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 (tetracyanoethene), 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)](PF6)(2) (PhCN = benzonitrile) and {(mu-L)[Ru(NH3)(5)](2)}(PF6)(4) (L = 1,4-dicyanobenzene (terephthalodinitrile) or pyrazine) were also investigated for comparison and were found to be essentially diamagnetic. Despite the even electron count, both the ruthenium and manganese tetranuclear complexes are paramagnetic, albeit with different spin-spin exchange coupling patterns. The manganese systems are characterized by exchange-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, identified as Ru-III/Ru-II mixed-valence pairs.