Polymorphism and the configuration of the cation in dichlorobis-1,10-phenanthroline- and dichloro-bis-2,2′-bipyridyl-chromium(III) and -cobalt(III) compounds

Abstract

A variety of polymorphs of cis-[M chelate₂Cl₂]X,xH₂O (M = Co or Cr; chelate = 1,10-phenanthroline, 2,2′-bipyridyl; X = Cl or ClO₄) are described, as well as other solids of low crystallinity; solid-state interconversions between some of these materials are also described. These, together with other salts of the cations, have variable colour in sunlight, with violet, red-brown, and green predominating. However, the configuration of the cation in all of the solids is cis. There is no evidence in such cations for a trans-configuration, which is expected to be destabilised by steric effects. Evidence for the cis-configuration relies on (a) the observation that the cations are non-labile, which allows a correlation in the phenanthroline series with α-cis-[Co phen₂Cl₂]Cl,3H₂O, the crystal structure of which is known; and (b) the d–d bands in the electronic spectra of the solids. Some i.r. data also are presented, which show that methods, previously used to assign structure in these compounds, are unreliable. Reactions of the chelate ligands with CrCl₃,6H₂O in dimethyl sulphoxide show that a stable bisphenanthroline complex is formed readily, but only monobipyridyl species [Cr bipy (Me₂SO)Cl₃] and [Cr bipy (Me₂SO)₂Cl₂]⁺. The cation cis-[Cr bipy₂Cl₂]⁺ in this solvent decomposes when heated according to the reaction, [Cr bipy₂Cl₂]⁺+ 2Me₂SO →[Cr bipy (Me₂SO)₂Cl₂]⁺+ bipy.