Chemistry Reference
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Table 2.5 p-Acceptor indices.
Entry Ligand p-Acceptor index
1 NMe
3
2
2 NH
3
4
3 Pyridine 15
4 PPh
3
16
5 PH
3
17
6 PMe
3
18
7 P(OMe)
3
22
8 NF
3
38
9 PF
3
38
10 PCl
3
48
11 CO 100
Adapted with permission from T. Leyssens, D. Peeters, A. G.
Orpen and J. N. Harvey, Organometallics, 2007, 26, 2637.
Copyright 2007 American Chemical Society.
Experimental studies aimed at measuring the p-accepting ability of
ligands have produced mixed results. Particularly, several studies
using photoelectron spectroscopy (PES) indicated that alkylphosphines are
poor p-acids;
25
however, more recent studies provide evidence that
alkylphosphines can act as p-acceptors with early transition metals in low
oxidation states (electron-rich metals), e.g., TiMe
2
(dmpe)
2
[dmpe
¼
1,2-
bis(dimethylphosphino)ethane].
26
2.2.3 Bite Angle Considerations (Chelating Ligands)
As mentioned previously, the seminal work of Kumada's group highlighted
the beneficial effect of a bidentate ligand, dppf, in cross-coupling reactions
of organobromides with secondary alkyl Grignard reagents relative to the
PPh
3
system.
7
In fact, many catalytic applications utilize chelating ligands
due to the increased stability, unique reactivity and selectivity that they
provide.
27
The P-M-P angle created by the metal chelate is commonly
referred to as the ''bite angle'' (Figure 2.6).
Casey and Whiteker introduced the concept of ''natural bite angle''
to quantify the strain imposed by chelating bisphosphine ligands.
30
In
this model, molecular mechanics calculations are used to determine
the preferred bite angle (P-M-P angle) imposed by ligand backbone
constraints - not by metal valence effects. In this simulation, a dummy ''metal''
atom is used at a distance of 2.315 Å (M-P distances of similar complexes
determined crystallographically), the purpose of which is to maintain
bidentate coordination during the calculation. The P-M-P force constant is set
to zero to exclude any contributions from the metal center (Figure 2.7).
Ligand bite angle effects can be classified as electronic or steric in nature.
Electronic bite angle effects are described as electronic changes at the metal
center imposed by the natural bite angle of the ligand. Dierkes and van
Leeuwen introduced the concept of ''metal-preferred bite angle'', which they
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