Redox properties of the phthalocyanine (pc) complexes of the second-ro
w transition metals zirconium [Zr-IV(pc)], molybdenum [(MoO)-O-V(pc)],
ruthenium [Ru-II(pc)], rhodium [Rh-III(pc)] and palladium [Pd-II(pc)]
are correlated with the energy levels of the phthalocyanine ring and
of the central metal. Oxidation and reduction occur exclusively at the
phthalocyanine ring in Zr-IV(pc), Ru-II(pc) and Pd-II(pc) complexes,
suggesting that the metal orbitals in these complexes are buried insid
e the filled phthalocyanine levels. (MoO)-O-V(pc) shows metal redox ac
tivity before any oxidation or reduction can be observed at the ring.
Hence the Mo-V metal orbitals must lie between the highest occupied mo
lecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
of the phthalocyanine ring. The site of redox activity in Rh-III(pc) d
epends on the nature of the axial ligands.