Advanced Inorganic Chemistry/Associative Mechanism

Associative substitution mechanism follows a common pathway for ligand substitution in a metal complex. Resembling an SN2 reaction, the intermediate is formed when the incoming ligand momentarily becomes part of the complex; increasing the coordination of the metal [1]. The leaving ligand detaches from the metal complex leaving the complex with a new ligand which was the incoming ligand. Complexes that are coordinatively unsaturated (metals that have less the 18 electrons) or have ligands that can change its bonding to the metal (metals that have 18 electrons) are prime candidates to follow an associative substitution reaction.

Mechanism
In a regular square planar the mechanism follows X becoming part of the complex momentarily and Y as the leaving group.

The rate determining step of the is the formation of the fully coordinated complex



If the solvent is nucleophilic enough then the solvent is part of the associative mechanism. As shown in figure 2. when the solvent is part of the mechanism the solvent becomes part of the complex then the leaving ligand leaves and the solvent is substituted by the incoming ligand [3].



The rate of an associative reaction depends if the solvent is involved in the reaction. If the rate of the reaction that only involves the incoming ligand is comparable to the reaction involving the solvent therefore:
 * kobs= k1[solvent] + k2[Y]

However, if the solvent isn't nucleophilic enough then only the incoming ligand is involved in the overall reaction such as the top reaction in figure 2.
 * kobs= k[Y]

Spectator Ligand Influence
The effect of the ligands trans to the leaving ligand from the complex plays an important role in the rate of reaction. This could come about from the stabilization of the transition state (TS) or the destabilization of the ground state (GS).

Trans Effect
Lowers the enrgy of the TS, starting material needing less energy to go through with the reaction. This is a kinetic effect. This involved back bonding. The more bonding between the pi start orbital with the d orbital, makes the metal have less electron density because density from the d orbital is being removed. The metal becomes more electrophilic which makes it facile for trigonal planar to form overall decreasing the transition state's energy.

Trans Infleunce
Raises the energy of the ground state which in fact decreases the energy the starting material has to overcome to start the reaction. This is a thermodynamic effect.

Several ligands such as
 * R- ~ H- > PR3 > CO ~ C=C ~ Cl- ~NH3

in order of decreasing order of being strong sigma donors have a higher trans influence respectively. The stronger the sigma bond integration of the trans ligand sigma orbital with the metal d orbital, the less contribution of the d orbital available for the leaving group sigma orbital. This leads in an increase bond length from the rest of the ligands making the complex less stable and higher in energy. This makes the position of the ground state closer in energy to the transition state.[1]

Indenyl Effect
Also known as ring slippage. Associative substitutions aren't usually observed in 18 electron compounds. This is due to the fact that the intermediate would be a compound that has more than n18 electrons. This would need more energy and yield an unstable compound. However, it is possible for an 18 electron compound to go through an associative mechanism. As mentioned before the complex needs to have ligands that can change its bonding to the metal. It is usually seen in cyclical compounds because the matal can change its coordination number with the ligand and avoid making a complex that excess the 18 electrons [1][2][3][4].

Reference

 * 1) Pfenning, B.W (2015) Principles of Inorganic Chemistry.Hoboken, New Jersey. John Wiley & Sons Inc.
 * 2) Miessler, G. L. (2014) Inorganic Chemistry. Pearson.
 * 3) https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Organometallic_Chemistry/Fundamentals_of_Organometallic_Chemistry/Associative_Ligand_Substitution