Geometrical isomer preference of RuII and RuIII: chemistry and structure of a RuS4P2 family. Crystal structure of [Ru(PhCH2SCS2)2(PPh3)2]

Abstract

The syntheses of mixed thioxanthate (trithiocarbonate)–phosphine complexes of types cis-[Ru(RSCS₂)₂(PPh₃)₂](1) and trans-[Ru(RSCS₂)₂(PPh₃)₂]PF₆(2⁺) are reported (R = Et, Prⁱ, or PhCH₂). These along with trans-[Ru(RSCS₂)₂(PPh₃)₂](2) and cis-[Ru(RSCS₂)₂(PPh₃)₂]⁺(1⁺) constitute an electrochemically observable redox isomerisation cycle. The formal potentials of the couples (1⁺)–(1) and (2⁺)–(2) are ≈ 0.65 and ≈ 0.33 V respectively vs. saturated calomel electrode (s.c.e.). The isomerisation steps are (1⁺)→(2⁺)(fast) and (2)→(1)(k≈ 7.0 × 10^–2 s^–1). The X-ray structure of complex (1; R = PhCH₂) has been determined: space group C2/c, a= 18.676(7), b= 10.511(2), c= 24.432(8)Å, and Z = 4. The superior stability of the cis configuration of Ru^II for complexes (1) arises primarily from the strong back-bonding (cis > trans) within the Ru(PPh₃)₂ fragment. For Ru^III, in which back-bonding is unimportant, the stable geometry is trans owing to favourable steric disposition of the PPh₃ molecules. The frozen glass e.s.r. spectra of complexes (2⁺) have a sizable axial distortion (Δ≈ 8 500 cm^–1) with a relatively small rhombic component (V≈ 2 200 cm^–1). One of the two predicted transitions (ν₁≈ 7 500 and ν₂≈ 9 900 cm^–1) within the t_2g shell is observable (ν₁≈ 6 500 cm^–1) in the near-i.r. region.