Linkage isomerism

In chemistry, linkage isomerism or ambidentate isomerism is a form of isomerism in which certain coordination compounds have the same composition but differ in their metal atom's connectivity to a ligand.

Typical ligands that give rise to linkage isomers are:

cyanide, CN⁻ – isocyanide, NC⁻
cyanate, OCN⁻ – isocyanate, NCO⁻
thiocyanate, SCN⁻ – isothiocyanate, NCS⁻
selenocyanate, SeCN⁻ – isoselenocyanate, NCSe⁻
nitrite, NO−2
sulfite, SO2−3

Examples of linkage isomers are violet-colored [(NH₃)₅Co-SCN]²⁺ and orange-colored [(NH₃)₅Co-NCS]²⁺. The isomerization of the S-bonded isomer to the N-bonded isomer occurs intramolecularly.^[1]

The complex cis-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) (RuCl₂(dmso)₄) exhibits linkage isomerism of dimethyl sulfoxide ligands due to S- vs. O-bonding. Trans-dichlorotetrakis(dimethylsulfoxide)ruthenium(II) does not exhibit linkage isomers.

History

Linkage isomerism was first noted for nitropentaamminecobalt(III) chloride, [Co(NH₃)₅(NO₂)]²⁺. This cationic cobalt complex can be isolated as either of two linkage isomers. In the yellow-coloured isomer, the nitro ligand is bound through nitrogen. In the red linkage isomer, the nitrito is bound through one oxygen atom. The O-bonded isomer is often written as [Co(NH₃)₅(ONO)]²⁺. Although the existence of the isomers had been known since the late 1800s, only in 1907 was the difference explained.^[2] It was later shown that the red isomer converted to the yellow isomer upon UV-irradiation. In this particular example, the formation of the nitro isomer (Co-NO₂) from the nitrito isomer (Co-ONO) occurs by an intramolecular rearrangement.^[3]

Structures of the two linkage isomers of [Co(NH₃)₅(NO₂)]²⁺.

References

^ Buckingham, D. A.; Creaser, I. I.; Sargeson, A. M. (1970). "Mechanism of Base Hydrolysis for Co^III(NH₃)₅X²⁺ Ions. Hydrolysis and Rearrangement for the Sulfur-Bonded Co(NH₃)₅SCN²⁺ Ion". Inorg. Chem. 9 (3): 655–661. doi:10.1021/ic50085a044.
^ Werner, A. (1907). "Über strukturisomere Salze der Rhodanwasserstoffsäure und der salpetrigen Säure". Ber. (in German). 40 (1): 765–788. doi:10.1002/cber.190704001117.
^ Basolo, Fred; Hammaker, G.S (1 February 1962). "Synthesis and Isomerization of Nitritopentammine Complexes of Rhodium(III), Iridium(III), and Platinum(IV)". Inorganic Chemistry. 1 (1): 1–5. doi:10.1021/ic50001a001.

[1] Buckingham, D. A.; Creaser, I. I.; Sargeson, A. M. (1970). "Mechanism of Base Hydrolysis for Co^III(NH₃)₅X²⁺ Ions. Hydrolysis and Rearrangement for the Sulfur-Bonded Co(NH₃)₅SCN²⁺ Ion". Inorg. Chem. 9 (3): 655–661. doi:10.1021/ic50085a044.

[2] Werner, A. (1907). "Über strukturisomere Salze der Rhodanwasserstoffsäure und der salpetrigen Säure". Ber. (in German). 40 (1): 765–788. doi:10.1002/cber.190704001117.

[3] Basolo, Fred; Hammaker, G.S (1 February 1962). "Synthesis and Isomerization of Nitritopentammine Complexes of Rhodium(III), Iridium(III), and Platinum(IV)". Inorganic Chemistry. 1 (1): 1–5. doi:10.1021/ic50001a001.

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