Delocalization tunable by ligand substitution in [L₂Al] ^n- complexes highlights a mechanism for strong electronic...

Ligand-based mixed valent (MV) complexes of Al(iii) incorporating electron donating (ED) and electron withdrawing (EW) substituents on bis(imino)pyridine ligands (I₂P) have been prepared. The MV states containing EW groups are both assigned as Class II/III, and those with ED functional groups are Class III and Class II/III in the (I₂P^-)(I₂P^2-)Al and [(I₂P^2-)(I₂P^3-)Al]^2- charge states, respectively. No abrupt changes in delocalization are observed with ED and EW groups and from this we infer that ligand and metal valence p-orbitals are well-matched in energy and the absence of LMCT and MLCT bands supports the delocalized electronic structures. The MV ligand charge states (I₂P^-)(I₂P^2-)Al and [(I₂P^2-)(I₂P^3-)Al]^2- show intervalence charge transfer (IVCT) transitions in the regions 6850-7740 and 7410-9780 cm^-1, respectively. Alkali metal cations in solution had no effect on the IVCT bands of [(I₂P^2-)(I₂P^3-)Al]^2- complexes containing -PhNMe₂ or -PhF₅ substituents. Minor localization of charge in [(I₂P^2-)(I₂P^3-)Al]^2- was observed when -PhOMe substituents are included.