UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation

Abstract

Transition metal complexes (NH₃)₅CoX²⁺ (X = CH₃, Cl) and L(H₂O)MX²⁺, where M = Rh or Co, X = CH₃, NO, or Cl, and L is a macrocyclic N₄ ligand are examined by both experiment and computation to better understand their electronic spectra and associated photochemistry. Specifically, irradiation into weak visible bands of nitrosyl and alkyl complexes (NH₃)₅CoCH₃²⁺ and L(H₂O)M^IIIX²⁺ (X = CH₃ or NO) leads to photohomolysis that generates the divalent metal complex and ˙CH₃ or ˙NO, respectively. On the other hand, when X = halide or NO₂, visible light photolysis leads to dissociation of X⁻ and/or cis/trans isomerization. Computations show that visible bands for alkyl and nitrosyl complexes involve transitions from M–X bonding orbitals and/or metal d orbitals to M–X antibonding orbitals. In contrast, complexes with X = Cl or NO₂ exhibit only d–d bands in the visible, so that homolytic cleavage of the M–X bond requires UV photolysis. UV-Vis spectra are not significantly dependent on the structure of the equatorial ligands, as shown by similar spectral features for (NH₃)₅CoCH₃²⁺ and L¹(H₂O)CoCH₃²⁺.