SUBSITITUTION MRCHANISM

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V. The Substitution Reactions of Square-Planar Complexes
-Square-planar complexes are those in which the metal ion contains
eight electrons in its outer d orbitals. These include Pt(II), Pd(II),
Au(III), Ni(II), Rh(I) and Ir(I). These complexes are diamagnetic
(spin paired).
-Square-planar complexes with a d8 metal centre are coordinatively
unsaturated with a sixteen-electron valence shell. Therefore, it
would be reasonable to assume that substitution reactions of these
metal centres would follow an associatively activated mechanism
since this would generate an eighteen-electron valence shell, five-
coordinate transition state. Square-planar complexes are also not
sterically hindered thus the incoming ligand can attack the complex
from above or below the plane.
These low-spin complexes also have a vacant pz orbital of
relatively low energy. This would help accommodate the pair of
electrons donated by the entering ligand.

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This trend has been supported by numerous evidence such as:
i) large and negative values of DS≠ and negative values of DV≠
ii) substitution occurs with steric retention
iii) many five- and six-coordinated d8 systems have been
isolated
iv) the rate of reaction is dependent on the entering ligand
v) steric influences on the rate
The associative pathway would therefore be energetically
more feasible than the dissociative pathway, which involves
three-coordinate, fourteen-electron valence shell
intermediates. There are cases that have recently been reported
where the preferred mechanism is dissociative in nature.

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V.1 The Kinetics and Mechanism of Substitution
Most kinetic studies of square-planar complexes are carried
out in coordinating solvents.
Therefore for the given reaction:
almost all substitutions follow the rate law:
Pseudo-first-order conditions are employed with at least a 10-
fold excess of Y to ensure the kinetics remain first-order and
to drive the reaction to completion.
The rate constant, kobs, obtained from Equation 2.7 is

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Where kobs = k1 + k2[Y] (2.9)
Equation 2.9 is seen as ‘the typical rate law for square-planar
substitution’. A plot of kobs versus [Y] will produce a straight
line graph with an intercept of k1 and a slope of k2.
Figure 7 Plots of the
observed rate constant,
kobs, for trans-[Pt(py)2Cl2]
in methanol at 30 °C as a
function of the
concentrations of different
nucleophiles.

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the value of k1 (incoming nucleophile Y- independent path) is the
same for each of the different nucleophiles reacting with the same
complex, in this case trans-[[Pt(py)2Cl2]. The value of k2,
represented by the slope, will usually be different since this is the
nucleophile dependent path. Care must also be taken to not
mistakenly assign a positive intercept to k1 (Equation 2.9). A
positive intercept may represent the reverse rate constant of
Reaction 2.6.
If a reaction is performed in a solvent which can act as an incoming
nucleophile (coordinating solvent), overwhelming experimental
evidence supports the mechanism proposed in Figure 8, where both
pathways are associative.

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the two term rate law in Equation 2.7 requires a two-path reaction
mechanism and it also indicates that both pathways are sensitive to
the same factors. The k2 route represents direct attack of the
nucleophile Y, at the platinum centre resulting in a reaction
intermediate with a trigonal bipyramidal conformation. The
product will have the same stereochemistry as the starting complex
with Y having displaced X. The k1 route represents the rate-
limiting attack by the solvent followed by the rapid replacement of
the solvent ligand by Y in a further associative pathway. This is the
case where the reverse reaction is negligible. This pathway also
proceeds via a trigonal bipyramidal intermediate with retention of
stereochemistry.
Equation 2.9 can then be written as the solvent pathway (k1
becomes kS) and reagent pathway (kY replaces k2).

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VI. Factors Affecting the Rate of Substitution
In an associative process all of the ligands involved in the
substitution can affect the stability and activation energy of the
five-coordinate activated complex
VI.1 Effect of the Entering Nucleophile
Since an associative mechanism is dependent on the nature of the
incoming ligand the nucleophilicity of this ligand will certainly
influence the rate of reaction. Studies done on complexes of Pt(II)
established the nucleophilic reactivity order:
R3P > tu > I- ~ SCN- ~ N3
- > NO2
- > Br- > py
> aniline ~ olefin ~ NH3 ~ Cl- > H2O > OH- (2.11)
It must be noted that the reactivity of the entering ligand is a
function of its polarisability and not of its base strength. The above
scale of nucleophilicity does not correlate with the corresponding
pKa scale.

