ligand (ML) interactions which are playing a key role in determining the diverse
properties and rich chemistry of organometallic compounds. We introduce in this
article the metal–ligand electronic parameter (MLEP), which is based on the local
vibrational ML stretching force constant, fully reflecting the intrinsic strength of this
bond. We discuss how local vibrational stretching force constants and other local
vibrational properties can be derived from the normal vibrational modes, which are
generally delocalized because of mode–mode coupling, via a conversion into local
vibrational modes, first introduced by Konkoli and Cremer. The MLEP is ideally
suited to set up a scale of bond strength orders, which identifies ML bonds with
promising catalytic or other activities. The MLEP fully replaces the Tolman electronic parameter (TEP), an indirect measure, which is based on the normal vibrational CO stretching frequencies of [R n M(CO) m L] complexes and which has been
used so far in hundreds of investigations. We show that the TEP is at best a
qualitative parameter that may fail. Of course, when it was introduced by Tolman
in the 1960s, one could not measure the low-frequency ML vibration directly, and
our local mode concept did not exist. However, with these two problems solved, a
new area of directly characterizing the ML bond has begun, which will open new
avenues for enriching organometallic chemistry and beyond.
Keywords Local vibrational mode · Tolman electronic parameter · Transition
metals · Vibrational spectroscopy
Abbreviations
ACS
Adiabatic connection scheme
BDE
Bond dissociation energy
BSO
Bond strength order
CEP
Computational electronic parameter
DFT
Density functional theory
LEP
Lever electronic parameter
LTEP Local Tolman electronic parameter
MC
Metal carbon
MD
Molecular dynamics
ML
Metal ligand
MLEP Metal–ligand electronic parameter
NHC
N-heterocyclic carbene
[NiFe] Nickel iron hydrogenase
PES
Potential energy surface
QALE Quantitative analysis of ligand effects
TEP
Tolman electronic parameter
ZPE
Zero-point energy
228
E. Kraka and M. Freindorf
properties and rich chemistry of organometallic compounds. We introduce in this
article the metal–ligand electronic parameter (MLEP), which is based on the local
vibrational ML stretching force constant, fully reflecting the intrinsic strength of this
bond. We discuss how local vibrational stretching force constants and other local
vibrational properties can be derived from the normal vibrational modes, which are
generally delocalized because of mode–mode coupling, via a conversion into local
vibrational modes, first introduced by Konkoli and Cremer. The MLEP is ideally
suited to set up a scale of bond strength orders, which identifies ML bonds with
promising catalytic or other activities. The MLEP fully replaces the Tolman electronic parameter (TEP), an indirect measure, which is based on the normal vibrational CO stretching frequencies of [R n M(CO) m L] complexes and which has been
used so far in hundreds of investigations. We show that the TEP is at best a
qualitative parameter that may fail. Of course, when it was introduced by Tolman
in the 1960s, one could not measure the low-frequency ML vibration directly, and
our local mode concept did not exist. However, with these two problems solved, a
new area of directly characterizing the ML bond has begun, which will open new
avenues for enriching organometallic chemistry and beyond.
Keywords Local vibrational mode · Tolman electronic parameter · Transition
metals · Vibrational spectroscopy
Abbreviations
ACS
Adiabatic connection scheme
BDE
Bond dissociation energy
BSO
Bond strength order
CEP
Computational electronic parameter
DFT
Density functional theory
LEP
Lever electronic parameter
LTEP Local Tolman electronic parameter
MC
Metal carbon
MD
Molecular dynamics
ML
Metal ligand
MLEP Metal–ligand electronic parameter
NHC
N-heterocyclic carbene
[NiFe] Nickel iron hydrogenase
PES
Potential energy surface
QALE Quantitative analysis of ligand effects
TEP
Tolman electronic parameter
ZPE
Zero-point energy
228
E. Kraka and M. Freindorf