Chemistry Reference
In-Depth Information
Table 2.3 Phosphonium pK
a
values.
Entry
Ligand
pK
a
(phosphonium)
8.65
20a
1
PMe
3
8.69
21
2
P(Et)
3
6.50
20a
3
PPhMe
2
4.57
20b
4
PPh
2
Me
2.73
20a
5
PPh
3
4.46
20a
6
P(p-OMePh)
3
1.03
20b
7
P(p-ClPh)
3
1.97
20c
8
P(p-FPh)
3
3.08
20c
9
P(o-tol)
3
9.7
20a
10
PCy
3
7.97
21
11
P(i-Bu)
3
8.43
21
12
P(n-Bu)
3
11.4
20c
13
P(t-Bu)
3
Table 2.4 v
CO
values for [Ni(CO)
3
L].
Entry Ligand (L) v
CO
(cm
1
)
22
1 P(t-Bu)
3
2056
2 PCy
3
2056
3 P(i-Pr)
3
2059
4 P(n-Bu)
3
2060
5 PEt
3
2062
6 PMe
3
2064
7 PPhMe
2
2065
8 P(4-OMeC
6
H
4
)
3
2066
9 P(o-tol)
3
2067
10 P(p-tol)
3
2067
11 PPh
2
Me 2067
12 PPh
3
2069
13 P(p-FPh)
3
2071
14 P(p-ClPh)
3
2073
15 P(OMe)
3
2079
16 P(OPh)
3
2085
17 P(C
6
F
5
)
3
2091
18 PCl
3
2097
19 PF
3
2111
Adapted with permission from C. A. Tolman, J. Am. Chem. Soc.,
1970, 92, 2953. Copyright 1970 American Chemical Society.
electron-donating ability. Thus, a lower v
CO
stretching frequency correlates
with a stronger s-donor ability of the ligand. See Table 2.4.
These methods do have some limitations. For instance, contributions
from p-backbonding are underrepresented. A computational method using
natural bond orbital (NBO) analysis
23
to measure the relative p-accepting
abilities of different ligands was reported by Leyssens et al.
24
This scale
sets the p-accepting ability of CO at 100; several phosphine and amine
ligands have been evaluated. Electron-rich donor ligands feature increased
s-donation, whereas electron-deficient donor ligands increase the degree of
p-backbonding (Table 2.5).
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