investigations with contributions from Fischer [121], Horrocks [122, 123], and
Cotton [124–127]. Strohmeier ordered transition metals according to their π-donor
ability: Cr > W > Mo > Mn > Fe. The overall proof of concept which emerged
from these more or less scattered studies was that L acts as a σ-donor and/or a πacceptor according to which the electron density at M is changed. This change can be
monitored by the metal spectator ligand stretching frequency, which provides
indirect evidence on the nature of L and the ML bond. However, it was the
pioneering work of Tolman who combined and systemized these findings, culminating in the Tolman electronic parameter (TEP) as ML bond strength measure
[128–130]. Originally, Tolman focused on tertiary phosphines (L ¼ PR 3 ) interacting
with a nickel–tricarbonyl rest, where the three CO ligands take the role of a spectator
group measuring the interaction of L with Ni. Tolman defined the TEP as the A 1 -
symmetrical CO stretching frequency of the nickel–tricarbonyl–phosphine complex
according to the following relationship:
TEP ¼ ω Ni; CO, A 1
ð
Þ¼2, 056 þ p L
ð1Þ
with P(t-Bu) 3 as a suitable reference with p L ¼ 0 and ω(CO, A 1 ) ¼ 2,056 cm
À1 .
Tolman considered P(t-Bu) 3 as the most basic phosphine because of its strong σdonor and absent π-acceptor ability. This leads to an increase of the electron density
at Ni, which is transferred via the d-orbitals into the antibonding π
⋆ (CO) orbitals as
sketched in Fig. 1a, b.
The CO bond length is increased, and the A 1 -symmetrical CO stretching mode is
redshifted to the value of 2,056 cm
À1 compared to the CO stretching frequency in
carbon monoxide of 2,071 cm
À1 [132]. Any other, less basic phosphine leads to a
lower electron density at Ni and thereby to a higher CO stretching frequency
ω(L) and the ligand-specific increment p L ¼ ω(L) À 2,056. In this way, the basicity
of phosphine ligands can be estimated by simply measuring the vibrational spectra
(infrared or Raman) of the corresponding nickel–tricarbonyl–phosphine complex.
Tolman used phosphine ligands because they cover a wide range of distinct electronic and steric properties, seldom participate directly in the reactions of a transition
metal complex, and can they be used to modulate the electronic properties of the
adjacent metal center. In addition, he relied on the following important assumptions:
1. ω(CO, A 1 ) is well separated from other frequencies, so it can be easily measured
and identified in the IR spectrum.
2. The ω(CO, A 1 ) stretching mode does not couple with other vibrational modes,
i.e., can be considered as local mode.
3. There is a general correlation between ω(CO, A 1 ) and ω(ML).
In literally hundreds of studies on transition metal–carbonyl complexes, the
original Tolman concept has been applied, and in some studies, its general applicability has been tested. For example, Otto and Roodt [133] fitted the CO frequencies
measured by Strohmeier for trans-[Rh(CO)ClL 2 ] (Rh-Vaska) complexes with the
CO frequencies of Tolman’s nickel–tricarbonyl–phosphines and obtained a quadratic relationship, which suggests that besides the σ-donor activity of the trialkyl
232
E. Kraka and M. Freindorf
Cotton [124–127]. Strohmeier ordered transition metals according to their π-donor
ability: Cr > W > Mo > Mn > Fe. The overall proof of concept which emerged
from these more or less scattered studies was that L acts as a σ-donor and/or a πacceptor according to which the electron density at M is changed. This change can be
monitored by the metal spectator ligand stretching frequency, which provides
indirect evidence on the nature of L and the ML bond. However, it was the
pioneering work of Tolman who combined and systemized these findings, culminating in the Tolman electronic parameter (TEP) as ML bond strength measure
[128–130]. Originally, Tolman focused on tertiary phosphines (L ¼ PR 3 ) interacting
with a nickel–tricarbonyl rest, where the three CO ligands take the role of a spectator
group measuring the interaction of L with Ni. Tolman defined the TEP as the A 1 -
symmetrical CO stretching frequency of the nickel–tricarbonyl–phosphine complex
according to the following relationship:
TEP ¼ ω Ni; CO, A 1
ð
Þ¼2, 056 þ p L
ð1Þ
with P(t-Bu) 3 as a suitable reference with p L ¼ 0 and ω(CO, A 1 ) ¼ 2,056 cm
À1 .
Tolman considered P(t-Bu) 3 as the most basic phosphine because of its strong σdonor and absent π-acceptor ability. This leads to an increase of the electron density
at Ni, which is transferred via the d-orbitals into the antibonding π
⋆ (CO) orbitals as
sketched in Fig. 1a, b.
The CO bond length is increased, and the A 1 -symmetrical CO stretching mode is
redshifted to the value of 2,056 cm
À1 compared to the CO stretching frequency in
carbon monoxide of 2,071 cm
À1 [132]. Any other, less basic phosphine leads to a
lower electron density at Ni and thereby to a higher CO stretching frequency
ω(L) and the ligand-specific increment p L ¼ ω(L) À 2,056. In this way, the basicity
of phosphine ligands can be estimated by simply measuring the vibrational spectra
(infrared or Raman) of the corresponding nickel–tricarbonyl–phosphine complex.
Tolman used phosphine ligands because they cover a wide range of distinct electronic and steric properties, seldom participate directly in the reactions of a transition
metal complex, and can they be used to modulate the electronic properties of the
adjacent metal center. In addition, he relied on the following important assumptions:
1. ω(CO, A 1 ) is well separated from other frequencies, so it can be easily measured
and identified in the IR spectrum.
2. The ω(CO, A 1 ) stretching mode does not couple with other vibrational modes,
i.e., can be considered as local mode.
3. There is a general correlation between ω(CO, A 1 ) and ω(ML).
In literally hundreds of studies on transition metal–carbonyl complexes, the
original Tolman concept has been applied, and in some studies, its general applicability has been tested. For example, Otto and Roodt [133] fitted the CO frequencies
measured by Strohmeier for trans-[Rh(CO)ClL 2 ] (Rh-Vaska) complexes with the
CO frequencies of Tolman’s nickel–tricarbonyl–phosphines and obtained a quadratic relationship, which suggests that besides the σ-donor activity of the trialkyl
232
E. Kraka and M. Freindorf