Introduction - Organic Synthesis Using Transition Metals - page 5

Carbon monoxide may also be a bridging ligand between two metal atoms. Some of the CO ligands in the
complexes Fe 2 (CO) 9 ,Co 2 (CO) 8 and Fe 3 (CO) 12 can behave in this way.
Heteroatoms may also be ligands. These include oxygen, nitrogen, sulfur and halogen atoms. Some of
these, such as oxygen, may form double bonds to the metal, as in OsO 4 . A variety of nitrogen species may
complex to the metal including the rather special case of the nitrosyl ligand, NO + , which can replace CO.

1.2.1 Phosphines

The most widely employed heteroatom ligands are the phosphines. Although they are largely spectators and
do not participate directly in bond formation (and when they do, the result is often highly undesirable), they
are not innocent bystanders. The size and electronic nature of the three groups attached to phosphorus have
a profound effect on the course of the reaction and may make the difference between success and failure. An
example is with the Grubbs catalyst (Chapter 8). The bis(triphenylphosphine) complex is of little use. The
bis(tricyclohexylphosphine) complex is Nobel-prize winning.
Triphenylphosphine 1.1 has always been the most commonly used ligand, due to cost, availability, ease of
handling and habit. While triphenylphosphine 1.1 remains commonly used, it no longer has its old ubiquity.
An entire field of research, which might be termed “ligand engineering”, has grown up, centred on the design
of new ligands with tailor-made electronic and steric properties (Figure 1.8). In a great many of the early
applications of transition metals to organic synthesis, triphenylphosphine was used almost exclusively. An
early exception is the use of a modified version, tri- o -tolylphosphine 1.2 , in Heck reactions. 17 This was done
to suppress quaternization of the phosphine by adding steric hindrance, though its success may actually be due
to formation of Herrmann's catalyst in situ . 18 Addition of one or more sulfonate groups to the phenyl rings
gives water-soluble analogues, such as 1.3 . Triphenylphosphine has also been modified by changing the donor
atom. Both triphenylarsine 1.4 and triphenylstibine 1.5 have been employed. Changing the phenyl groups
to furyl groups giving the more electron-rich tri-(2-furyl)phosphine 1.6 can also be beneficial. Alternatively,
adding fluorine atoms gives an electron-poor ligand in tris(pentafluorophenyl)phosphine 1.7 . One or more of

Me

Ph Ph

SO 3 - Na + P

P

P

Ph Me Me

1.1 1.2 1.3

C 6 F 5 C 6 F 5

Ph Ph Ph

Sb Ph
Ph

O O

P P

As P

Ph C 6 F 5

O

1.4 1.5 1.6 1.7 1.8

t -Bu t -Bu t -Bu t -Bu

n -Bu n -Bu t -Bu t -Bu

P P P P

P t -Bu 2

Me Me

n -Bu t -Bu

Me

1.9 1.10 1.11 1.12 1.13

Me

Figure 1.8 Phosphine ligands.