Ligand field photochemistry of trans-tetraamminechromium(III) complexes with one strong-field and one weak-field axial ligand

Abstract

The ligand field photolysis of three trans-Cr(NH/sub 3/)/sub 4/(CN)X/sup z+/ ions, with X = H/sub 2/O, NCS/sup -/, and F/sup -/, has been studied in acidic aqueous solution (10/sup -3/ M HClO/sub 4/). These complexes are electronically unusual, in that the CH/sup -/ and X axial groups act in opposition on the splittings of the octahedral states and, moreover, are respectively ..pi..-acceptors and ..pi..-donors. In each case photoaquation of all three types of ligands is observed. For X = H/sub 2/O, trans ..-->.. cis 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 = H/sub 2/O, NCS/sup -/, and F/sup -/, respectively, upon irradiation of the long-wavelength maximum. This is consistent with the small energy separation between the lowest excited quartet states, /sup 4/E and /sup 4/B/sub 2/. The results are compared with the predictions of various photolysis models and are analyzed particularly in terms of excited-state sigma-and ..pi..-bonding changes. All three complexes phosphoresce from the lowest doublet state under photochemical conditions, with 20/sup 0/C lifetimes of 1.1, 30, and 0.2more » ..mu..s, respectively. Luminescence and photoreaction quenching by Cr(C/sub 2/O/sub 4/)/sub 3//sup 3 -/ have been investigated in the case of trans-Cr(NH/sub 3/)/sub 4/(CN)(NCS)/sup +/. 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.« less