(PYRIDINE)PENTAAMMINECHROMIUM(III) - SYNTHESIS, CHARACTERIZATION, ANDPHOTOCHEMISTRY

The Cr(NH3)(5)(py)(3+) ion has been obtained by metathesis of Cr(NH3)( 5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and cha racterized by absorption (lambda(max) at 467, 352, and 260 nm) and emi ssion spectra (lambda(max) at 668 nm, tau = 2.0 mu s at 20 degrees C i n water) and by the py aquation rate (k = 5 x 10(-4) s(-1) at 80 degre es C). Ligand-field (LF) band irradiation in acid aqueous solution (10 (-2) M HClO4) induces photoaquation of py (Phi = 0.26) and NH3 (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH3)(4)(py)(H2O)3+, with the predominance of the cis iso mer. This is the first Cr(NH3)(5)X(z+) species where Phi(x) > Phi(NH3) : the result is compared with the predictions of various photolysis mo dels and is taken as chemical evidence for pi-acceptance by the py lig and. The photostereochemistry is also discussed. The phosphorescence i s totally quenched by Cr(C2O4)(3)(3-)(k(q) = 2.7 x 10(9) M(-1) s(-1)), while the photoreactions are only in part. On 470-nm excitation, the Phi(py)/Phi(NH3) ratio is similar to 1 and similar to 2 for the unquen chable and the quenchable contributions, respectively. Such a differen ce, suggesting at least two reactive precursors, can be interpreted in terms of the photochemistry proceeding from either the lowest doublet and quartet excited states or, alternatively, from the (4)E and B-4(2 ) states. Irradiation of the very distinct absorption of coordinated p yridine results in both doublet-state emission and loss of py and NH3. Comparison of this photobehavior with the LF results gives an efficie ncy of 0.7 for conversion of the py-localized pi pi states into the C r-localized LF states, confirmed by the wavelength dependence of the r elative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pi pi states.