Ultraviolet irradiation of the complexes trans-[Co(N2O2)(N-ax)(2)](+)
and cis-beta-[Co(N2O2)(acac)] in methanol (where N2O2 = open-chain qua
dridentate Schiff base dianionic ligands of acacen, salen or benacen-t
ype; N-ax = N-donor axial ligands NH3 or pyridine; acac = 2,4-pentadio
nate anion) gives rise to photosubstitution of one axial ligand (in tr
ans-complexes) forming trans-[Co(N2O2)(N-ax)(CH3OH)](+), and photoredu
ction of Co(III) to Co(II) (in cis-beta-complexes), respectively. The
complexes trans-[Co(N2O2)(py)(CH3OH)](+) are are photochemically stabl
e at 313 nm irradiation, they undergo, however, the photoreduction of
Co(Ill) to Co(II) coupled to the oxidation of methanol to formaldehyde
at 254 nm irradiation. The efficiency of both the heterolytic and hom
olytic breaking of a bond between the central atom Co(III) and a monod
entate or the bidentate ligand is influenced by the peripheral groups
of the quadridentate N2O2-ligands. The electronic properties of these
groups (expressed by means of their Hammett parameters) do not correla
te with the quantum yields of the photochemical reactions.