on A and B, e is the dielectric constant of the fluid, a is the distance of closest approach of A and B, and the other constants have their usual meanings.hexachloro complex. (17) See, for example, C. H. Langford and J. P. H. Tong, Acc. Chem. Res., 10, 258 (1977). (18) This ratio was calculated by use of the data given by Palmer and Harris' for the cis tetrachloro complex, by W. Robb and M. M. de V. Steyn, Contribution from the Irradiation of 50% v/v water-ethanol solutions of Cr(acac)j at 16 wavelengths between 230 and 730 nm leads to replacement of one acetylacetonate ligand by two water molecules. The photoproduct is cationic, neutral, or anionic depending on solution p H and was characterized by absorption spectra, chemical analysis, ion exchange, and conductivity. The photoaquation quantum yields are pH independent between p H 0 and p H 13, are wavelength independent between 350 and 730 nm, and are atypically low for a Cr(II1) complex (4 = 0.01). A fourfold increase in quantum yield is observed upon 250-nm excitation, with no evidence for redox processes. The system is photochromic, as photoinduced acetylacetonate labilization is opposed by thermal anation of acetylacetone, and photostationary states can occur. The results are interpreted in terms of two distinct reactive excited states: a relatively unreactive ligand field (LF) state and a higher energy ligand-localized (LL) state.
IntroductionThe remarkable stability,] t h e well-characterized structural properties,* absorption3 and emission4 spectra, and the variety of electronic transitions make tris(acety1acetonato)chromium(II1) especially interesting from a photochemical point of view.5 Previous investigations of Cr(acac), and of other, variously substituted, P-diketonatochromium(II1) chelates have focused on p h o t o i s~m e r i z a t i o n~~~ and on partial photoresolution.8-12 These studies, however, have been performed in organic, noncoordinating media such as h e~a n e ,~.~ benzene,'O and c h l o r~b e n z e n e ,~~~~~~ where only intramolecular processes (either twisting or bond rupture and recoordination) are possible.An u n d e r s t a n d i n g of t h e photochemical behavior of Cr-(acac), in a coordinating solvent is i m p o r t a n t for a n u m b e r of reasons. (i) The solvent is t h o u g h t to play a crucial role in t h e photochemical reactivity of many Cr(II1) c~m p l e x e s , '~ so studies in noncoordinating solvents exclude a n i m p o r t a n t reactant. (ii) The exceptional inertness' and extensive Telectron system'-3 of the acetylacetonato ligand introduce factors which are rather u n u s u a l in photochemical studies of chromium(III).'3 (iii) The diversity of t h e absorption spectrum gives access to several types of electronic excitation, corresponding t o ligand field (LF), charge-transfer (CT), a n d ligand-localized (LL) transitions.s Selective irradiation may
