Magnetic Properties of some Aryl-Carboxylates of Copper II

Abstract

NYHOLM¹ has pointed out that in the d⁹ configuration Fe^−I, Co⁰, Ni^I, Cu^II, the presence of metal–metal interaction with the formation of binuclear complexes is common. In marked contrast to the other ions of this series, however, many compounds of copper II are known where no such interaction appears to occur. The variation found for the copper complexes has been correlated with the ligand polarizability, and it has been suggested from consideration of the isoelectronic ions that such interaction occurs more readily with the more polarizable ligand groups, so reducing the effective charge on the copper ions and allowing favourable overlap of the appropriate metal orbitals^1–3. Martin et al.^3,4 have investigated a series of alkyl carboxylic acid complexes of copper II by magnetic and cryoscopic methods and shown that the dimeric copper acetate structure in which four acetate groups bridge the two copper ions and there is a direct interaction between the copper ions, with the formation of a δ-bond, appears to apply to this series with the exception of the formate complexes. The difference in the magnetic properties of the acetate and the formate compounds has been attributed to a difference in the structure of the compounds which has been related to the lower polarizability of the formate group³. Thus a carboxylic acid may bond to metal ions to form polynuclear complexes in one of three ways³: The interaction between the metal ions will be greatest in structure (a). If the repulsion between the metal ions is significant this will increase as the effective positive charge on the metal increases. Any bonding between orbitals of the two ions M will also tend to decrease as the charge on the metal increases., as the orbitals will be contracted by any increase in effective nuclear charge reducing the overlap. Thus structures (b) and (c) will be favoured by the decrease in the polarizability of the acid. In agreement with this copper acetate has a structure corresponding to (a) whereas the copper formate tetra-hydrate (ref. 5) has a structure (b) and one form of anhydrous copper formate⁶ a structure of type (c).