The behaviour of the magnetic properties of the *TI, term under simultaneous perturbation by spin-orbit coupling and an axial ligand-field is presented. The theory is applied to the variation with temperature of fifteen octahedral cobalt(1i) complexes in the range 80--300°K. The values of k, the orbital reduction factor, deduced for the present complexes are determined mainly by the absolute magnitudes of the mean susceptibilities, while those of the axialfield splitting, A, and spin-orbit coupling coefficient are determined more by the temperature variation of the moments. It is found that for these compounds, where the variation of the magnetic moment with temperature is not large, there is usually uncertainty concerning the magnitude, and even the sign, of A.IN previous papers it has been shown that the theory of the magnetic properties of the cubic-field 2T2,1 5T2,2 and 3T1,3 terms under perturbation by spin-orbit coupling and a tetragonal or trigonal ligand-field component can be useful in accounting for the magnetic behaviour of complexes in which they are the ground terms. In this paper we now report the results of a similar treatment on the remaining orbital triplet groundterm which can arise for transition-metal complexes, namely 4T1.t The data has been applied to a number of six-co-ordinate cobalt (11) complexes and estimates of the values of the axial ligand-field component, A, the spinorbit coupling constant, A, and the amounts of t,, orbital reduction have been made.The treatment and parameters are identical to those for the 3T1 terms3 The matrices of 4T1, under the combined action of spin-orbit coupling and an axial ligand-field component, are given in the Appendix. As in previous papers 1-3 A is defined as the splitting of the orbital degeneracy of the 4T1 term by the asymmetric ligand-field component, in the absence of any spint The term is 4T,, in the octahedral and *TI in the tetrahedral case, but in order to simplify the text, we will use 4T, as the name of the term.1 See ref. 4 and 5 for further discussion of k and A.
