General theoretical analysis of three-connected polyhedral molecules and their capped derivatives

The synthesis of the half-sandwich compound Na[(C₅H₅)Ni{P(S)(CH₃)₂}₂] is described. The anions [(C₅H₅)Ni{P(S)R₂}₂]^?, 1a (R = OCH₃) and 1b (R = CH₃) react as bidentate sulfur ligands with [Ni₂(C₅H₅)₃]⁺, giving nickelocene and weakly paramagnetic dinuclear complexes of the type [(C₅H₅)Ni{P(S)R₂}₂Ni(C₅H₅)] (2a,b). In these compounds, the P(S)R₂ units form NiPSNi bridges in such a fashion as to generate a (C₅H₅)NiP₂ and a (C₅H₅)NiS₂ unit. A temperature-dependent singlettriplet spin equilibrium is observed, which is essentially localized on the (C₅H₅)NiS₂ side. Accordingly, the position of the cyclopentadienyl peak of the (C₅H₅)Ni unit bound to the two sulfur donor centers displays a very large temperature dependence in the ¹H NMR spectra. MO model calculations (EHT) for P(S)H₂^?, [(C₅H₅)Ni{P(S)H₂}₂]^? (1c), [(C₅H₅)Ni{P(S)H₂}₂Ni(C₅H₅)] (2c) and its isomer 3c allow the observed spin crossover to be explained as a consequence of the pronounced π-donor properties of the sulfur centers and allow predictions for related complexes.The green complexes 2a,b isomerize completely and irreversibly in a first-order reaction to yield the diamagnetic red compounds [{(C₅H₅)NiP(S)R₂}₂] (3a,b), in which each (C₅H₅)Ni unit is coordinated to one P and one S donor atom. The rate constant of isomerization of 2a, k (7.6 ± 0.3) × 10^?4s^?1 at 306 K, and the energy of activation, E_a 76 kJ mol^?1, have been determined. The rate of isomerization is independent of the solvent, and crossover experiments verify that the isomerization is an intramolecular process without involvement of the monomeric units [(C₅H₅)NiP(S)R₂].