Halogen complexes of ruthenium cyclopentadienyl [CpRu(PTA)(2)X]; [CpRu(PTA)(PPh3)X]; [CpRu(PPh3)(2)Cl], and [CpRu(mPTA)(PPh3)X](+) (Cp = C5H5; PTA = 1,3,5-triaza-7-phosphaadamantane; mPTA(+) = [1-methyl-1,3,5-triaza-7-phosphaadamantane](+) ; X = Cl-, I-) were investigated by electrospray mass spectrometry (ESI-MS), in flow-cell cyclic voltammetry, by microelectrodes, and by combined online electrochemistry and electrospray mass spectrometry (EC/ESI-MS) in dimethyl formamide solution. Coordination changes and the structures of the initial compounds and the products of the electrooxidation of the Ru(II) complexes were traced by in situ EC/MSn experiments which revealed their fragmentation pathways. ESI-MS collision-induced dissociation fragmentations of the initial reactants and the oxidation products were explained by soft acid-hard base considerations taking into account the different nature of Ru(II)-Ru(IV) centers. The electrochemical studies show that it is possible to tune the formal potentials for the oxidation of [CpRuL2X] complexes by over 300 mV by proper selection of the ligands. The increase of the redox potential by the different ligands follows the order PTA < PPh3 < mPTA(+). We demonstrate a similarity between the propensity of the ligand to fragment out in the gas phase and its relationship to the formal potential of the complex. (c) 2006 The Electrochemical Society.