Tuning Reactivity of Platinum(II) Complexes

Table 2.2: The effect of the leaving group on the lability of [Pt(dien)X] +
Leaving group (X) kobs (s -1)
NO3 ¯ Very fast
H2O 1900
Cl ¯ 35
Br ¯ 23
I ¯ 10
N3 ¯ 0.83
SCN ¯ 0.30
NO2 2- 0.050
CN ¯ 0.017
This produces a leaving group dependence of the order:
NO3 ¯ > H2O >> Cl ¯ > Br ¯ > I ¯ >>N3 ¯ >SCN ¯ > NO2 2- > CN ¯
In general it easier to substitute leaving groups which have a low nucleophilicity. For
example the replacement of CN ¯ as the leaving group by Cl ¯ results in the reduction of
the rate by a factor of 2 x 10 3. But when CN ¯ is the entering group, it is 10 4 times better
than Cl ¯. 18 Therefore for an associative mechanism, a substantial amount of metal-ligand
bond breaking occurs in the transition state which depends on a specific reaction. 26,35
2.5.3: Effect of Steric Hindrance
Steric effects are generally space-filling effects 37 and are categorised as, steric bulky and
steric hindrance. Steric bulky is due to mutual repulsion of electron densities brought
about by overcrowding of a group of atoms around the metal atom. The magnitude of
resulting steric effect is proportional to the spatial size (i.e. a volume effect) of the
substituent(s) that causes it. On the other hand, steric hindrance is the shielding of the
29