Prediction\nand Rationalization of Different Photochemical\nBehaviors of <i>mer</i>- and <i>fac</i>-Isomers\nof [Ru(pyridyltriazole)<sub>3</sub>]<sup>2+</sup>

Facial and meridional\nisomerism of metal complexes is known to\nresult in fundamental differences in photophysical properties. One\nmay also envisage differences in their photochemical reactivity and\ntherefore predict different outcomes of their light-triggered transformations.\nThe <i>fac</i>- and <i>mer</i>-isomers of the\ncomplex [Ru(pytz)<sub>3</sub>]<sup>2+</sup> (<i>fac</i>-<b>1</b> & <i>mer</i>-<b>1</b>, pytz = 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole)\nwere separated and isolated. <i>mer</i>-<b>1</b> undergoes\na predicted pytz photodechelation process in acetonitrile to yield <i>trans</i>-[Ru(κ<sup>2</sup>-pytz)<sub>2</sub>(κ<sup>1</sup>-pytz)(NCMe)]<sup>2+</sup> (<b>2</b>) whereas unfavorable\ninterligand steric interactions are predicted to, and indeed do prevent\ncomparable photoreactivity for <i>fac</i>-<b>1</b>. Reversible photoisomerization of <i>fac</i>-<b>1</b> and <i>mer</i>-<b>1</b> is also observed, however.\nThe differences in photochemical reactivity of the two isomers can\nbe rationalized based on structural programming of the preferential\naccessibility of particular <sup>3</sup>MC excited states due to differences\nin their interligand steric interactions. Here we present an initial\npredictive thought experiment, subsequent experimental verification,\nand computational rationalization of the differences in photochemical\nreactivity of these two isomeric complexes.