Sequential Bond Energies of Fe(CO)_x⁺(x = 1–5): Systematic Effects on Collision-Induced Dissociation Measurements

Collision-induced dissociation (CID) of Fe(CO)_x⁺(x = 1-5) is studied by using guided ion beam mass spectrometry. A flow tube source is used to produce thermalized iron carbonyl ions. We discuss in detail our threshold modeling procedures and several possible sources of systematic error that can affect accurate determination of bond energies from CID threshold measurements. Careful analysis of CID thresholds provides the following 0 K bond dissociation energies: D°[(CO)₄Fe⁺-CO] = 1.16 ± 0.04 eV (26.8 • 0.9 kcal/mol); D°[(CO)₃Fe⁺-CO] = 1.07 ± 0.07 eV (24.7 ± 1.4 kcal/mol); D° [(CO)₂Fe⁺-CO] = 0.69 ± 0.05 eV (15.9 ± 1.2 kcal/mol); and D°[(CO)Fe⁺-CO] = 1.61 ± 0.15 eV (36.1 • 1.8 kcal/mol). We also measure D°[Fe⁺•CO] = 1.59 ± 0.08 eV (36.6 ± 1.8 kcal/mol), but this dissociation may correspond to production of excited Fe⁺(⁴F), in which case the D°[Fe⁺-CO] for dissociation to Fe⁺(⁶D) is 1.36 ± 0.08 eV (31.3 ± 1.8 kcal/mol). The sum of the five bond energies, 6.12 ± 0.09 eV (140.1 ± 3.1 kcal/mol), is in excellent agreement with literature thermochemistry, and the individual bond strengths are in reasonable accord with prior measurements and theoretical calculations.