Tuning Reactivity of Platinum(II) Complexes

eaction site from direct attack by an incoming reagent. The magnitude of this type of
steric effect depends on
(i) the spatial size or volume occupied,
(ii) relative spatial orientation or configuration of the substituent with respect to the
target metal centre,
(iii) the position of the steric imposing substituents relative to the leaving group. 44
Generally, the larger the entering groups the slower the reaction.
The transition state of an associated-type of mechanism is accompanied by an increase
in the coordination number. This results from the bonding between the metal centre
and the incoming ligand. It is expected that steric hindrance is increased in the
transition state. This slows down the substitution process because the transition state is
destabilized by the increased steric interactions. The retardation effect on the rate of
substitution is more prominent if the steric effect imparting substituent is located in a
cis-position relative to the leaving group rather than in the trans-position on a square-
planar geometry. 15
Example is the substitution of pyridine into a series of complexes of the form cis-/trans-
[Pt(PEt3)2(R)X] given in Equation 2.45. The data is summarised in Table 2.3. 15
cis-/trans-[Pt(Et 3 P) 2 (R)Cl] + py cis-/trans-[Pt(Et 3 P) 2 (R)py]+ + Cl - (2.45)
30