Metal-metal bonding and structures of metal-string complexes: Tripyridyldiamido pentanickel and pentacobalt from IR, Raman, and surface-enhanced Raman scattering spectra

We recorded infrared, Raman, and surface-enhanced Raman scattering (SERS) spectra of metal-string complexes Ni₅(tpda)₄X₂ and Co₅(tpda)₄X₂ (tpda = tripyridyldiamido, X = Cl^-, NCS^-) and free ligand tripyridyldiamine (H ₂tpda) to determine their vibrational wavenumbers and the strength of the metal-metal bonds. For SERS measurements, these complexes were adsorbed on silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium. The spectra of SERS and Raman modes show insignificant deviation in spectral features and band positions. We observe a single breathing band of pyridyl in Co₅(tpda)₄X₂, indicating the existence of only the symmetric form, whereas split pyridyl lines are observed for Ni ₅(tpda)₄X₂ and assigned to arise because of a varied environment of coordination: square planar for the inner nickels and square pyramidal for the outer nickels in the complexes. From our analysis of the vibrational normal modes, we assign lines at 257/266 and 302/313 cm ^-1 to Ni₅, at 287/284 and 355/360 cm^-1 to Co₅ symmetric stretching modes, and at 255/267 and 297/305 cm ^-1 and 319/323 and 391/392 cm^-1 to Ni₅ and Co₅ asymmetric stretching, respectively, for complex with axial ligand Cl/NCS. The bonding in Ni-Ni is weaker than for Co-Co, consistent with the prediction from molecular-orbital theory.