Metal−Metal Bonding and Structures of Metal−String Complexes: Tripyridyldiamido Pentanickel and Pentacobalt from IR, Raman, and Surface-Enhanced Raman Scattering Spectra

Abstract

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 (H2tpda) 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 Ni5(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.