Mononuclear and Binuclear Metal(II) Complexes of Ethane-1,2-Dithiolate. Preparation, Equilibrium and Redox Properties of [Ni(S2C2H4)2]2- and [Ni2(S2C2H4)3]2-, and the Structures of Ni2(S2C2H4)3]2- and [Pd(S2C2H4)2]2-

Abstract

The diamagnetic ethane-1,2-dithiolate complexes [Ni( edt )2]2- and [Ni2( edt )3]2- were isolated as Ph4P+ salts from reaction systems with K2( edt ) : NiCl2 mol ratios of 10 : 1 and 2 : 1, respectively. (Ph4P)2[Ni2( edt )3] crystallizes in triclinic space group Pī with a 10.409(2)Ǻ, b 12.872(4)Ǻ, c 21.235(6)Ǻ, α 76.59(2)°, β 84.65(2)°, γ 70.12(2)°, and Z 2. The anion consists of a Ni( edt ) fragment bridged to [Ni( edt )2]2- through cis sulfur atoms of different chelate rings, resulting in a complex of idealized CS symmetry. Each NiS4 coordination unit shows small deviations from planarity. Their mean S4 planes are inclined at a dihedral angle of 118.0° across the S-S vector of bridging sulfur atoms, which leads to a Ni-Ni separation of 2.914(1)Ǻ. Terminal Ni-S bond lengths average to 2.177(12)Ǻ. (Ph4P)2[Pd( edt )2].4H2O crystallizes in monoclinic space group C2/c with a 10.470(3)Ǻ, b 24.264(9)Ǻ, c 19.691(7)Ǻ, β 93.05(3)°, and Z 4. The PdS4 unit of the anion deviates from planarity by a dihedral angle of 7.7° between chelated PdS2 groups. The mean Pd-S distance is 2.318 Ǻ. In protic and aprotic solvents [Ni( edt )2]2- converts into the binuclear complex in the reaction 2[Ni( edt )2]2- → [Ni2( edt )3]2- + edt2-, which is first order in the mononuclear complex. Reactions are faster in protic solvents; in HCONMe2 at 40°C k ≈ 4×10-5 s-1. In acetonitrile [Ni2( edt )3]2- shows an irreversible oxidation at Ep,a -0.21 V v . s.c.e .; for [Ni( edt )2]2- under the same conditions Ep,a -0.61 V and E1/2 for the 2-/1- couple is estimated as -0.68 V. These values demonstrate that potentials of classical anionic NiII tetrathiolate complexes are substantially reduced relative to those of non- thiolate species (excluding dithiolenes ), and are in the region of NiII /III potentials in hydrogenases.