Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 125
teract strongly and cannot be treated separately from each other. This effects
so-called static or non-dynamical correlation which is especially important
in transition metals already at the atomic level where several electronic configurations
are nearly degenerate. Moreover in transition metal complexes
characterized by a high density of electronic states in a limited domain of
energy the computation of reliable absorption spectra has to include this effect
at the zero-order level, namely when computing the reference wave
function. The static correlation energy is also very important at the molecular
dissociation limit where several states may be necessary to describe the
electronic structure of the products of dissociation. This is illustrated in
Scheme 1 by the formation of di-radical fragments arising from metal-hy-
Scheme 1
drogen, metal-alkyl or metal-metal bonds homolysis (in L nMR complexes)
described by two degenerate states at dissociation, namely the triplet T 1 corresponding
to the (s g ) 1 (s u) 1 electronic configuration and the singlet S 0 corresponding
to the two-determinantal (s g) 2 (s u) 0 Ÿ(s g) 0 (s u) 2 configuration
(the s g and s u orbitals being the metal-hydrogen, metal-metal or metal-alkyl
bonding and anti-bonding orbitals, respectively).
Finally, when handling photochemical reactions near degeneracy effects
may also occur along the reactive pathways at critical geometries (avoided
crossings, conical intersections) where the nature of the electronic wave
function changes abruptly as a function of the nuclear displacement by mixing
with upper electronic states. The multiconfiguration approach is necessary
to describe correctly these areas of the potential energy surfaces (PES).