In this reacion the incoming group Y is attached to the reaction center forming an intermediate with a coordination number one unit greater than in the starting compound. The leaving group X is subsequently separated.
The reaction intermediate, with a coordination number one unit larger than in the starting compound, has a trigonal bipyramid structure. Both groups Y and X, incoming and outgoing, are bonded to the central atom. La subsequent dissociation of the leaving group X restores the tetrahedral geometry in the final compound.
The reaction intermediate, with a coordination number one unit larger than in the starting compound, has a square piramidal structure.
The reaction intermediate, has a coordination number 7, one unit larger than in the starting compound. These heptacoordinated species are fluxional so we present one of the possible geometries for the reaction intermediate.