to sufficient overlap. Apart from that, 4e repulsive interaction with the occupied Ni
3d-orbitals is also possible.
Other effects such as steric bulk (increasing from PH 3 to PAt 3 and thereby
weakening the Ni–L bond) or the relativistic contraction of the 5s(I),5p(I) or 6s
(At),6p(At) orbitals, which leads to a smaller 3pπ(P) coefficient in the pseudoπ
⋆ (PX 3 ) orbital and reduced back donation from Ni to L, also play a role. These
effects cannot be captured by the TEP, which decreases from 2,140 (PF 3 ) to 2,135
(PCl 3 ), 2,133 (PBr 3 ), 2,129 (PI 3 ), and 2,127 cm
À1 (PAt 3 ), thus suggesting an
increase rather than decrease of the Ni–L bond strength. The unusually strong Ni–
P bond for the PF 2 H ligand (BSO n ¼ 0.638, Fig. 9) is the result of a favorable
compromise between a limited weakening of the σ-donation effect due to just two
electronegative F substituents and still strong π-back donation into the pseudoπ
⋆ (PHF 2 ) orbitals.
5.3 Special Role of Carbene Ligands
A prominent example of the increasing popularity of the TEP has been its application
to transition metal complexes containing as ligand an N-heterocyclic carbene (NHC)
[135, 137, 292, 293]. In recent reviews on N-heterocyclic carbene (NHC), Nelson
and Nolan [294] and Dröge and Glorius [295, 296] discussed the TEP as a tool for
experimentalists who investigate the electronic properties of metal–NHC complexes.
For the purpose of studying the bonding properties of NHC ligands, usually the cis[MCl(CO) 2 (NHC)] M ¼ Rh, Ir model complexes are synthesized, because of the
toxicity of the corresponding [Ni(CO) 3 (NHC)] complexes [294–297]. Using linear
regression schemes proposed by Dröge and Glorius [296], TEP values obtained from
different metal complexes can be correlated. This has led to a large compilation of
TEP data for hundreds of NHC ligands, all using the same TEP scale. TEP values for
NHCs generally stretch from 2,030 cm
À1 for electron-rich NHCs to 2,060 cm
À1 for
electron-poor NHCs [294–296]. This seems to be a small range of TEP values,
considering the large variety and complexity of NHC compounds stretching from
NHCs with extended poly-aromatic substituents [176]; planar chiral
imidazopyridinium-based NHCs, which can function as Lewis acids and ligands
for transition metal complexes [185], nano-sized Janus bis-NHC ligands based on a
quinoxalinophenanthrophenazine core [298], and NHCs with O-functionalized
triazole backbones [299] and to cyclic alkyl amino carbenes as strongly donating
ligands at the lower end of the NHC-TEP scale [183].
Singlet carbenes possess a lone pair for σ-donation and an empty pπ-orbital for
accepting negative charge from Ni. As shown in Fig. 1c, d, the full effect of these
interactions cannot be captured by the TEP focusing just on the CO bonds. However,
the data shown in Fig. 10 reveals that the BSO n(ML) recover the full effect. The
BSO value of L ¼ CH 2 is with 1.229 the largest of all neutral ligands included in the
set of 181 [Ni(CO) 3 L] complexes. Replacement of the H atoms by hyperconjugative
or π-donor substituents R ¼ R
0 leads to a reduction of the NiC bond strength: R¼CH 2
group (vinylidene; 1.142), Me (0.900), Cl (0.907), F (0.885), OMe (0.653), NH 2
252
E. Kraka and M. Freindorf
3d-orbitals is also possible.
Other effects such as steric bulk (increasing from PH 3 to PAt 3 and thereby
weakening the Ni–L bond) or the relativistic contraction of the 5s(I),5p(I) or 6s
(At),6p(At) orbitals, which leads to a smaller 3pπ(P) coefficient in the pseudoπ
⋆ (PX 3 ) orbital and reduced back donation from Ni to L, also play a role. These
effects cannot be captured by the TEP, which decreases from 2,140 (PF 3 ) to 2,135
(PCl 3 ), 2,133 (PBr 3 ), 2,129 (PI 3 ), and 2,127 cm
À1 (PAt 3 ), thus suggesting an
increase rather than decrease of the Ni–L bond strength. The unusually strong Ni–
P bond for the PF 2 H ligand (BSO n ¼ 0.638, Fig. 9) is the result of a favorable
compromise between a limited weakening of the σ-donation effect due to just two
electronegative F substituents and still strong π-back donation into the pseudoπ
⋆ (PHF 2 ) orbitals.
5.3 Special Role of Carbene Ligands
A prominent example of the increasing popularity of the TEP has been its application
to transition metal complexes containing as ligand an N-heterocyclic carbene (NHC)
[135, 137, 292, 293]. In recent reviews on N-heterocyclic carbene (NHC), Nelson
and Nolan [294] and Dröge and Glorius [295, 296] discussed the TEP as a tool for
experimentalists who investigate the electronic properties of metal–NHC complexes.
For the purpose of studying the bonding properties of NHC ligands, usually the cis[MCl(CO) 2 (NHC)] M ¼ Rh, Ir model complexes are synthesized, because of the
toxicity of the corresponding [Ni(CO) 3 (NHC)] complexes [294–297]. Using linear
regression schemes proposed by Dröge and Glorius [296], TEP values obtained from
different metal complexes can be correlated. This has led to a large compilation of
TEP data for hundreds of NHC ligands, all using the same TEP scale. TEP values for
NHCs generally stretch from 2,030 cm
À1 for electron-rich NHCs to 2,060 cm
À1 for
electron-poor NHCs [294–296]. This seems to be a small range of TEP values,
considering the large variety and complexity of NHC compounds stretching from
NHCs with extended poly-aromatic substituents [176]; planar chiral
imidazopyridinium-based NHCs, which can function as Lewis acids and ligands
for transition metal complexes [185], nano-sized Janus bis-NHC ligands based on a
quinoxalinophenanthrophenazine core [298], and NHCs with O-functionalized
triazole backbones [299] and to cyclic alkyl amino carbenes as strongly donating
ligands at the lower end of the NHC-TEP scale [183].
Singlet carbenes possess a lone pair for σ-donation and an empty pπ-orbital for
accepting negative charge from Ni. As shown in Fig. 1c, d, the full effect of these
interactions cannot be captured by the TEP focusing just on the CO bonds. However,
the data shown in Fig. 10 reveals that the BSO n(ML) recover the full effect. The
BSO value of L ¼ CH 2 is with 1.229 the largest of all neutral ligands included in the
set of 181 [Ni(CO) 3 L] complexes. Replacement of the H atoms by hyperconjugative
or π-donor substituents R ¼ R
0 leads to a reduction of the NiC bond strength: R¼CH 2
group (vinylidene; 1.142), Me (0.900), Cl (0.907), F (0.885), OMe (0.653), NH 2
252
E. Kraka and M. Freindorf