Tuning Reactivity of Platinum(II) Complexes

Leishmania parasites in addition to human ovarian carcinoma. 6 However, interactions of
Pt species with sulphur-donor ligands have been associated with negative phenomena
such as renal toxicity, neurotoxicity and other side effects limiting their widespread
clinical use. 7 Therefore, establishing appropriate properties of the Pt(II) complexes that
may satisfy the criteria as potential medicinal agents has attracted considerable
attention. This search includes the need to understand the kinetics of the interactions of
these compounds with biological sulphur-donor nucleophiles.
The monofunctional [Pt(terpy)Cl] + and related compounds are good models for studying
ligand substitution in square-planar d 8 complexes because only the fourth coordinated
ligand is labile. The tridentate terpy ligand is stable due to the “chelate effect” and
aromatic stability. The role of the π-backbonding in substitution reactions has been
shown to enhance the reactivity, for instance, chloride substitution in [Pt(terpy)Cl] + in
methanol is 10 2 to 10 4 times faster than for the cation [Pt(dien)Cl] + having saturated
diethylenetriamine (dien) ligand as the chelating ligand. 8-10 The reactivity difference has
been attributed to delocalisation of the π-system in the terpy ligand, which stabilises the
five-coordinate transition state relative to the ground state by accepting electron density
from platinum 5d orbitals into the π* molecular orbitals of the terpy ligand. 11,12 In a
related study, 13 addition of electron-donating groups on the ancillary positions of the
terpy ligand has been shown to retard the rate of substitution of chloride ligand. The
opposite is true for electron-withdrawing groups. 13-15
Limited information relating to the factors that influence the activity of these
compounds such as the role of extended π-conjugation has been reported. 16,17 A
systematic investigation involving the variation of the position of pyridine or amine
donors in the cis and trans positions of complexes of the type [PtL(OH2)] +2 where L =
tridentate ligand, has been documented. 16,18 The rates of substitution were shown to
increase in a stepwise manner by addition of π-acceptor ligands. The results also
indicated that the cis π-acceptors are superior to the trans π-acceptors in enhancing the
rate of ligand substitution.
2