Five- and six-coordinated complexes of cobalt(II) with 2,2',2'-terpyridyl: Unusual structure and magnetism

Cobalt(II) complexes of the type [Co(terpy)X2] (I), [Co(terpy)2]X2,nH2O (II), and [Co(terpy)2]Y2,nH2O (III), where X = Cl-, Br-, I-, NCS-, and Y = ClO4-, [BPh4]-, have been prepared and characterized by magnetic, spectroscopic, and conductance measurements. The ?mono? complexes (I) are 5-coordinated and monomeric, non-ionic in nitrobenzene, and have high-spin magnetic moments in the range 4.7-5.0 B.M. They are readily and reversibly converted into the ?bis? complexes (II) with the addition or loss of a molecule of terpyridyl: [Co(terpy)X2]+ terpy ↔ [Co(terpy)2]X2     The bis complexes (II) and (III) exhibit the characteristic features of the electronic cross-over phenomenon, their magnetic moments being strongly dependent on temperature, the anion X or Y, and water of crystallization. A model based on the 4T1-2E cross-over is set up which gives a good representation of the magnetic properties. Values of the difference between the zero-point energies of the 4T1 and 2E states are given for the various compounds.     Attempts to isolate halogeno complexes of the type [Co(terpy)X]+ were unsuccessful.