Cyclometallated Ru(II) complexes of the type [Ru(CO)(EPh3)2(L)] (E = P or As; L = tridentate hydrazone-derived ligand) have been obtained by refluxing an ethanolic solution of [RuHCl(CO)(PPh3)3] or [RuHCl(CO)(AsPh3)3] with the hydrazone derivatives H2php (2-[(2,4-dinitro-phenyl)-hydrazonomethyl]-phenol), H2phm (2-[(2,4-dinitro-phenyl)-hydrazonomethyl]-6-methoxy-phenol) and H2phn (2-[(2,4-dinitro-phenyl)-hydrazonomethyl]-naphthalen-1-ol). The formation of stable cyclometallated complexes has been authenticated by single crystal X-ray structure determination of two of the complexes, and the mechanism of C–H activation is discussed in detail. The spectral (IR, UV–Vis and 1H NMR) and electrochemical data for all the complexes are reported. Electrochemistry shows a substantial variation in the metal redox potentials with regard to the electronic nature of the substituents present in the hydrazone derivative.