Tuning Reactivity of Platinum(II) Complexes

[X and [Y ] = initial concentration of X and Y respectively
] 0
0
For the kinetic analysis of equation 2.35, the absorbance can be obtained from equation
2.39
[ X ] ⎛ o Ao
− A ⎞ ∞ ln = ln = k1t
[ X ] ⎜
t At
A ⎟
(2.39)
⎝ − ∞ ⎠
Often, the absorbance-time resolved data is used directly to evaluate the observed rate
constants using Equation 2.39. Apart from its use in monitoring the kinetics of
conventionally slow reactions, the UV-visible absorption spectroscopy is also used to
perform spectrophotometric titrations, especially in pKa determinations. The pKa values
of the coordinated aqua ligand are important thermodynamic set of data used as
indicator of the electrophilicity of the metal centre. 33
2.5 Factors Affecting the Rate of Substitution
2.5.1 Effect of the Entering Group
In an associative substitution reaction mechanism, the second order-rate constant, k2, is
strongly dependent on the nucleophilicity of the entering nucleophile. Nucleophilicity is
a measure of how readily the nucleophile is able to attack an electron-deficient atom, in
this case Pt metal centre, in exclusion of the proton. That is the stronger the nucleophile
the greater the rate of substitution reaction of a particular Pt(II) complex. It is derived
from the second-order rate constant, k2, for the substitution process. 5,15 The
nucleophilicity of the ligand is usually influenced by several factors: 6,26, 34
a. Basicity: Basicity of the entering nucleophile is characterised by its pKa
and correlates well with the nucleophilicity of the entering
nucleophile towards the metal centre.
b. Polarisability: Polarisability of the nucleophile is an important
consideration for the rates rather than for equilibria of
chemical reactions. 35 An increase in 1† polarisability reflects
the effectiveness of the ligand as an electron donor.
1 † The polarisability of a ligand is better explained by Pearson’s “Hard Soft Acid Base” theory which states
that hard acids, i.e. those metal ions that are small bear a high charge and possess a valence electron shell
23