Metal-Metal Bonding And Structures Of Trinickel And Tricobalt Dipyridylamido Complexes From Surface-Enhanced Raman Spectra

We recorded surface-enhanced Raman scattering (SEM) spectra of metal-string complexes Co(3)(dpa)(4) Cl(2) [di(2-pyridyl)amido (dpa)], Ni(3)(dpa)(4) Cl(2) and the oxidized form of the Ni(3) complex to determine their vibrational wavenumbers and to investigate their structures. For SERS measurements these complexes were adsorbed on silver nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and the complexes remain in thermal equilibrium. From our analysis of the vibrational normal modes we assigned the SERS lines at 242 and 276 cm(-1) to Ni(3) and Co(3) symmetric-stretching modes of the symmetric form. For Co(3) (dpa)(4)Cl(2) a Raman line at 383 cm(-1) was assigned to the Co-Co stretching mode of the unsymmetric form. The wavenumber of the Ni(3) symmetric-stretching mode of the oxidized form [Ni(3)(dpa)(4)](3+) is 274 cm(-1), greater than that for neutral Ni(3)(dpa)(4)Cl(2), in agreement with a prediction of delocalized molecular-orbital theory that an electron is removed from an antibonding orbital after oxidation. The experimental data show that the SERS technique serves as an excellent tool to observe the variation of metal-metal bonding during an oxidation or reduction reaction. Copyright (C) 2010 John Wiley & Sons, Ltd.