Electronic and geometrical structures of isomers of FenOH, FenOH-, and FenOH+ (n = 1-4) with associative and dissociative attachment of OH are computed using density functional theory with generalized gradient approximation. It is found that OH is associatively attached in the ground-state FeOH and FeOH+ species, but is dissociatively attached in FeOH-. The neutral and negatively charged species with n = 2-4 prefer dissociative attachment of OH. In the ground-state Fe3OH+ and Fe4OH+ cations, OH is attached associatively, while it is attached dissociatively in Fe2OH+. © 2005 Elsevier B.V. All rights reserved.