ciation of the ion pair will result in an apparent decrease of AV*. Such an effect is significant in those solvents with larger dielectric constants, e.g., PC. No information is available, however, concerning the pressure effect on the ion pair formation in organic solvents.DMF with the largest DN gives a negative AV*. The molecule is not as large as PC and can go deep into the coordination sphere to overcompensate the increase in volume at the transition state caused by the break of one end of the ligands. The complex undergoes solvolysis in this solvent. However, since the solvolysis proceeds more slowly than the decrease in CD strength by a factor of more than 10, the encountered experimental error will remain less than ca. 10%.If one of the acac" ligands leaves the coordination sphere to give bis(acetylacetonato)bis(dimethylformamide)germanium-(IV) ion on solvolysis, the product can be of either cis or trans isomerism. If the trans isomer were formed, it cannot be optically active and the experimental error comes only from the apparent molar extinction coefficient e, whenever the ratio Atm/e2v is used in place of the CD strength itself for the first-order kinetic plot. If ds-[Ge(acac)2(DMF)2] + were' formed with retention of the configuration, the product could contribute to the CD strength at 305 nm. However, bis(/3diketonato) complexes of metal ions have UV absorption maxima due to the exciton band at a lower wavenumber region than the tris-type complexes do.17 The CD strength at the negative exciton peak of this bis(S-diketonato) complex should shift to the lower wavenumber region for the present isomer, which has a negative CD peak at a lower wavenumber side. The ( decreases more steeply than the molar extinction coefficient does at the higher wavenumber side of the peaks. Therefore, the contribution of ds-[Ge(acac)2(DMF)2]+ to the <305/<287 ratio cannot be large. The In (Ae305/í287) vs. time plots, in fact, remained linear until 70% of the initial [Ge-(acac) 3] + racemized. This fact supports indirectly the legitimacy of our kinetic treatment.Conformity of the racemization mechanism in donating and less donating solvents was also discussed on the basis of the rather continuous change of AH* and AS* values.8 The present results support that the racemization proceeds via an intermediate with one unidentate ligand that was formed by a solvent-assisted bond break. However, a less donating solvent assists the bond break to only a very small extent.Acknowledgment. The authors thank the Ministry of Education of the Japanese Government for Grant-in-Aid No. 434029.Registry No. (-)589-[Ge(acac)3]Ci04, 76370-51-9.
Resonance Raman (RR) spectra are reported for (NH3)4Run complexes of riboflavin (RBF) and 10-methylisoalloxazine (10-MeIAlo), excited at 647.1 nm, in resonance with a strong absorption band that is assigned to a Ru11 -flavin charge transfer. RR bands II and III, which are associated with the pyrazine ring, shift down on complexation of RBF to (NH3)4Run, similar to the shifts in flavin derivatives with electron-donating C8 substituents, which stabilize a "quinoid" resonance structure, but opposite to the shift observed on semiquinone formation. The downshift is interpreted as a consequence of Ru11 -»• flavin back-bonding, which stabilizes a resonance form in which the N5-C4a bond is lengthened. The Ru11 complexes show especially strong enhancements of bands in the 1150-1350-cm"1 region, associated with pyrazine and uracil modes, which are plausibly involved in the molecular distortion of the charge-transfer excited state. Rich spectra are also observed below 1000 cm"1, involving in-plane deformation modes, several of which involve large uracil contributions, as evidenced by shifts on H/D exchange, ionization, and methylation at N3. A strong band of (NH3)4Run(RBF) at 275 cm"1 is tentatively identified as the Ru-N5 stretching mode, although in (NH)4Run(10-MeIAlo) this band is replaced by a 285-310-cm"1 doublet. There are a number of RR spectral differences between the RBF and 10-MeIAlo complexes that are not understood.
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