Replacement of 2,2′-bipyridine by 1,4-diazabutadiene acceptor ligands:: Why the bathochromic shift for [(N∧N)IrCl(C5Me5)]+ complexes but the hypsochromic shift for (N∧N)Ir(C5Me5)?

Replacement of 2,2'-bipyridine (bpy) by substituted 1,4-diazabutadiene (R-DAB) alpha-diimine ligands Nboolean ANDN leads to a substantial hypsochromic shift of about 0.8 eV for the long-wavelength absorption band in compounds (Nboolean ANDN)Ir(C5Me5) but to a bathochromic absorption shift of about 0.4 eV for the complex ions [(Nboolean ANDN)IrCl(C5Me5)](+). DFT calculations on model complexes based on experimental (R-DAB compounds) and geometry-optimized structures (bpy systems) reveal that the low-energy transitions of the cationic chloro complexes are largely of ligand-to-ligand charge-transfer character L'LCT (L = alpha-diimine, L' = Cl) whereas the neutral compounds exhibit pi --> pi* transitions between the considerably mixed metal d(pi) and alpha-diimine pi(*) orbitals. The much more pronounced metal-ligand orbital interaction for the R-DAB complexes causes the qualitatively different shifts on replacing the stronger basic bpy by the better pi-acceptors R-DAB. Only the LUMO of the neutral compounds is destabilized on replacement of bpy by R-DAB whereas the LUMO of [(Nboolean ANDN)IrCl(C5R'(5))](+) and both HOMOs are stabilized through this change.