Chemistry Reference
In-Depth Information
Table 2.2 %V
bur
for LAuCl complexes.
Entry
Ligand
%V
bur
(2.00 Å)
%V
bur
(2.28 Å)
1
PMe
3
27.3
23.3
2
PPhMe
2
30.5
26.2
3
PPh
3
34.8
29.9
4
P(p-tol)
3
34.2
29.3
5
P(C
6
F
5
)
3
42.6
37.3
6
P(o-tol)
3
44.8
39.4
7
P(mesityl)
3
50.5
45.0
8
PCy
2
Ph
38.0
32.7
9
PEt
3
31.7
27.1
30.4
a
25.9
a
10
P(n-Bu)
3
11
P(i-Pr)
3
39.1
24.0
12
PCy
3
38.8
33.4
13
P(t-Bu)
3
43.9
38.1
14
P(1-adamantyl)
2
(n-Bu)
41.9
36.3
15
P(OMe)
3
30.8
26.4
16
P(OPh)
3
36.5
31.9
17
2-(Di-tert-butylphosphino)biphenyl
(JohnPhos)
55.5
50.9
2-Dicyclohexylphosphino-2
0
,6
0
-
dimethoxybiphenyl (SPhos)
18
53.7
49.7
2-Dicyclohexylphosphino-2
0
,4
0
,6
0
-
triisopropylbiphenyl (XPhos)
19
57.4
53.1
a
Calculated from the crystal structure of the free phosphine.
Adapted from Ref. 15.
σ∗
R
R
Pd
Pd
R
P
R
P
R
R
P
Pd
P
Pd
P
−−
Pd
σ
-donation
Pd
−−
P
P
-back donation
Figure 2.5 Phosphine-palladium orbital interactions.
to examine the pK
a
of the corresponding phosphonium salt (see Table 2.3).
Despite the fact that the nature of the interaction between H
1
and a phos-
phine is hard-soft and that of a phosphine metal bond is soft-soft, re-
markably good correlations have been found.
19
Hard-soft acid-base theory
can also be invoked to explain why phosphines (soft Lewis bases) are sig-
nificantly better ligands for palladium (soft Lewis acid) than amines (hard
Lewis bases) in spite of their similar basicity.
The classic method for evaluating electronic contributions from donor
ligands was developed by Tolman.
9c
This method of quantifying ''nucleo-
philicity'' utilizes IR spectroscopy to measure the vibrational frequency of
the A
1
carbonyl stretching mode (v
CO
) of [Ni(CO)
3
L] complexes. The v
CO
value
is related to the degree of backbonding between Ni and CO, which is related
to the electron density on the Ni. This, in turn, is influenced by the ligand's
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