The formation of a new bond controls the rate of the reaction. Therefore, the second-order kinetics is: V = K [EX][Y] and the entropy of activation is negative. It is especially common in square-planar compounds but also occurs in octahedral and tetrahedral sites.
The incoming group forms an intermediate species X-E-Y, with a coordination number one unit greater than in the starting compound. Then the leaving group X is removed. Thus, this mechanism only occurs when the central atom of the substrate can increase the coordination number.
The reaction intermediate with a coordination number one unit higher than the starting compound has the structure of trigonal bipyramid. Both groups, incoming and outgoing X, are bonded to the central atom.La subsequent dissociation of the leaving group X restores the tetrahedral geometry of the compound.
The rate controlling step reaction involves formation of a E-Y bond while the geometry is approaching the trigonal bipyramid. Therefore, if the substrate was EXABC (chiral center), the product EYABC would have the opposite absolute configuration. The reaction is enantiospecific and leads to an inversion of the configuration: (R)-EXABC + Y ⇔ (S)-EYABC + X
The tetrahedral geometry is common in compounds of representative elements, EX4, and transtion metal elements, ML4. As the mechanism requires the central atom to increase the coordination number, this mechanism is not observed in carbon compounds CX4, but in Si, Ge, Sn or P, with empty d orbitals of proper energy to participate in bonds. Some examples are Ph3SiCl, A2XP=O (A = NR2, OR, R o X y X = halogen). In general, substitutions of type A in tetrahedral species occur when ligands do not present strong steric effects such as in [M(PR3)2X2] + PR`3 (M = Fe, Co, Ni).
The reaction intermediate with a coordination number one unit higher than the starting compound has a trigonal bipyramidal structure. In this animation ligands in cis or trans to the leaving group are labeled as AC and AT. Removal of the X group in the second step is usually fast and the pentacoordinate intermediate, normally fluxional, has no time to exchange the ligand positions. The reaction is stereospecific with the incoming ligand taking the same position previously occupied by the leaving group.
AT ligands that better stabilize the transition state and produce rapid sustitution (trans effect) are H- and ligands considered good π-acceptors, such as CO, phosphine or NO2-. Square-planar compounds EX 4 are rare for representative elements, but abundant in complexes of transition elements, particularly in d8 complexes ML4 [M = Co, Rh, Ir (I), Ni, Pd, Pt (II), Au (III)]. The ligand substitution reactions are slow in these compounds and occur by the mechanism A. For example, [PtCl4]2- + Py ⇄ [PtPyCl3]- + Cl-.
AT represents the ligand trans to the leaving group. Note that the step that controls the reaction rate is the formation of the heptacoordinate intermediate. This can have several geometries (the animation represents only the pointed octahedral) that is fluxional. If dissociation of M-X occurs before the intermediate heptacoordinated reorganizes, the reaction would stereospecific with incoming ligand taking the same position occupied by the leaving group.
Substitutions of type A are more likely when the central elements are able to form stable species with coordination numbers seven or higher, as configurations d0, d1, d2 (electron deficient) and especially with elements of the second and third transition series. One example is the sustitution of Me2S in [MCl4(SMe2)2] (M = Zr, Hf). However, in the absence of conclusive experimental evidence, we can say that A is a likely mechanism.