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Ligand nucleophilicity is influenced by several factors:
a) Basicity:
The basicity of an entering ligand often correlates well with ligand
nucleophilicity towards the metal centre.
b) Polarisability:
Since Pt(II) is polarised extensively it is a “soft” metal ion. Soft
nucleophiles are most effective towards soft substrates and
similarly hard nucleophiles (such as OH-) prefer hard substrates.
The “soft” approach is when repulsions between the electron pair
of the nucleophile and the electrons of the central metal ion are
reduced by the ability of the entering group to accept electrons
from the metal into a suitable empty π orbital. Therefore a
“polarisable” ligand is a good nucleophile. Polarisability in the
nucleophile is always more important for rates than for equilibria.

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c) Oxidability:
Ligands that readily lose their electrons are easily oxidised and are
therefore good nucleophiles. This ability is judged by their
standard reduction potentials or by polarographic half-wave
potentials. The lower the standard potential of a nucleophile the
stronger reducing agent it is and hence the better nucleophile it
will be.
d) Solvation Energy:
If a ligand is very solvated the nucleophile must be freed from the
bonded solvent before it can coordinate to the metal. This process
requires energy, therefore better solvated ligands will be weaker
nucleophiles.
e) Metal Centre:
In the transition state the heavier elements are polarised more than
the lighter elements. The corresponding rates of substitution follow
the trend Ni(II) > Pd(II) >> Pt(II).

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The order of reactivity of a series of isovalent ions is;
Ni(II) > Pd(II) >> Pt(II)
- This order of reactivity is the same order as the tendency to form
5-coordinate complexes.
- More ready the formation of a 5-coordinate intermediate
complex, the greater the stabilisation of the transition state and so
the greater the bimolecular rate enhancement.
M =Ni ky = 33 M-1 sec-1
Pd ky = 0.58 M-1 sec-1
Pt ky = 6.7 x 10-6 M-1 sec-1

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VI.2 Effects of Ligands Already Present
In the transition state of the associative mechanism for square-
planar complexes the configuration is trigonal bipyramidal with the
leaving group (X), entering group (Y) and the ligand trans to the
leaving group all in the trigonal plane. The two ligands cis to the
leaving group occupy the axial plane. Therefore in the final
substituted complex the stereochemistry is retained with the two cis
ligands and the trans ligand still in the T-shape. The consequence
of this is that the trans ligand will exert a totally different effect on
the rate of substitution than that from the cis ligands. The ligand
trans to the leaving group has a more pronounced influence than
the two cis ligands on the rate of departure of the leaving ligand.

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VI. 3 The trans Effect
The trans effect, as defined by Basolo and Pearson, is ‘the effect of
a coordinated group upon the rate of substitution reactions of
ligands opposite to it’ in a metal complex. Basically it is the
tendency of a spectator ligand to direct an incoming group (Y) to
the position trans to itself. Referring to the generalised reaction in
Figure 10 there are two possible products from the reaction,
depending on whether the incoming group replaces the chloro-
ligand trans to the spectator ligands L1 or L2. The proportion of the
two products formed mainly depends on the nature of the trans
(and not cis) spectator ligands L1 and L2, and not on the nature of
the entering nucleophile.
Figure 10 Possible products from the substitution reaction of a
square-planar Pt(II) complex with the general nucleophile, Y-.

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The trans effect sequence in increasing power is:
An example of this effect is shown in Reactions 2.16 and 2.17
where it has been used in synthesising cis and trans isomers.
The negative ligand Cl- has a greater labilising effect on a
group trans to it than it does to the cis ligands. This effect is
usually larger for a negative ligand than for a non-π-bonding
neutral group, e.g., NH3.

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In Reaction 2.16 the second ammonia is substituted to form a cis
isomer because the trans effect of the chloride is greater than that
of NH3. The least reactive chloro group in [PtNH3Cl3]- is the one
opposite the NH3 ligand. In Reaction 2.17 the entering ligand, Cl-,
replaces the NH3 group trans to the chloro ligand in [Pt(NH3)3Cl]+
since it is the most labile. The complex trans-[Pt(NH3)2Cl2] is thus
formed.

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The synthesis of cis- and trans-[Pt(NH3)(NO2)Cl2]-can be
achieved by introducing the entering ligands in a different order. It
can be seen in Reaction 2.19 that the NO2
- group has a greater
trans effect than the chloro ligand and so directs the incoming
group (NH3) trans to itself.

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It must also be noted however that bond strength also plays a role.
In Reaction 2.20 it can be seen that the position of substitution
depends on the trans effect series Br- > Cl- > py >NH3 as well as
the fact that the Pt-N bond strength is greater than the Pt-Cl bond
strength.