Crystal structures and interrelationships of the blue and green conformational isomers of tetrakis(trimethylarsine sulphide)cobalt(II) perchlorate

Abstract

Blue and green compounds, [Co(Me₃AsS)₄][ClO₄]₂, which are conformationally isomeric, crystallise when cobalt(II) perchlorate reacts with trimethylarsine sulphide in ethanol and acetone respectively. Their crystal structures show that the cation field spreading from the AsMe₃ groups permits side-by-side packing of the cations into columns with the methyl groups pointing outwards. The column packing, together with S–Co–S valence angles and S–Co–S–As torsion angles, differs in the two isomers. For the blue isomer the S–Co–S angles fall in pairs: 120.4, 117.0; 108.6,107.1; 102.5,102.0°. For the green isomer two angles are 90° and four are close to 120°, with the sulphur atoms forming a tetragonal sphenoid. The green isomer and the nickel compounds [Ni(Me₃AsS)₄][ClO₄]₂ and [Ni(Me₃AsS)₄][BF₄]₂ are isomorphous. For these nickel compounds the sphenoidal configuration is associated with ligand-field stabilisation. It is suggested that the ligand-field effect, though smaller for cobalt, helps stabilise the green isomer.