The Cr(CN)3(NH3)2' ion was synthesized by irradiation of Cr(CN)63' in methanol and reaction with NH3, isolated as the potassium salt, and characterized by analysis, chromatography, aquation, absorption spectroscopy (Xmax at 397 and 320 nm, as predicted theoretically), and phosphorescence (Xmax at 777 nm, = 32 ms at 20 °C in Me2SO). Ligand field excitation causes release of both NH3 and CN". The respective quantum yields are 0.08 and 0.06 in H20, 0.05 and 0.007 in Me2SO, and 0.11 and 0.003 in dimcthylformamide. The prevalence of NH3 loss is in agreement with the angular overlap model but not with previous photolysis theories. The medium markedly affects the absorption spectrum, the emission behavior, the photoreaction efficiencies, and especially the ,/ratio; the changes are interpreted in terms of solvent orientation and hydrogen bonding. The phosphorescence is observed in aprotic solvents but not in H20 and is quenched by Co(sep)3+. In Me2SO, at least 90% of each photoreaction mode remains unquenchcd upon complete doublet-state quenching. Both photoprocesses are concluded to proceed from the lowest quartet excited state prior to intersystem crossing. Results for Cr(CN)63" photolyzed in Me2SO in the absence and in the presence of Co(scp)3* are also reported, and the two systems are compared.

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The ligand field photolysis of three Zra/i$-Cr(NH3)4(CN)Xz+ ions, with X = H20, NCS", and F", has been studied in acidic aqueous solution (10"3 M HC104). These complexes are electronically unusual, in that the CN" and X axial groups act in opposition on the splittings of the octahedral states and, moreover, are respectively -acceptors and -donors. In each case photoaquation of all three types of ligands is observed. For X = H20, transcis photoisomerization is taken as a measure of water photoexchange. The total quantum yields are in the 0.2-0.4 range, virtually wavelength independent. Equatorial and axial reactivities are of comparable magnitude, their ratios being 0.32, 2.5, and 3.0 for X = HzO, NCS", and F~, respectively, upon irradiation of the long-wavelength maximum. This is consistent with the small energy separation between the lowest excited quartet states, 4E and 4B2. The results are compared with the predictions of various photolysis models and are analyzed particularly in terms of excited-stateand -bonding changes. All three complexes phosphoresce from the lowest doublet state under photochemical conditions, with 20 °C lifetimes of 1.1, 30, and 0.2 gs, respectively. Luminescence and photoreaction quenching by Cr(C204)33" have been investigated in the case of Zrani-Cr(NH3)4(CN)(NCS)+. The quantum yields for the three photoaquation modes decrease in parallel with emission and, upon complete doublet quenching, one-fourth of the photoreactivity remains unquenched. The proportion in ligand labilization is the same for both the unquenchable and the quenchable photochemistry. This suggests a common precursor (4E/4B2) for all photoreactions, hence, doublet deactivation mainly via back intersystem crossing to the lowest excited quartet state(s).

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The Cr(NH(3))(5)(py)(3+) ion has been obtained by metathesis of Cr(NH(3))(5)(Me(2)SO)(3+) in pyridine, isolated as the perchlorate salt, and characterized by absorption (lambda(max) at 467, 352, and 260 nm) and emission spectra (lambda(max) at 668 nm, tau = 2.0 &mgr;s at 20 degrees C in water) and by the py aquation rate (k = 5 x 10(-)(4) s(-)(1) at 80 degrees C). Ligand-field (LF) band irradiation in acid aqueous solution (10(-)(2) M HClO(4)) induces photoaquation of py (Phi = 0.26) and NH(3) (Phi = 0.16). HPLC indicates that the latter reaction gives rise to both cis- and trans-Cr(NH(3))(4)(py)(H(2)O)(3+), with the predominance of the cis isomer. This is the first Cr(NH(3))(5)X(z+)() species where Phi(x) > Phi(NH)3: the result is compared with the predictions of various photolysis models and is taken as chemical evidence for pi-acceptance by the py ligand. The photostereochemistry is also discussed. The phosphorescence is totally quenched by Cr(C(2)O(4))(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(NH)()3 ratio is approximately 1 and approximately 2 for the unquenchable and the quenchable contributions, respectively. Such a difference, 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 (4)B(2) states. Irradiation of the very distinct absorption of coordinated pyridine results in both doublet-state emission and loss of py and NH(3). Comparison of this photobehavior with the LF results gives an efficiency of 0.7 for conversion of the py-localized pipi states into the Cr-localized LF states, confirmed by the wavelength dependence of the relative emission yields. Some py release (Phi = 0.03) is concluded to originate in the pipi states.