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

Compounds ((n)Bu(4)P)[PtBr(3)(C(2)H(4))] (1), trans-[PtBr(2)(NH(2)Ph)(C(2)H(4))] (2), cis-[PtBr(2)(NH(2)Ph)(C(2)H(4))] (3), ((n)PBu(4))(2)[PtBr(4)] (4), ((n)PBu(4))[PtBr(3)(NH(2)Ph)] (5), and cis-[PtBr(2)(NH(2)Ph)(2)] (6), as well as the trichlorido analogue of 1, ((n)Bu(4)P)[PtCl(3)(C(2)H(4))] (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)(CH(2)) 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)(CH(2)) A(1) 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.