Experimental (IR, Raman) and Computational Analysis of a Series of PtBr2 Derivatives: Vibrational Coupling in the Coordinated Ethylene and Pt-Br Modes

Compounds ((Bu4P)-Bu-n)[PtBr3(C2H4)] (1), trans-[PtBr2(NH2Ph)(CH4)] (2), cis-[PtBr2(NH2Ph)(C2H4)] (3), ((PBu4)-P-n)(2)-[PtBr4] (4), ((PBu4)-P-n)[PtBr3(NH2Ph)] (5), and cis-[PtBr2(NH2Ph)(2)] (6), as well as the trichlorido analogue of 1, ((Bu4P)-Bu-n)[PtCl3(C2H4)] (1(Cl)), have been investigated experimentally by both IR and Raman spectroscopy, and theoretically by geometry optimization and normal-mode analysis by the DFT approach. An analysis of the normal modes of coordinated ethylene in compounds 1, 1(Cl), 2, and 3 followed by a potential energy distribution investigation shows extensive vibrational coupling between the nu(C=C) and delta(s)(CH2) A(1) modes in two bands at around 1510-1520 and 1230-1250 cm(-1), the latter one having greater nu(C=C) contribution. The rho(w)(CH2) A, mode, the contribution of which the above two bands is negligible is responsible for a lower-frequency band at 995-1005 cm(-1). A complete vibrational analysis, backed up by the DFT calculations, has also been carried out on the Pt-Br stretching vibrations of the tetrabromido complex 4, the tribromido complexes 1 and 5, and the dibromido complexes 2, 3, and 6, and on the Pt-Cl vibrations of the analogous complex 1(Cl). The study illustrates the advantages of coupling high-level computations to the vibrational analyses to make unambiguous band assignments in IR and Raman spectroscopy.