Structural and mechanistic studies of co-ordination compounds. Part 32. Different photochemical pathways of some trans-dihalogenobis(ethylenediamine)ruthenium(III) cations: Ligand-field versus ligand-to-metal charge-transfer excited states

It has been demonstrated that the ligand-field (l.f.) excitation of trans-[Ru(en)₂I₂]⁺ (en = ethylenediamine) leads to iodide aquation with extensive stereochemical change (>85%), whereas ligand-to-metal charge-transfer (l.m.c.t.) excitation leads to iodide aquation with complete retention of configuration. For trans-[Ru(en)₂X₂]⁺ (X = Cl or Br), where l.f. and l.m.c.t. bands accidentally overlap with each other, the observed photochemical behaviour of halide aquation with extensive stereochemical change is taken to occur from the I.f. excited states, whether these states are populated by direct absorption into the l.f. bands or by internal conversion from the l.m.c.t. states. The gradual decrease in the isomerization quantum yields from X = Cl through Br to I (6.2 × 10^-3, 3.2 × 10^-3, and 2.3 × 10^-3 respectively) is discussed.