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Major disadvantages of the use of the anisotropic Ising model to describe strong antiferromagnetic coupling in infinite linear chains of copper(I1) ions are discussed with particular reference to cupric oxalate. EXAMINATION of the several recent reviews on antiferromagnetically coupled CuII ions reveals that the Ising model has been fairly generally accepted as the most convenient basis for the interpretation of infinite linearchain systems.1 This model involves the simplification of the effective spin-exchange Hamiltonian [equation (l)] by setting y = 0.2This practice considerably simplifies the sums-indeed it is the only way that analytical equations for the magnetic susceptibility have been obtained3-but it contrasts dramatically with the methods used for finite polynuclear systems (the Heisenberg model) in which y is set equal to unity. Few workers appear to have appreciated what a Machine calculations on chains with up to 11 members have been carried out by Bonner and Fisher4 using different values of in equation (1). This is a much neglected paper, for it shows that the predictions of the king and Heisenberg models do not converge significantly as chain-length increases and, secondly, i t provides empirical data for the approximate description of infinite linear chains with the preferred model of Heisenberg. We have fitted these data [modified to be consistent with equation ( l ) ] to the polynomial expression given in equation (2). The agreement is better than 1% over the whole range studied by these workers and the equation therefore provides a convenient basis for the interpretation of appropriate experimental data. (N.b. this expression cannot be used for ferromagnetic coupling.)where P = RT/ I J I TABLE. Magnetic data for copper(I1) dicarboxylatesa [Equations Ising [Equation Dimer model Heisenberg model (3) and (4) ] model (3)] CompoundRef.-J (cm-1) 10% -J (cm-l) 10%-J (cm-l)106u -J (cm-l) lO6o Cupric oxalate,0.3H20