UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation
In this paper, transition metal complexes (NH 3 ) 5 CoX2 + (X = CH 3 , Cl) and L(H 2 O)MX 2+ , where M = Rh or Co, X = CH 3 , NO, or Cl, and L is a macrocyclic N 4 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 (NH 3 ) 5 CoCH 3 2+ and L(H 2 O)M III X 2+ (X = CH 3 or NO) leads to photohomolysis that generates the divalent metal complex and ˙CH3 or ˙NO, respectively. On the other hand, when X = halide or NO 2 , 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 NO 2 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 (NH 3 ) 5 CoCH 3 2+ and L 1 (H 2 O)CoCH 3 2+ .