Photochemistry and Photophysics of Coordination Compounds

6 V. Balzani et al.
Fig. 3 Molecular orbital diagram for an octahedral complex of a transition metal. The
arrows indicate the four types of transitions based on localized MO configurations. For
more details, see text
contributions as: (1) strongly bonding, predominantly ligand centered σL orbitals;
(2) bonding, predominantly ligand-centered πL orbitals; (3) essentially
nonbonding, metal-centered πM orbitals of t2g symmetry; (4) antibonding,
predominantly metal-centered σ ∗ M orbitals of eg symmetry; (5) antibonding,
predominantly ligand-centered π∗ L orbitals; and (6) strongly antibonding,
predominantly metal-centered σ ∗ M
orbitals. In the ground electronic configu-
ration of an octahedral complex of a d n metalion,orbitalsoftypes1and2are
completely filled, while n electrons reside in the orbitals of types 3 and 4.
As for organic molecules, excited configurations can be obtained from the
ground configuration by promoting one electron from occupied to vacant
MOs. At relatively low energies, one expects to find electronic transitions of
the following types (Fig. 3): metal-centered (MC) transitions from orbitals of
type 3 to orbitals of type 4; ligand-centered (LC) transitions of type 2 →5;
ligand-to-metal charge-transfer (LMCT) transitions, e.g., of type 2 →4; and
metal-to-ligand charge-transfer (MLCT) transitions, e.g., of type 3 →5. The
relative energy ordering of the resulting excited electronic configurations depends
on the nature of metal and ligands in more or less predictable ways.
Low-energy metal-centered transitions are expected for metals of the first
transition row, low-energy ligand-to-metal charge-transfer transitions are expected
when at least one of the ligands is easy to oxidize and the metal is
easy to reduce, low-energy metal-to-ligand charge-transfer transitions are expected
when the metal is easy to oxidize and a ligand is easy to reduce,