A theoretical study of the [FeCN](+) system has been carried out. Density functional methods, as well as single- and multi-reference techniques, were employed. Both cyanide and isocyanide isomers are very close in energy, with FeCN+ lying about 0.9 kcal mol(-1) below FeNC+. A non-linear minimum, with iron formally bonded just to the nitrogen atom according to a topological analysis of the charge density, is also found just 1.3 kcal mol(-1) above the cyanide isomer. The estimated energy barrier for the isomerization process is just 5 kcal mol(-1), suggesting that rearrangement of [FeCN](+) should take place easily. (C) 2007 Elsevier B.V. All rights reserved.