Intermediates in chromium(III) photochemistry

A review of proposed intermediates and transition states for Cr(III) photosubstitution processes shows that several competing ideas have been presented. A survey of the available experimental evidence reveals, however, only one instance of a well characterized intermediate with measurable lifetime, that for cis-Cr(cyclam)(NH3)(2)(3+). Study of this and other hexam(m)ine Cr(III) complexes by laser flash photolysis with conductivity detection found other examples of transient conductivity increases associated with intermediate chemistry but confirms that the cis-Cr(cyclam)(NH3)(2)(3+) intermediate is so far unique in having a low-pH conductivity decay lifetime longer than that of its parent's doublet state. The behavior of the complexes has been modeled via numerical integration of the differential equations for the proposed kinetics after laser excitation, and the unique nature of cis-Cr(cyclam)(NH3)(2)(3+) is attributed to initial loss of a cyclam ring N atom with subsequent recombination and displacement of an ammonia ligand, consistent with the predictions of the angular overlap model of Cr(III) photosubstitution. The analysis indicates that about 30% of the reaction occurs directly from the quartet state, and that the ring-N mode constitutes about two-thirds of the photochemistry. For all the systems studied, the observed 'intermediates' were assigned as metastable six-coordinate primary photoproducts. (C) 2001 Elsevier Science B.V. All rights reserved.