Topics in Current Chemistry

Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes 141
3.1
Electronic Spectroscopy of Transition Metal Carbonyls
3.1.1
Ni(CO)4, Cr(CO)6, HM(CO)5 (M=Mn, Re) and Mn2(CO)10
Transition metal carbonyls constitute a very important class of complexes in
laser chemistry as reactive precursors of catalytic processes, substitution reactions,
chemical vapour deposition of thin films or layers on surfaces. They
are among the most reactive transition metal complexes and their electronic
structure has long been a matter of considerable interest stimulating theoretical
research. Even though their experimental spectra have been known since
the early 1970s, relatively little attention has been given to the electronic
spectroscopy of the molecules reported in Table 1. A fundamental aspect of
the theoretical study is related to the electronic correlation effects which are
very important already at the electronic ground state level for a good description
of the metal-CO bonding in these molecules. The results reported in Table
1 have been obtained by the means of correlated methods able to describe
correctly the dp-pp back-bonding interaction in this class of molecules. The
experimental spectrum in gas phase of Ni(CO) 4 (a d 10 system) exhibits three
bands attributed to MLCT 1 A 1! 1 T 2 transitions [109]. The TD-DFT [90],
CASPT2 [110], SAC-CI [111] and EOM-CCSD [12] approaches give rise to
three allowed transitions in the energy range of 4.0–6.5 eV in agreement with
experiment leading to a reasonable assignment. The deviations on the transition
energies never exceed 7%. The assignment of the upper bands in terms
of one electron excitations in the principal configurations is quite sensitive to
the level of calculation and is still a purpose of controversy.
The spectrum of Cr(CO) 6 (a d 6 system) is dominated by two very intense
absorption bands assigned to MLCT 1 A 1g! 1 T 1u transitions and by low-lying
shoulders originally attributed to weak MC transitions [112]. This assignment
was confirmed by semi-empirical INDO/S CI [109] and SCF calculations
[113, 114]. The more recent studies reported in Table 1 and based on
DSCF [115], TD-DFT [88] and CASSCF/CASPT2 [110] methods reinterpreted
the electronic spectrum of Cr(CO) 6. According to this new analysis the
lowest part of the spectrum does not correspond to MC transitions but to
orbitally and spin-forbidden MLCT states of low intensity not reported in
Table 1 where only the intense bands observed at 4.43 eV and 5.41 eV are
presented. If the TD-DFT and CASPT2 results are in excellent agreement
with the experimental values the DSCF method overestimates by more than
20% the transition energies. These poor results are attributed to the inability
of this approach to account for the configuration mixing which characterizes
these two states.
The main features of the experimental spectra of HMn(CO) 5 and HRe
(CO) 5 are alike with three main bands of increasing intensity when going
from the low to the high energies: 4.31 eV [HMn(CO) 5] vs 4.63 eV [HRe