This mechanism is characterized by the formation of a reaction intermediate where E center has one bond less (lower coordination number). This implies that the rate determining step in the process is the bond cleavage. Therefore, we have a first order reaction rate v = K [EX] and a positive activation entropy. It is common in octahedral sites and less common in tetrahedral and rare in square-planar complexes.
There is a prior dissociation of a group X followed by coordination of the incoming group Y.
There is first a slow cleavage of the B-P bond and a fast entrance of the NH3.
The slow and initial step is the cleavage of the E-X bond. The resulting intermediate EABC, triangular-planar, quickly combines with the incoming group Y. This Y group may enter either side of the intermediate. In the case of a compound of formula EXABC the product would be chiral and a mixtura of R-EYABC and S-YABC. Ther eactio is not enantioselective.
This mechanism is rare in tetrahedral compounds, but occurs for example in the halogen substitutions of Ph3CX. The resulting triangular- planar carbocation is stable and has been isolated as [Ph3C]+BF4-.
The pentacoordinated intermediate EA5 resulting from the cleavage of the E-X bond is fluxional. In this example, the incoming ligand occupies the vacancy generated by X.
Most octahedral complex of elements of the first transition series with intermediate oxidation states (II or III) are labile (fast substitution reaction occurs). However, complexes of Cr(III), Co(III) and the elements of the second and third series are inert (slow substitution). The most frequent mechanisms are D o Id.
Starting from the cis isomer the cleavage of the E-X bond gives a square pyramidal EXA4 intermediate with the X group in the base of the pyramid. If the recomposition of the C.N. 6 with the coordination of Y is faster than the rearrangement of the intermediate, the incoming Y will occupy the vacancy left by X. In that case, the reaction is stereospecific. Notice that the cleavage of the E-X bond yields a pentacoordinated intermediate EXA4. Again, if the entrance of Y is faster than the reorganization of the intermediate, the reaction will be stereospecific.(SS)
Notice that if the reorganization of pentacoordinated intermediate EXA4 to a trigonal-bipyramidal geometry, is faster than the entrance of Y, the reaction will be non-stereospecific (NSS). A mixture of cis and trans isomers will be formed.