Tuning Reactivity of Platinum(II) Complexes

Table 2.3: Rate constants for the substitution of Cl¯ in [Pt(PEt3)2LCl] by pyridine. 15,44
31
kobs ( s -1 )
L—Pt cis (0 o C) trans (25 o C)
phenyl
o-tolyl
mesityl
C
H 3
CH 3
Pt
Pt
CH 3
CH 3
Pt
8.0 x 10 -2 1.2 x 10 -4
2.0 x 10 -4 1.7 x 10 -5
1.0 x 10 -6 (25 oC) 3.4 x 10 -6
When the ligand L cis to the leaving group increases in bulk from phenyl to mesityl, the
rate decreases by a factor of 1/80 000 while for the trans-isomer it drops by 1/35. 15,34,35
In the transition state of the cis-isomer the bulky group occupies an axial position as
shown in Figure 2.10. This causes greater repulsions between its ortho-methyl groups,
the leaving group and the incoming ligand. In the case of the trans-isomer the phenyl
group lies in the equatorial position at an angle of 120°. 35 As a result, repulsions
between the ortho-methyl groups, the leaving group and the incoming ligand are
reduced. Thus, the corresponding reduction in rate of reaction is less affected. 34 It can
then be concluded that steric hindrance from a substituent in the cis-position to the
leaving group exerts higher effect on the rate of substitution than in the trans-position
of d 8 square-planar geometry.