Structural and mechanistic studies of coordination compounds. 36. Electronic spectra and photochemistry of some trans-(tetraamine)ruthenium(III) complexes

The electronic absorption spectra of an extensive series of tetraamine complexes of the type trans-[RuLX₂]⁺ (X = Cl, Br, or I) have been analyzed. In general, two ligand-to-metal charge-transfer (CTTM) transitions of (p_π)_x → d_π* origin are observed. The wavelength of the lowest energy band increases gradually with increasing chelation and steric congestion of the amine ligand L around the ruthenium(III) ion. The splitting of the (p_π)_x → d_π* transition is largest for the two sterically congested teta and tetb complexes, where teta and tetb represent C-meso- and C-rac-5,5,7,12,12,14-hexa-methyl-1,4,8,11-tetraazacyclotetradecane, respectively, but decreases with the nature of X, Cl > Br > I. The spectra of trans-[Ru(cyclam)X₂]X (X = Cl or Br) have been resolved at low temperature (∼30 K), and (p_σ)_x → d_π* transitions have been assigned to some of these resolved bands. Photochemistry of trans-[RuLIX]⁺ (L = (en)₂, X = Cl, Br, or I; L = 2,3,2-tet or cyclam, X = I; en, 2,3,2-tet, and cyclam represent ethane-1,2-diamine, 3,7-diazanonane-1,9-diamine, and 1,4,8,11-tetraazacyclotetradecane, respectively) has been investigated. Irradiation at the lowest CTTM band (λ_irr > 500 nm) leads to stereoretentive aquation of X^- with quantum yields independent of X but decreasing with increasing chelation of L. Domination of the Ru^II-I• entity in the CTTM excited state has been discussed. Irradiation of the second CTTM band leads to photoaquation with concomitant stereochemical change. The results are interpreted in terms of a quartet ligand field state as the photoreactive precursor, and a dissociative model previously proposed to explain the photostereochemistry of d⁶ metal complexes has been useful also for this d⁵ system. © 1983 American Chemical Society.