Antiferromagnetism in transition-metal complexes. Part II. The importance of metal–metal bonding in the antiferromagnetism of copper(II) acetate monohydrate and some of its homologues

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Abstract
A model which incorporates spin-exchange and metal–metal bonding is used to interpret the magnetic susceptibility data of cupric acetate monohydrate and some of its homologues. Experimental values of g and the Néel parameters are used to determine a unique set of values for J, the spin-exchange integral, and 2γa, the covalency parameter. Although the overall pattern of energy levels leads to antiferromagnetism, spin-exchange appears to be ferromagnetic, and therefore direct in origin, for the alkanoates, except for formates in which direct exchange and superexchange through the bridging groups appear to be approximately equal. The separation between the ground-state singlet and the spin-triplet corresponds closely to the singlet–triplet separation of earlier models. A very good fit between calculated and experimental data is obtained with the new model. The ferromagnetic spin-exchange parameter J and the covalency parameter 2γa appear to increase together along the series: formate < acetate < propionate < butyrate, H2O < pyridine < picolines. This pattern is correlated only with the overall stability of the complexes in view of the weakness of the metal–metal bonds. The effect of errors in the value of g or of the temperature-independent susceptibility is not of great importance, but random errors in the Néel temperature or a systematic error in χN as a result of paramagnetic impurities could lead to serious errors in the computed values of J and 2γa.

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