Intermediates in chromium(III) photochemistry

A review of proposed intermediates and transition states for Cr(III) photos ubstitution processes shows that several competing ideas have been presente d. A survey of the available experimental evidence reveals, however, only o ne 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 interme diate chemistry but confirms that the cis-Cr(cyclam)(NH3)(2)(3+) intermedia te is so far unique in having a low-pH conductivity decay lifetime longer t han that of its parent's doublet state. The behavior of the complexes has b een 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 w ith subsequent recombination and displacement of an ammonia ligand, consist ent with the predictions of the angular overlap model of Cr(III) photosubst itution. The analysis indicates that about 30% of the reaction occurs direc tly from the quartet state, and that the ring-N mode constitutes about two- thirds of the photochemistry. For all the systems studied, the observed 'in termediates' were assigned as metastable six-coordinate primary photoproduc ts. (C) 2001 Elsevier Science B.V. All rights reserved.