Solution properties of the cuboidal mixed-metal cluster [Mo3NiS4(H2O)10]4+ including complexing (at Ni) and redox properties

Abstract

The green-blue complex [Mo₃NiS₄(H₂O)₁₀]⁴⁺ has been prepared by NaBH₄ reduction of a 1:10 solution of [Mo₃S₄(H₂O)₉]⁴⁺ and NiCl₂·6H₂O in 0.5 M HCl, and purified by Dowex 50W-X2 cation-exchange chromatography. Solutions (0.1–2.0 mM) in 2 M HClO₄ obey Beer's law, consistent with the single cube formulation. Complexation equilibration reactions, Mo₃NiS₄⁴⁺+ X^–⇌ Mo₃NiS₄X³⁺, studied by stopped-flow spectrophotometry (25°C), give forward and back rate constants for X^–= Cl^– of k_f= 9.4 M^–1 s^–1, k_b= 0.097 s^–1, and for X^–= NCS^– of k_f= 45 M^–1 s^–1, k_b= 0.062 s^–1. Complexation, assigned to substitution at the tetrahedrally ligated (and metal–metal bonded) nickel centre, is unusually slow, and slower than substitution on [Ni(H₂O)₆]²⁺. Redox reactions, e.g. Mo₃NiS₄⁴⁺+ 2Co^III→ Mo₃S₄⁴⁺+ Ni²⁺+ 2Co^II, with [Co(dipic)₂]^–(dipic = pyridine-2,6-dicarboxylate) and [Fe(H₂O)₆]³⁺ as oxidants, have also been studied. Rate laws are in both cases first order in the concentration of each reactant. Whereas at 25°C the [Co(dipic)₂]^– reaction is [H⁺]-independent and outer sphere (0.35 M^–1 s^–1), the reaction with [Fe(H₂O)₆]³⁺ gives a dependence k_a+k_b[H⁺]^–1, with k_a= 5.4 × 10^–4 M^–1 s^–1 and k_b= 5.3 × 10^–4 s^–1, assigned to outer- and inner-sphere mechanisms respectively. All studies were in aqueous perchloric acid solutions, I= 2.00 M (LiClO₄).