inorganic chemistry

the 2eg(p) and not to the b1g(dx (^2) – y 2 ), which would give a d^10 con-
figuration. NiTPP is diamagnetic (closed shell) and now both
a1g(dz 2 ) and 1eg(dxzand dyz) metal orbitals also raise above the
porphyrin a2uand a1uorbitals, with the a1g(dz 2 ) becoming the
HOMO and the b1g(dx (^2) – y 2 ) the LUMO. Experimental oxidation
potentials suggested that the first electron is removed from the
metal, but calculations show that [NiIIITPP]þis less stable than
the [NiIITPP]þradical cation and that the electron must come
from the porphyrin a2uor a1uorbitals (15,22). In the reduction
of NiTPP, the first three electrons go to the porphyrin 2eg(p)
orbitals because the occupation of this orbital unstabilizes the
b1g(dx (^2) – y 2 ) orbital and inverts their energy order. The 1eg(dxz
and dyz) orbitals of low-spin CoTPP are situated above the a2u
or a1uorbitals of the porphyrin and are the HOMO, whereas
the a1g(dz 2 ) orbital is placed below and is singly occupied. The
b1g(dx (^2) – y 2 ) is higher in energy than the 2eg(p*) orbitals, which
are the LUMOs of CoTPP. Initial oxidation removes an electron
from the porphyrin ring leading to [CoIITPPþ] followed by elec-
tron redistribution yielding high-spin [CoIIITPP]þ where the
a1g(dz 2 ) becomes doubly occupied and 1eg(dxz and dyz) orbitals
are each singly occupied. Reduction adds an electron to the
a1g(dz 2 ) orbital, as expected. All the four occupied 3d-like orbitals
b2g(dxy), a1g(dz 2 ), and 1eg(dxzand dyz) of FeTPP lie above the por-
phyrin a2u and a1uorbitals, and the unoccupied b1g(dx (^2) – y 2 ) is
much higher in energy. The first oxidation takes place from the
central metal (dz 2 ), and the first and second reductions populate
the low-lying half-filled metal d orbitals.
Metalloporphyrins that undergo a metal atom oxidation show
a linear dependence of the corresponding oxidation potentials
with the third ionization potentials of their metal atoms( 15 ).
In general, the reduction potentials become more negative as
the electron affinity increases ( 23 ). Changes in ligand oxidation
or reduction potentials have been correlated with electronegativ-
ity and covalent radius changes of the central atom ( 24 ). A simi-
lar correlation also holds for electronic transition energies ( 24 ).
The stability of axial ligands depends on the electron configu-
ration of the metal. Diamagnetic ZnII(TPP) binds to a variety of
neutral or charged axial ligands, but the five-coordinate 1:1
complexes are very labile. Paramagnetic CuII(TPP), like other
copper porphyrin complexes, has very little tendency to add axial
ligands because of the population of the dz 2 orbital. NiII(TPP) has
a low-spin d^8 configuration, and axial ligands are usually
repelled by the filled dz 2 orbital. The low-spin d^7 configuration
of CoII(TPP) supports one or two axial ligands, with an odd elec-
tron occupying the dz 2 orbital. CoIII(TPP) is diamagnetic and
196 LUIS G. ARNAUT