When compared with the established palladium and nickel catalyst systems, simple iron salts turn out to be highly efficient, cheap, toxicologically benign, and environmentally friendly precatalysts for a host of cross-coupling reactions of alkyl or aryl Grignard reagents. The inorganic Grignard reagent [Fe(MgX)(2)], where X corresponds to Br or I, is a good catalyst for cross-coupling reactions. The present study reports a thorough theoretical analysis of the mechanisms of the [Fe(MgBr)(2)] catalyzed cross-coupling reaction between 4-chlorobenzoic acid methyl ester and n-hexylicmagnesium bromide using density functional theory (DFT) calculations. Our calculations show that the overall catalytic cycle includes three basic steps: oxidation of [Fe(MgBr)(2)] to obtain [Ar-Fe(MgBr)], addition to yield [Ar-(n-hexyl)-Fe(MgBr)(2)], and reductive elimination to return to [Fe(MgBr)(2)]. The energy barrier is lower if n-hexylicmagnesium bromide attacks the intermediate of the oxidative addition directly before [Cl-Mg-Br] dissociates to form the middle product [Ar-Fe(MgBr)] than if the attack occurs after the dissociation of [Cl-Mg-Br]. The solvation effect in this step dearly leads to a lowering of the energy barrier. The rate-limiting step in the whole catalytic cycle is the reductive elimination of [Ar-(n-hexyl)-Fe(MgBr)(2)] to regenerate the catalyst [Fe(MgBr)(2)], where the electronic energy barrier Delta E is 29.74 kcal/mol in the gas phase and the Gibb's free energy in solvent THF Delta G(sol) is 28.13 kcal/mol computed using the C-PCM method.