The facet-dependent redox reactions of diruthenium metal–string complexes by gold nanoparticles (AuNPs) are explored by using the surface-enhanced Raman scattering (SERS) technique. Gold nano-rhombic dodecahedrons (AuRDs), gold nanocubes (AuNCs), and gold octahedrons (AuOhs) with exclusive facets {110}, {100}, and {111}, respectively, were synthesized. These AuNPs linked face-to-face by metal–string complexes Ru 2 M(dpa) 4 Cl 2 (dpa = dipyridyl amino, M = Ni, Cu) with chloride axial ligands serve as both SERS substrates and reducing agents in the reactions. We employ the diruthenium core in these complexes with multiple redox states to study the reduction ability of varied AuNP facets upon plasmonic excitation. In Ru 2 Ni(dpa) 4 Cl 2 , the Ru–Ru stretching mode ν Ru–Ru str. lies at 327 cm –1 on the SERS substrate AuOh, but this band shifts to 313 cm –1 on the AuRD and AuNC. The diruthenium moiety was reduced to [Ru 2 ] 4+ by the AuRD facet {110} and the AuNC {100}. The gold nanorods in the solution prepared with metal–string complexes bridging head-to-head on {111} facets were used for the SERS substrate. The SERS curves of the complexes in these self-assembled head-to-head rods display ν Ru–Ru str. at 327 cm –1 , which is assigned to having an [Ru 2 ] 5+ core. Hence, facets {110} and {100} have a reduction reactivity greater than that of {111}. In Ru 2 Cu(dpa) 4 Cl 2 , the ν Ru–Ru str. is observed to lie at 312 cm –1 on AuRD, but shifts to 320 cm –1 on the AuNC and AuOh. In the latter cases, the diruthenium moiety was reduced to having a charge of 4+ with electronic configuration π* 2 δ* 2 , whereas the former case band at 312 cm –1 with a weaker Ru–Ru bonding is also attributed to [Ru 2 ] 4+ but with electron configuration π* 4 . π* 4 lies at an energy greater than π* 2 δ* 2 . The electrochemical SERS spectra of diruthenium complexes were recorded to verify their oxidation states. Conclusively, these results yield the reduction reactivity of the following facet: {110} > {100} > {111}. According to the results of the redox reactions, the valence states of the diruthenium metal–string complexes are verified. In the [Ru 2 ] n + core, n = 4 π* 4 , 4 π* 2 δ* 2 , 5 π* 2...

We report the bonding strength of nickel ions in trinickel extended metal atom chains (EMACs) and dinickel complexes using Raman, surface-enhanced Raman scattering (SERS), and electrochemical SERS (ECSERS). By using the redox ability of gold and silver nanoparticles during plasmonic excitation, the bonding strength and the valence state of metal ions can be determined. For dinickel complexes, we assign the Raman band at 322 cm–1 to Ni2+–Ni3+ stretch in [Ni2(TPG)4]­BF4 (TPG = N,N′,N″-triphenylguanidinate, [Ni2]5+) and 327 cm–1 for Ni2+–Ni1+ stretch of [Ni2]3+ moieties in Ni5(camnpda)4. For trinickel EMACs, no band is assigned to Ni3 symmetric stretch νNi3 sym in the neutral form Ni3(dpa)4X2 (dpa = dipyridyl amido and X = NCS, Cl). In the reduced form, the ECSERS curves show the band at 242 cm–1, which also appeared at gold nanoparticle SERS measurement, assigned to νNi3 sym for [Ni3]5+ core. The trinickel complexes were reduced by gold nanosphere, and this νNi3 sym band is further enhanced with SERS measurements when gold nanorods were used and the trinickel EMACs served as bridging compounds on both ends. On increasing the applied voltage in ECSERS to +1.3 V, complexes were oxidized and one additional band at 351 cm–1 appeared. This new band is assigned to νNi3 sym of [Ni3]7+ in [Ni3(dpa)4X2]+. Great vibrational frequency indicates that one electron from the metal σ* orbital instead of ligand was removed, leading to a three metal center bond. Distinct from the vibrational band wavenumber obtained in dinickel complexes, we confirm that [Ni3]5+,7+ has delocalized electronic structures.