Tuning Reactivity of Platinum(II) Complexes

with π*-orbitals of the ligand. This allows electron density flow from the filled 5dπ and
6pz-orbitals of the metal into the empty π*-orbitals of the ligand, 37 making the trigonal
bipyramidal transition state in pyPhenPtCl more stable and the Pt(II) centre more
electrophilic, thereby enhancing the departure of the chloride. This is supported by the
DFT-calculations which show that this system has the smallest HOMO-LUMO energy gap.
This effect is far reduced in the case of CH3PhisoqPtCl. Thus, the extensive π-
conjugation in the fused-ring system of the Phen ligand, accompanied by enhanced
electronic communication in the aromatic system around the Pt-metal centre, results in
increased reactivity of the pyPhenPtCl complex compared to PtCl.
The trend of reactivity observed in this study between CH3PhPtCl and PtCl has been
reported before in the literature. 14, 15 The positive σ-inductive effect associated with the
4′-(2'"-CH3-phenyl) group decreases the electrophilicity of the metal while increasing
the energy gap between the HOMO-LUMO orbitals, 9,13,1517 resulting in decreased
reactivity as compared to PtCl.
The substitution of chloride by the nucleophiles TU, DMTU and TMTU decreases
according to the increasing steric hindrance of the nucleophiles for all the complexes
with the most sterically hindered TMTU, reacting significantly slower. The reactivity of
the anionic nucleophiles follows the order I ¯ > SCN ¯ > Br ¯; an order that depicts the
nucleophilicity and polarisability of the incoming ligands and correlates well with
previous studies. 16, 38 The iodide ion being the most polarisable reacts faster than the
other nucleophiles, and is aided by its strong electrostatic attraction forces with the
monocationic complexes. It is a well documented fact that soft (polarisable) nucleophiles
have a high affinity for softer substrates like Pt(II) ion. 39
The activation enthalpies (ΔH ≠ ) as tabulated in Table 3.2 are low and positive, and
activation entropies (ΔS ≠) are large and negative. The sensitivity of the second-order
rate constants to different nucleophiles and the significantly negative intrinsic entropies
of activation (ΔS ≠ ) values as a result of bond formation in the transition state, support
the associative mechanism well known for d 8 square-planar metal complexes. 39-41
23