η¹-coordination of phosphinine C₅H₅P and arsenine C₅H₅As to ruthenium(II) and osmium(II)

Reductive complexation of RuCl₃ and OsCl₃ in the presence of phosphinine C₅H₅P and arsenine C ₅H₅As yields the species trans-Cl₂(η ¹-C₅H₅E)₄M^II (M = Ru, E = P, As; M = Os, E = P). trans-Cl₂(η¹-C ₅H₅As)₄Ru constitutes the first arsenine complex of a late transition metal. Further reduction and isolation of the binary complexes (C₅H₅E)_nM⁰ (n = 2, 4, 5) failed, presumably because of metal-ligand bond cleavage. According to X-ray diffraction analysis, trans-Cl₂(η¹-C ₅H₅P)₄Ru (3) features two pairs of coplanar trans-phosphinine ligands, which adopt eclipsed and staggered orientations, respectively, with regard to the Cl-Ru-Cl backbone. DFT calculations indicate a flat curvature of the potential governing conformational change and predict a structure for 3 that differs from that observed in the crystal. This illustrates the need to include intermolecular Ru-Cl⋯H hydrogen bonding as a structural director.