PARAMAGNETISM OF TETRANUCLEAR COMPLEXES BETWEEN TCNX-LIGANDS (TCNE, TCNQ, TCNB) AND 4 PENTAAMINERUTHENIUM OR DICARBONYL(PENTAMETHYLCYCLOPENTADIENYL)MANGANESE FRAGMENTS

The tetranuclear complexes {(mu(4)-TCNX)[Ru(NH3)(5)](4)}(A)(8) and (mu (4)-TCNX)[Mn(CO)(2)(C5Me5)](4) [A = PF6 or CF3SO3; TCNX = TCNE (tetrac yanoethene), TCNQ (7,7,8,8-tetracyano-p-quinodimethane), or TCNB (1,2, 4,5-tetracyanobenzene)] were studied by variable-temperature (2-300 K) SQUID susceptometry. Mono- and dinuclear species [(PhCN)Ru(NH3)(5)](P F6)(2) (PhCN = benzonitrile) and {(mu-L)[Ru(NH3)(5)](2)}(PF6)(4) (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also inves tigated for comparison and were found to be essentially diamagnetic. D espite the even electron count, both the ruthenium and manganese tetra nuclear complexes are paramagnetic, albeit with different spin-spin ex change coupling patterns. The manganese systems are characterized by e xchange-coupled S = 1 states at the individual metal centers, whereas the magnetic behavior of the tetranuclear ruthenium compounds results from an exchange-coupling interaction between two S = 1/2 sites, ident ified as Ru-III/Ru-II mixed-valence pairs.