1ST FERROMAGNETIC INTERACTION IN A HETEROPOLY COMPLEX - [COII4O14(H2O)2(PW9O27)2]10- - EXPERIMENT AND THEORY FOR INTRAMOLECULAR ANISOTROPIC EXCHANGE INVOLVING THE 4 CO(II) ATOMS

The four coplanar Co(II) atoms in K10[Co4O14(H2O)2(PW9O27)2].22H2O occupy the vertices of a rhomb in a Co4O16 entity. Each Co atom is somewhat off-center in its edge-sharing CoO6 octahedron. The corrected effective magnetic moment is essentially constant at approximately 10.4 mu(B) down to 40 K, corresponding to four independent Co(II) atoms each having spin = 3/2 and significant spin-orbit coupling. Below 20 K the complex exhibits strong ferromagnetic coupling, mu(eff) rising to over 14.4 mu(B) and the Curie-Weiss theta becoming approximately +6 K. Below 9 K mu(eff) tends toward leveling out again, in accordance with the isolated nature of the intramolecular ferromagnetic coupling in the heteropoly system. This is the first report of ferromagnetic coupling in a heteropoly complex and apparently the first case of a polynuclear Co(II) system showing intramolecular ferromagnetic exchange. Furthermore, the complex provides a novel geometrical arrangement of the four edge-sharing CoO6 octahedra, which increases interest in the explanation of the magnetic interaction. The results emphasize the potential value of heteropoly complexes for magnetic studies because of the magnetic isolation of the groups of interacting atoms and the existence of isomorphous series of complexes containing a wide variety of combinations of paramagnetic atoms. Owing to the combined effects of spin-orbit coupling and distortion, each octahedral Co(II) atom behaves below approximately 30 K as an anisotropic spin doublet, S = 1/2, as confirmed by the low-temperature ESR spectrum of the complex doped into the diamagnetic isomorph wherein Zn replaces Co. The two structural types of Co require different g tensors. Evaluation of the exchange and Zeeman Hamiltonians leads to a best fit to the data below 22 K requiring 2J(parallel-to) = +19 cm-1, J(perpendicular-to)/J(parallel-to) = 0.321, g(parallel-to) = 7.9 and 6.15, and g(perpendicular-to) = 2.04 and 5.1. Emphasis is placed on the necessity for anisotropy if the data are to fit, more than on the specific values of the parameters. The magnetic results and the structure lead to suggestion of an explanation of the ferromagnetism involving some overlap of magnetic orbitals of adjacent Co's and the orbital degeneracy of the Co, thereby allowing electronic transfers in their t2g orbitals, which keep a parallel spin alignment on the virtual Co(III)-Co(I) excited state, thus providing a pathway for stabilization of the ferromagnetic state. Work on isomorphs is in progress aimed at elucidating this point.