Question

Explain substitution reactions of square planar complexes

Answer 1



Directing effects in inorganic substitution reactions. Part I. A hypothesis to explain the trans-effect

J Chatt, LA Duncanson, LM Venanzi

Journal of the Chemical Society (Resumed), 4456-4460, 1955

The hypothesis that trans-directing ligands in square planar and octahedral complex compounds exert their directing effects in substitution reactions because they co-ordinate by double bonds is developed, and it is shown how the general phenomenon of the trans-effect can be explained. It is supposed that the forces responsible for the directing effect originate mainly in the electron withdrawal, occasioned by dative x-bond formation, from ad,,-orbital of the metal towards the ligand A of high trans-effect (Fig. 2). This electron withdrawal will increase the electron affinity of the metal atom and perhaps the strength of a-bonding to all the other ligands B. However, the electrons are withdrawn mainly from the antinodes remote from A of the d-orbital, thus decreasing any tendency of the ligand B in trans-position to A to form a dative x-bond. The whole M= B bond will thus be drastically weakened (ze, more subject to heterolytic fission) if the dative x-bond is an essential component. However, if B, tvans to A, has little tendency to form dative x-bonds its attachment to the metal could be strengthened by the presence of A. The operation of the rapid and readily reversible elimination of groups trans to ligands A of high trans-effect (i.~., high double-bonding capacity) is readily explained if we suppose that trans-substitution occurs by an S, 2 (bimolecular) mechanism. Increasing double bonding by A increases the electron affinity of the metal atom and hence the ease of nucleophilic attack. Also, because the electron withdrawal occasioned by A occurs from the antinodes remote from A of the d,,-orbital, the attack takes place there, preferentially displacing the ligand trans to A (Fig. 2). Electronically platinum (I1) is uniquely constituted for the origin and transmission of a strong trans-effect.

Answer 2

The substitution reactions of square planar complexes mean the replacement of a ligand bonded to the central metal with the incoming nucleophilic ligand.

Explanation:

• Substitution reactions are those in which a group or ligand is replaced by an incoming nucleophile or ligand.
• In square planer complexes, the ligand attached to the central metal is replaced by an incoming ligand or nucleophile.
• The substitution follows the associative mechanism.
• In this mechanism, the incoming ligand binds to the central metal in the beginning and a square pyramidal intermediate is formed.
• This intermediate further changes into a trigonal bipyramidal intermediate and then the weak ligand opposite to the trans-directing groups are replaced by the incoming ligand.
• Trans substitution is more favoured in comparison to the cis-substitution.
• The substitution reactions depend upon the following two factors:

1. Kinetic effect
2. Thermodynamic effect

• Usually, trans substitution takes place as shown in the image.
• However, the substitution reaction in square planer complexes giving cis product is given in the following figure which is the formation of cisplatin.
• In the given reaction, in tetrachloroplatinate (II), the substitution of ammonia occurs that replaces the leaving chlorine ligand due to the trans directing effect of chlorine.
• Similarly, other chlorine ligands that are trans to chlorine atom are substituted by other incoming ammonia ligands resulting in the formation of cisplatin.

#SPJ2