Preparation, properties, and reactions with sulphur dioxide of triphenylphosphine oxide complexes of manganese(II) thiocyanate. Crystal structures of [{Mn(OPPh3)2(NCS)(µ-NCS)}2], [Mn(OPPh3)4(NCS)2], and (PPh3O)(PPh3OH)(HSO4), a derivative of sulphuric acid formed under ambient air oxidation of sulphur dioxide

Abstract

Manganese(II) thiocyanate reacts with triphenylphosphine oxide to yield the complexes [Mn(OPPh₃)_n(NCS)₂](n= 1,2,3, or 4) which have been characterised by elemental analyses, i.r. and e.s.r. spectroscopy, and magnetic susceptibility measurements. These complexes are non-electrolytes in nitromethane solution. The crystal structures of the bis, [{Mn(OPPh₃)₂(NCS)(µ-NCS)}₂], and tetrakis, [Mn(OPPh₃)₄(NCS)₂], complexes are reported. The former is centrosymmetric comprising two Mn(OPPh₃)₂(NCS)₂ units bridged by two NCS ligands, the geometry around each manganese atom being square pyramidal with an OPPh₃ ligand in the axial position; the latter is a trans-pseudo-octahedral monomer with inversion symmetry about the central manganese atom. The reactions of the complexes with sulphur dioxide in both the solid state and in toluene suspensions have been investigated. The tris and tetrakis complexes react reversibly to give 1:1 adducts, [Mn(OPPh₃)_n{NCS)₂(SO₂)], whereas the mono and bis complexes are inactive towards sulphur dioxide. Dilute solutions of [Mn(OPPh₃)₄(NCS)₂(SO₂)] in the presence of air result in the oxidation of sulphur dioxide to sulphuric acid, as evidenced by the isolation and X-ray crystallographic characterisation of (PPh₃O)(PPh₃OH)(HSO₄). This derivative of sulphuric acid is characterised by extremely strong hydrogen bonds, 2.351 and 2.492 Å.