Tuning Reactivity of Platinum(II) Complexes

constants of CH3PhisoqPtCl decrease by a factor of about three or lower depending on
the nucleophiles. The expectation would be an increase in the rate of substitution due to
the extended π-conjugation, which should enhance π-backbonding effect. 16,17 In the case
of pyPhenPtCl and PtCl the expected trend is observed with pyPhenPtCl being more
reactive than PtCl.
To provide an explanation to this trend it is important that the properties of the
coordinated ligand to the platinum metal is understood. Looking at the structures of the
Pt(II) complexes which are shown in Scheme 3.1, one notes that they are all
characterised by the presence of a delocalised π-system that allows for facile electron
transfer from the metal to the ligand. This giving rise to metal-to-ligand-charge transfer
(MLCT) absorption spectra that are usually found in the wavelength range 350-450
nm, 33-35 and is also seen in Figure 3.5 Since the π-acceptor property of the complex can
be viewed as transfer of electron density from the HOMO orbital of the complex into the
LUMO, any difference in energies of the d-orbital HOMOs of the complex and the π*-
LUMOs localised on the ligands, would determine the HOMO-LUMO energy gap and
hence the energy of the MLCT absorption, and hence the reactivity.
Absorbance
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
CH3PhPtCl
CH3PhisoqPtCl
200 250 300 350 400 450 500
21
393 nm
Wavelength (nm)
399 nm
Figure 3.5: Absorption spectra of CH3PhPtCl and CH3PhisoqPtCl in acetonitrile