-SR)_2 bridges.Thus the angle formed by the intersection of NiN_2S_2/WS_2C_2 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 deg.Variable-temperature ~(13)C NMR studies show that the spatial orientations of the ligands remained fixed with respect to the W(CO)_4 moiety to temperatures of 100 deg C.