_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

General reactions of transition metal complexes
Pd, Pt and Au, however, often show little or no tendency tobe converted into 18-
electron products. In these cases the 16-electron configuration may often be
viewed as being essentially 'saturated'. Sixteen electron complexes are common
for (Co), Rh and Ir and for Ni. Pd and Pt. Even lower electron configurations
(16, 14 and 12) are normal for Ag and Au, for which I 8-electron complexes are
rare.
Some general types of reaction which are followed by the essentially covalent
transition metal complexes which are the subject of this book are listed in
Table 5.8. The simplest way in which an unsaturated species ML 11 _ 1 can be
become saturated is by addition of a Lewis base L, in other words by revers al of the
dissociation mentioned above. This is shown in Fig. 5.16 by horizontal arrows
( ~) (Table 5.8, 1 (a), 1 (b) ). Examples are provided by the substitution of carbon
monoxide in Ni(C0) 4 or Fe(CO), (p. 168) which follows a dissociative (D)
mechanism, 1(a) followed by l(b). Ligand substitution in 16-electron square
planar complexes of Pd, Pt and Au for example, normally follows an associative
mechanism, 1(b) followed by 1(a). An intermediate reaction type, synchronous
interchange of ligands, in which the leaving group moves away as the incoming
group enters (Id, Ia), involves no change in electron configuration. 18-Electron
complexes never react by an associative mechanism, as the first step would
require the formation of a 20-electron intermediate. Where overall second order
kinetics are observed, an interchange process is likely. This probably accounts for
the small ligand dependent contribution to the rate of substitution of the
carbonyls M(C0) 6 (M = Cr, Mo, Wl (Rate= kJM(C0) 6 ] + kJM(C0) 6 ] [L]) which is
observed especially with strong (J donors such as trialkylphosphines.
Coordinatively unsaturated transition metal complexes may therefore behave
as electrophiles (Lewis acids) by virtue of empty low lying orbitals centred on the
metal atom. Complexes, however. especially those with 18-electron configurations,
may also possess electrons in essentially non-bonding d-orbitals. Where
the ligands are rather electronegative (N. O. halogen donors). or where the
complex carries an overall positive charge, such orbitals lie rather low in energy.
With relatively electropositive donor atoms (P. As. C. H donors) or in anionic
species ( e.g. Mn( CO);) the energy of these d-orbitals is rai sed and the compound
becomes 'electron rich'. Such complexes can behave as bases; this property is
noted above for carbonyl anions (p. l 7 3) which are the conjugate bases of
hydridocarbonyls. Replacement of CO by phosphites or phosphines increases the
base strength; while HCo(C0) 4 is a strong acid (pK ~ 2), HCo(C0) 1 PPh 1 is rather
weak (pK ,."., 7). HCo{P(OMe)J 4 is converted into the anion only by very strong
bases such as KH in tetrahydrofuran. while HCo(PMe 1 ) 4 cannot be deprotonated
at ali. Neutra! complexes ML" (L = P(OR) 1 or PRJ are quite strong bases. In some
cases crystalline salts such as HFeL:x- ha ve been isolated, especially with large
weakly coordinating anions (X= BF 4 • PF 6 ) which ha ve little tendency to displace
a ligand from the coordination sphere.
A few adducts between metal complexes and Lewis acids such as boron
trifluoride (p. 179) have been isolated.
175