SPECTROSCOPIC, MAGNETIC AND STRUCTURAL STUDIES ON THE MIXED-VALENCE CUBOIDAL CLUSTERS [MO(4)E(4)(EDTA)(2)](3-) (E=S OR SE, H(4)EDTA=ETHYLENEDIAMINETETRAACETIC ACID) AND [MO4S4(H2O)(12)](5+)

The Se-77 NMR spectrum of the mixed-valence cluster [Mo4Se4(edta)(2)]( 3-) (H(4)edta = ethylenediamine-N,N,N',N'-tetraacetic acid) was record ed and appears to be consistent with the presence in solution of a val ence-trapped species (Mo3MoIV)-Mo-III with a lifetime in excess of 10( -5) s. However this was not supported by Mo-95 and Se-77 NMR studies o n other [Mo4Se4(edta)(2)](n-) (n = 2 or 4) clusters. Overall it appear s that multiple NMR signals arise from the presence in solution of spe cies which differ slightly from the above formulae in the extent of co -ordination of H2O and edta. The studies do not preclude the possibili ty of valence-trapped species with lifetimes shorter than 10(-6) s. A model in which intervalence electron exchange within the core clusters Mo(4)E(4)(5+) (E = S or Se) is slower than formation/decay (approxima te to 10(9) s(-1)) of ion-pair precursors is proposed to account for t he biphasic kinetic behaviour previously observed for substitution of H2O on [Mo4S4(H2O>(12)](5+) The crystal structure of a hydrated sodium salt of [Mo4Se4(edta)(2)](3-) has been determined, and is compared to that previously obtained for [Mo4S4(edta)(2)](3-). Other techniques e xplored include magnetic moment measurements on a solid sample of [Mo4 S4(H2O)(12)][pts](5) . 14H(2)O (pts = toluene-p-sulfonate) which show a Curie-like behaviour (chi = C/T) for an S = 1/2 ground state, with a corresponding magnetic moment independent of temperature over the ran ge 300-4.2 K. Metal-ligand stretching frequencies from Fourier-transfo rm IR studies on the chloride salt of the same 5+ cluster have also be en obtained.