The coordinatively unsaturated ruthenium(II) complexes [Ru(eta (6)-C6R6(eta -amidinate)]X-+(-) (R = H, Me, X = TFPB, PF6), being isoelectronic with Ru(eta (5)-C5Me5)(eta -amidinate), have been isolated and characterized by spectroscopy and crystallography. A weak pi -coordination of the amidinate ligands in the solid state was observed by X-ray crystallography. DFT calculations also suggest that such a coordination mode contributes to the stabilization of these complexes. These complexes behave as highly reactive transition metal Lewis acids in the reactions with various two-electron donor ligands.