Electronic, Raman and resonance Raman spectroscopy of [NBu4][RuBr4(MeCN)(2)]

Raman spectra taken at resonance with the e(u)(Br p pi) --> b(2g)(Ru d pi) charge-transfer (CT) transition of the trans-[RuBr4(MeCN)(2)](-) ion, as it s [NBu4](+) salt at ca. 80 K, consist of long overtone progressions in v(1) (a(1g)), the symmetric RuBr stretch (at 188.5 cm(-1)), together with combin ation band progressions in which v(1) is the progression-forming mode and t he enabling modes include v(2)(b(1g)), v(as)(RuBr), and v(4)(b(2g)), delta( s)(BrRuBr). The excitation profile of the v(1) band approximately follows t he contour of the e(u) --> b(2g) CT transition referred to above, consisten t with the operation of the A-term scattering mechanism. The spectroscopic data allow the determination of the harmonic wavenumber (omega(1)) and anha rmonicity (x(11)) of the v(1)(a(1g)) mode to be 188.8 cm(-1) and -0.15 cm(- 1), respectively Some comparative data on the analogous chloride ion, trans -[RuCl4(MeCN)(2)](-), are also given.