process. Accordingly, it includes any (de)stabilization effects of the products to be
formed. The magnitude of the BDE reflects the energy needed for bond breaking but
also contains energy contributions due to geometry relaxation and electron density
reorganization in the dissociation fragments. Therefore, the BDE is not a suitable
measure of the intrinsic strength of a chemical bond as it is strongly affected in
non-predictable ways by the changes of the dissociation fragments. Accordingly, its
use has led in many cases to a misjudgment of bond strength [69–74]. Also the ML
bond length is not a qualified bond strength descriptor. Numerous cases have been
reported illustrating that a shorter bond is not always a stronger bond [75–79]. Other
computational approaches utilized to determine the strength of the ML bond include
molecular orbital approaches [80, 81] or energy decomposition methods [82–
84]. However, also these approaches provide more qualitative rather than quantitative results [69, 85]. On the other hand, detailed information on the electronic
structure of a molecule and its chemical bonds is encoded in the molecular normal
vibrational modes [86]. Therefore, vibrational spectroscopy should provide a better
basis for a quantitative bond strength descriptor, which will be discussed in the next
section.
2 The Tolman Electronic Parameter (TEP)
Experimentalists have used vibrational properties to describe chemical bonding
including metal and transition metal catalysts for a long time [87–113] despite the
fact that the rationalization of this use was never derived on a physically or
chemically sound basis. Vibrational force constants seemed to be the best choice
for describing the strength of chemical bonds, because they are independent of the
atomic masses. However, it turned out that force constants derived from normal
vibrational modes are dependent on the coordinates used to describe the molecule
[114–118]. Therefore, the use of normal vibrational frequencies, which are directly
available from experiment, was suggested. Because of the relatively large mass
of M, ML vibrational frequencies appear in the far-infrared region, which was
experimentally not accessible in the early 1960s. Therefore, the idea of a spectator
ligand came up, which should have a high stretching frequency, i.e., easy to measure,
and which was well-separated from all other frequencies in the spectrum. The metal
spectator stretching frequency had to be sensitive to the strength of the ML bond and
any electronic changes at M resulting from modifications of L, and it had to be
common to most transition metal complexes. This idea was realized in several
investigations on transition metal complexes, whereas suitable spectator and sensor
ligands such as nitriles, isonitriles, and nitrosyl and carbonyl groups were tested,
assuming that the CN, NC, NO
+ , or CO stretching frequencies are sensitive with
regard to the electronic configuration of M in the transition metal complex and a
given ML bond, so that a spectroscopical (indirect) description of the latter seemed
to be possible. Strohmeier’s work on chromium, vanadium, manganese, tungsten,
and other complexes [119, 120] made the lead in the field of metal–ligand
Characterizing the Metal–Ligand Bond Strength via Vibrational Spectroscopy:. . .
231
formed. The magnitude of the BDE reflects the energy needed for bond breaking but
also contains energy contributions due to geometry relaxation and electron density
reorganization in the dissociation fragments. Therefore, the BDE is not a suitable
measure of the intrinsic strength of a chemical bond as it is strongly affected in
non-predictable ways by the changes of the dissociation fragments. Accordingly, its
use has led in many cases to a misjudgment of bond strength [69–74]. Also the ML
bond length is not a qualified bond strength descriptor. Numerous cases have been
reported illustrating that a shorter bond is not always a stronger bond [75–79]. Other
computational approaches utilized to determine the strength of the ML bond include
molecular orbital approaches [80, 81] or energy decomposition methods [82–
84]. However, also these approaches provide more qualitative rather than quantitative results [69, 85]. On the other hand, detailed information on the electronic
structure of a molecule and its chemical bonds is encoded in the molecular normal
vibrational modes [86]. Therefore, vibrational spectroscopy should provide a better
basis for a quantitative bond strength descriptor, which will be discussed in the next
section.
2 The Tolman Electronic Parameter (TEP)
Experimentalists have used vibrational properties to describe chemical bonding
including metal and transition metal catalysts for a long time [87–113] despite the
fact that the rationalization of this use was never derived on a physically or
chemically sound basis. Vibrational force constants seemed to be the best choice
for describing the strength of chemical bonds, because they are independent of the
atomic masses. However, it turned out that force constants derived from normal
vibrational modes are dependent on the coordinates used to describe the molecule
[114–118]. Therefore, the use of normal vibrational frequencies, which are directly
available from experiment, was suggested. Because of the relatively large mass
of M, ML vibrational frequencies appear in the far-infrared region, which was
experimentally not accessible in the early 1960s. Therefore, the idea of a spectator
ligand came up, which should have a high stretching frequency, i.e., easy to measure,
and which was well-separated from all other frequencies in the spectrum. The metal
spectator stretching frequency had to be sensitive to the strength of the ML bond and
any electronic changes at M resulting from modifications of L, and it had to be
common to most transition metal complexes. This idea was realized in several
investigations on transition metal complexes, whereas suitable spectator and sensor
ligands such as nitriles, isonitriles, and nitrosyl and carbonyl groups were tested,
assuming that the CN, NC, NO
+ , or CO stretching frequencies are sensitive with
regard to the electronic configuration of M in the transition metal complex and a
given ML bond, so that a spectroscopical (indirect) description of the latter seemed
to be possible. Strohmeier’s work on chromium, vanadium, manganese, tungsten,
and other complexes [119, 120] made the lead in the field of metal–ligand
Characterizing the Metal–Ligand Bond Strength via Vibrational Spectroscopy:. . .
231