Electrochemical, SERS and SPR results indicated that the SAM of [Ru(NH3)(4)(CNpy)(pyS)](2+) complex was successfully formed on gold. At first, the modification procedure was achieved by the self-assembling of the sulphate starting complex, [Ru(SO4)(NH3)(4)(pyS)](+). Then, a potential of -0.4V vs. Ag/AgCl was applied in order to reduce the metal center and induce the substitution reaction of (SO4)(2-) by H2O which, in turn, was replaced by CNpy. The surface pKa of the SAM formed with [Ru(NH3)(4)(CNpy)(pyS)](2+) was calculated as 3.05 indicating an increasing in the pi-back-bonding interaction upon adsorption since a value of 2.66 is observed for the complex in solution. This behavior was reinforced by the observation of the shift of the nu(C N) mode to lower frequency (2190 cm(-1)) in the SERS spectrum in comparison to the spectrum in the solid state (2201 cm(-1)). The enhancement of the back-bonding interaction upon adsorption is consistent with the existence of gold adatoms in 1+ oxidation state that is assumed to act as a withdrawing group attached to the pyS moiety. The mass of the adsorbed complexes per unit area, calculated by SPR, indicated an excess of CNpy which is consistent with the SERS results where it was observed the frequency assigned to the C N mode of CNpy free of coordination. (C) 2013 Elsevier Ltd. All rights reserved.