2-Mercapto- and 4-mercaptopyridine (2- and 4MPy) react with the [Fe(CN)(5)(H2O)](3-) complex, forming the S-coordinated [Fe(CN)(5)(2Mpy)](3-) and the N-coordinated [Fe(CN)(5)(4MPy)](3-) complexes. The rates of formation and dissociation of the [Fe-II(CN)(5)(2Mpy)](3-) complex were determined as k(i) = 294 dm(3) mol(-1) s(-1) and k(d) = 0.019 s-1 by means of stopped-flow technique. The equilibrium constants for the iron(II) and -(III) species were calculated as K-f(II) = 1.5 x 10(4) mol(-1) dm(3) and K-f(III) = 1.3 x 10(6) mol(-1) dm(3), in comparison with 2.6 x 10(5) and 3.4 x 10(1) mol(-1) dm(3), respectively, for the 4MPy isomer. In the presence of gold nanoparticles, both 2- and 4MPy can displace the stabilizing citrate species, leading to substantial aggregation in aqueous solution, as deduced from the surface-enhanced Raman spectroscopy effect and from the decay of the 520-nm plasmon band accompanied by the rise of the characteristic exciton band at 650 nm. The [Fe(CN)(5)(4Mpy)](3-) complex promotes strong stabilization of the gold nanoparticles by interacting through the S atom. On the other hand, the labile [Fe(CN)(5)(2Mpy)](3-) complex induces aggregation, delivering the 2MPy ligand to the gold nanoparticles.