Pd–Phosphine Precatalysts for Modern Cross-Coupling Reactions - New Trends in Cross-Coupling: Theory and Applications - page 95

There are a number of exceptions to the 18-electron rule, one of which is
illustrated in Figure 3.3. The electron count of dppfPdCl 2 only adds up to 16,
which in theory would make this an unsaturated reactive complex. Instead, it
is an air-stable precatalyst. The most common exception to the rule is that
these Pd(II) complexes have a low-spin d 8 configuration. They adopt a
square-planar conformation where the splitting of the d-orbitals is such that
a 16-electron complex is the most stable.
Other exceptions to the rule arise due to the nature of the ligands
bonded to the metal center. Very bulky ligands, e.g., t-Bu 3 P, dtbpf, Q-Phos [Q-
Phos ¼ 1,2,3,4,5-pentaphenyl-1 0 -(di-tert-butylphosphino)ferrocene)], radical
ligands or p-donating ligands are all able to stabilize a low-electron count
complex better. Figure 3.4 shows three examples of precatalysts that are
very ecient in a number of coupling reactions. The classical coupling
catalyst (Ph 3 P) 4 Pd is an 18-electron species, yet it is air sensitive as it
can easily dissociate two ligands to form a 14-electron species. As expected,
(t-Bu 3 P) 2 Pd is an air-sensitive complex as it has a 14-electron configuration,
whereas (Ph 3 P) 2 PdCl 2 and Q-PhosPd(crotyl)Cl are air-stable complexes
with a 16-electron configuration. Interestingly, the dimeric complex
[(t-Bu 3 P)Pd(m-Br)] 2 , despite its 16-electron configuration, is fairly air

sensitive, due to the þ 1 oxidation state of Pd.
In order to account for all these anomalies, Landis and Weinhold pro-
posed an alternative theoretical treatment, where it is suggested that tran-
sition metals do not employ p-orbitals for bonding. 18 They therefore have
only a valence shell of one s- and five d-orbitals. These can accommodate up
to 12 electrons with two-center-two-electron bonds. If the metal has co-
ordination numbers beyond this, the bonds will have to be formed by linear
three-center-four-electron bonds, where only one metal orbital is used for

Neutral Count Ionic Count

Pd (0) d 10 ( t -Bu) 3 P Pd P( t -Bu) 3 10 10

PR 3 4 4

14 14

( t- Bu) 2
P

Pd (II) d 8
PR 3

10
2

8
2
2

Cl

Fe Pd

Ph
Ph

Ph
Ph

Cl 1

h 3-allyl 3 4

Ph

16 16

Pd (I) d 9
PR 3

10
2

9
2
4

Br

( t -Bu) 3 P Pd Pd P( t -Bu) 3

Br

2Br 3

Pd-Pd 1 1

16 16

Figure 3.4 Electron count for (t-Bu 3 P) 2 Pd, Q-PhosPd(crotyl)Cl and [(t-Bu 3 P)Pd(m-Br)] 2 .