Photosolvation of [Co(CN)₆]³⁻, trans-[Cr(NH₃)₂(NCS)₄]⁻, and trans-[Cr(en)₂NCSF]⁺ in aqueous mixed solvent systems

Abstract

Can. J. Chem. 54, 3791 (1976). Photosolvation of [ C~( c N )~] j -( l ) , frur1s-[Cr(NHj)~(NCS)~]-(2) and tratls- [Cr(en),NCSF]+(3) has been studied in various water;methanol, ethanol. acetonitrile. eth1,lene gllcol, glycerol, and acetone mixtures and in aqueous solutions of pol~\ii~ylp)rolidolle. The quantun~ yield for thiocyanate loss from 3 was found to be fairly independent of solveiit, while for thiocyanate loss from 2. large, and at higher concentrations of organic solvent, specific reductions of quantum yield were observed. The study of 1 using spectrophotometric analysis for [CO(CN)~H~O]'-yielded similar data to some in the literature. but the larger range of bysteins studied here did not support the reported reduction of quantum yield with bulk vixosity. Furthermore evidence from this and other work suggests that niany of the obserbed quantum jield red~~ctions are only apparent, based on a false assutnption that the photo~olvatioii is cjualitativeiy the same in all mixtures. The data are reinterpreted in terms of competitive sol\ation by both solvent components. It is argued that this study supports a dissociative model of reaction for 1, but an associative model for the chroniium con~plexes 2 and 3. aconduit B des donnees semblablec celles que 1' 011 peut retrouver dans la littirature; toutefois les etelldues plus grandes des systkmes CtudiCs dans le present travail n'apportent pas de support pour la reduction des rendetnents quantiques qui a Ct C rapportees en fonction de l'augmentation de la viscositC globale. De plus des donnees rapportee dans ce travail et dans d'autres travaux suggkrent que pl~lsieurs reductions dans les rendements quantiques ne sont qu'apparentes et basees uniquement sur I'hypothkse fausse que la photosolvatation est qualitativement la m&me dans tous les melanges. On rCinterprkte les donnees en terme de solvatation compktitive par les deux composants du solvant. On prCsente des arguments suggirant que notre etude supporte un modkle dissociatif pour la reaction de 1 mais un modkle a,sociatif pour les complexes 2 et 3 du chrome.[Traduit par le journal]

Photochemistry of the ruthenium(II) ammine complexes, Ru(NH3)5(py-X)2+. Variation of systemic parameters to modify photochemical reactivities

“…For example, quantum yields for the LF excitation of Co(CN)63~h ave been shown to be insensitive to solvent characteristics such as dielectric constant or the nature of the solvent's coordinating atom.llc Within experimental uncertainty, reactions in H20, acetonitrile, DMF, ethanol, and pyridine each gave the same -• However, a recent study of the photoreactions of iron5-Cr(NH3)2(SCN)4~h as shown that the photosubstitution reactions of this ion are sensitive to the solvent medium. 22 In the present case, these factors may each influence the quantum yields observed for various Ru(NH3)5py-X2+ complexes where the general order of photoreactivity ( ) in the various solvents is CH3CN > H20 > Me2SO.…”

Photophysics of hexakis(cyano)chromate(3-) and hexakis(cyano)cobaltate(3-) in polyalcohol-water solutions at room temperature

“…20 Wong and Kirk found a decrease in as Xh2o is reduced in several mixed solvents.8 They determined the extent of reaction spectrophotometrically, by assuming that the monoaquo complex was the principal photoproduct, whereas the actual extinctions are smaller when water is not the substituent.8,20 This incorrect assumption vitiates Wong and Kirk's quantitative results as well as those of Scandola et al 7 Unfortunately, Nakamura et al20 did not study Co(CN)63™ in ethylene glycol or glycerol and it is possible that, in glycerol-water and ethylene glycol-water, the reduction in is real. 8 Three reaction modes can be envisioned,21 dissociative (k¿), interchange (Ar), and associative (kj, and -r1 = k6 + ka + kd + ki. The reaction quantum yield is $r -4>i8c ak¿ + ka + k\ ke + kd + ka + ki -^ISC^r' (3) where a is the fraction of dissociated complexes that react.…”

“…solution of Co(CN)63" has been ascribed to viscosity enhanced cage recombination,7 a conclusion that has been disputed. 8 In spite of a substantial solvent effect on the photoreaction yields, the intersystem crossing efficiencies in Cr(NH3)2(NCS)4" and Cr(NCS)6~a re invariant to solvent. 9 We now report the effect of solvent composition in water-polyalcohol mixtures on the lifetimes and intersystem crossing efficiencies of Co(CN)63" and Cr(CN)63".…”

Pentacyano(pyridine)chromate(III): Synthesis, Characterization, and Photochemistry

“…The nucleophilicity of both solvents is comparable, but the higher viscosity makes ligand photosubstitution less favored in Me 2 SO (η = 1.98 cP at 25 °C) than in H 2 O (η = 0.89 cP). Such an effect, that indeed appears to be peculiar to negatively charged complexes, has been systematically investigated with the trans -Cr(NH 3 ) 2 (NCS) 4 - anion. , …”

“…Such an effect, that indeed appears to be peculiar to negatively charged complexes, has been systematically investigated with the trans-Cr(NH 3 ) 2 (NCS) 4 -anion. 67,68 Another aspect of interest is the marked increase of the Φ py /Φ CN ratio on going from H 2 O to Me 2 SO (Table 2), which results from the fact that Φ CN decreases far more than does Φ py . Two types of explanation may be advanced for the higher value of Φ CN in aqueous medium.…”