Bonding between chromium atoms in metal-string complexes from Raman spectra and surface-enhanced Raman scattering: Vibrational frequency of the chromium quadruple bond

By measuring the vibrational wavenumbers of their stretching modes in Raman and surface-enhanced Raman scattering (SERS) spectra, we investigated the strength of the Cr-Cr bonds in metal-string complexes Cr₅(tpda) ₄X₂ and Cr₇(teptra)₄(NCS) ₂ (tpda = tripyridyldiamido; teptra = tetrapyridyltriamido; X = Cl^-, NCS^-). The bands in SERS and Raman differ insignificantly in spectral positions, indicating no major structural variation between the solid and solution forms. For SERS measurements, these complexes were bound to silver or gold nanoparticles in aqueous solution to eliminate the constraint of a crystal lattice and to maintain the complexes in thermal equilibrium; this method is convenient to identify the stable structure. We identified both penta-and heptachromium complexes in both symmetric (s) and unsymmetric (u) forms. For pentachromium complexes, our data agree with the results obtained from structural determination of the crystalline form, but for the heptachromium complex, this experimental evidence is the first for the existence of the u-form structure. From our analysis of the vibrational normal modes, we assign the band at 280 cm^-1 to the Cr-Cr symmetric stretching mode of the s-form pentachromium complex. According to comparisons of SERS spectra obtained at either high temperatures or under oxidizing conditions, we assign 570 cm^-1 to the stretching mode of the Cr-Cr quadruple bond in the u-form for the pentachromium complex and 554/571 cm ^-1 analogously for the heptachromium complex. The bands for metal-related modes in SERS spectra might be enhanced because of interaction with the metal nanoparticles. The metal-string complexes with a linear arrangement of metal ions have an increased absorption coefficient in the visible spectra and, consequently, an increased resonance Raman intensity for the metal-metal stretching modes, yielding information about the strength of chromium-chromium multiple bonding.