The new terdentate ligand 2,6-di(diphenylphosphinomethyl)pyridine forms a series of crystalline complexes, MU2, with iron(l1). cobalt(1i). and nickel(l1) halides and thiocyanates. Distorted square pyramidal structures are suggested on the evidence of their electronic spectra and other physical properties. Magnetic susceptibility measurements as a function of temperature classify the cobalt and nickel compounds as low-spin, high-spin. or as having an anomalous temperature-dependence. For the latter group a low-spin + high-spin equilibrium is suggested.All the iron complexes are fully high-spin over the temperature range studied. The suggested magnetic cross-over in these compounds is discussed in relation to that in previously reported compounds having a distorted trigonal bipyramidal structure.

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