​Among the following complexes, the one showing the highest rate of substitution of a CO ligand on heating with one equivalent of PPh₃ in decalin, is

Correct option is D

​In inorganic substitutions, the limiting mechanisms are dissociative (D), in which the intermediate has a lower coordination number than the starting complex, and associative (A), in which the intermediate has a higher coordination number.

The substitution of a CO ligand by another 2-electron donor (e.g. PR₃) may occur by photochemical or thermal activation, either by direct reaction of the metal carbonyl and incoming ligand, or by first replacing a CO by a more labile ligand such as THF or MeCN. 

​The substitution steps are dissociative. The outgoing ligand leaves, creating a 16-electron metal centre which is coordinatively unsaturated. The entry of a new 2-electron ligand restores the 18-electron count. 

In the given question, all the metal complexes contain the metal atom Mn and 3 CO ligands.  In each metal complex, Mn ([Ar] 4s²3d⁵) has zero oxidation state. A CO ligand is a two electron donor. Cyclopentadienyl ligand contributes five electrons. Indenyl ligand contributes five electrons. The total electrons present in each metal complex is =5+7+6 = 18 electrons.

The indenyl effect refers to an explanation for the enhanced rates of substitution exhibited by η⁵-indenyl complexes vs the related η⁵-cyclopentadienyl complexes.

Associative substitution occurs by the addition of a ligand to a metal complex followed by dissociation of an original ligand. Associative pathways are not typically seen in 18-electron complexes due to the requisite intermediates having more than 18 electrons associated with the metal atom. 18 electron indenyl complexes; however, have been shown to undergo substitution via associative pathways quite readily. This is attributed to the relative ease of η⁵ to η³ rearrangement due to stabilization by the arene. This stabilization is responsible for substitution rate enhancements of about 10⁸ for the substitution of indenyl complexes compared to the corresponding cyclopentadienyl complex.

​η⁵ and η³ isomers exist in rapid chemical equilibrium. The rate-limiting step occurs with the attack of the nucleophile on a η³ isomer. The nature of the substituents of the allyl group can strongly affect the kinetics and regiochemistry of the nucleophilic attack.