Chemistry · Ch 9 — Coordination Compounds
Structural isomers (Constitutional isomers)
Structural isomers (Constitutional isomers)
Structural (constitutional) isomers have genuinely different atom-to-atom linkages, even though their overall chemical formula is identical, and this chapter recognises four distinct sub-types. (a) Linkage isomers arise specifically from an AMBIDENTATE ligand (section 9.2.3) coordinating through a different one of its two donor atoms in each isomer. The nitrite ion NO2- is the standing example: [Co(NH3)5(NO2)]2+ has a Co-N bond (the 'nitro' isomer) while [Co(NH3)5(ONO)]2+ has a Co-O bond (the 'nitrito' isomer) -- same overall formula, different metal-ligand bond. (b) Ionization isomers arise when a ligand and a counter ion literally swap roles -- a species that is coordinated (inside the brackets) in one isomer instead sits outside as a free counter ion in the other, and vice versa. [Co(NH3)5SO4]Br (with SO4(2-) coordinated to cobalt and Br- as the free counter ion) and [Co(NH3)5Br]SO4 (with Br- coordinated to cobalt and SO4(2-) as the free counter ion) are ionization isomers of each other; because the coordinated species is different in each, the two isomers give DIFFERENT ions when dissolved -- versus . (c) Coordination isomers arise in a salt built from TWO different complex ions (one cationic, one anionic, each with its own separate metal centre), where the two metals literally exchange ligand sets between their two coordination spheres. [Co(NH3)6][Cr(CN)6] (cobalt bound to the six ammine ligands, chromium bound to the six cyanide ligands) and [Cr(NH3)6][Co(CN)6] (the exact ligand assignment swapped between the two metals) are coordination isomers of each other. (d) Solvate isomers (specifically called hydrate isomers when the solvent involved is water) are structurally similar to ionization isomers, but involve the SOLVENT molecule itself moving in or out of the coordination sphere …