RELATIVE STEREOCHEMICAL AND ELECTRONIC EFFECTS OF OXYGEN AND SULFUR DONOR ATOMS IN PLANAR AND TETRAHEDRAL COMPLEXES OF NICKEL(2)

Reactions of primary amines with the 3-phenyl-1,2-dithiolium cation and with O-ethyl-β-ketoamine cations have led to the synthesis of three series of α,β-unsaturated, N-substituted β-aminothiones RΓC(S)CH= C(NHR)Rα (H(R-SRΓHRα) : RΓHRα = PhHH, PhHMe, MeHMe). Structures of the compounds of the PhHH series have been established by mass spectroscopy and synthesis; the predominant tautomer of all compounds in chloroform solution has been shown by pmr to be the aminothione form. Bis-chelate nickel(II) and zinc(II) complexes M(R-SRΓHRα)2 can be readily prepared by nonaqueous chelation reactions. Spectral, magnetic, and proton resonance studies reveal that Ni(R-SRΓHRα)2 species in chloroform and carbon tetrachloride solutions are involved in a rapid planar→ tetrahedral equilibrium presumably identical with that displayed by the precisely analogous bis(β-ketoamine) complexes Ni(R-RΓHRα)2. Thermodynamic data (∆F, ∆H, ∆S) pertaining to the structural equilibria of eight pairs Ni(R-RΓHRα)2-Ni(R-SRΓHRα)2, the members of which differ only in their donor atom sets, have been obtained from analysis of the temperature dependencies of contact shifts. In all cases it is observed that the planar stereoisomer of Ni(R-SRΓHRα)2 is much more stable than that of Ni(R-RΓHRα)2. The relative equilibrium positions are shown to be controlled mainly by enthalpy effects, which in turn are ascribed to differences primarily in Ni-O and Ni-S bonding in the two stereoisomers. Contact shifts in β-ketoamine and β-aminothione complexes arise from π derealization in the highest filled MO of the ligands produced by ligand-to-metal antiparallel spin transfer. Experimentally determined spin densities in the mixed ligand complexes Ni(t-Bu-PhHH)(t-Bu-SPhHH) and Ni(t-Bu-SPhH)(Et2-ati) (Et2-ati = N,Nʹ-diethylaminotroponeiminate) indicate that, with respect to tetrahedral Ni(II) as the common acceptor, the 7π-donor propensity of a β-aminothione is less than that of the analogous β-ketoamine ligand. Large contact shift differences between active and meso diastereoisomers of Ni(R-SPhHH)2, R = sec-Bu, MeCHCH2Ph (Amp), are observed, and it is shown for the R = Amp isomers that these differences arise mainly from inequalities in ∆Factive and ∆Fmeson, in agreement with a previous argument. © 1969, American Chemical Society. All rights reserved.