Novel ruthenium(III) dimers Na-2[{trans-RuCl4(Me2SO-S)}2(mu-L)] and [{mer,cis-RuCl3(Me2SO-S)(Me2SO-O)}2(mu-L)] (L = bridging heterocyclic N-donor ligand) closely related to the antimetastatic complex Na[trans-RuCl4(Me2SO-S)(Him)]