A thermochemical determination of the relative solid-state stabilities of the yellow [Ni(qnqn)Cl22] and violet [Ni(qnqn)Cl2] isomers (where qnqn is trans-2-(2-quinolyl)methylenequinuclidin-3-one) has indicated that the violet isomer is the metastable isomer at room temperature. D.t.a.-t.g. measurements have been obtained for the iron, cobalt, nickel, and copper chloride and nickel bromide complexes of qnqn and reveal in the case of the nickel chloride isomers an endothermic structural transformation of the yellow dimer to the violet monomer at 230 °C. No related transformation is observed in the remaining complexes. The unusual inter-related thermal properties of the nickel chloride isomers are discussed in terms of the various properties of the two complexes.