A neutralization—reionization (NR) case study for cationic iron complexes with simple ligands: NR mass spectra of Fe(C 2H 4) + and Fe(CO) +

Abstract

The cationic iron(I) complexes Fe(C 2H 4) + and Fe(CO) + were examined by neutralization—reionization mass spectrometry (NRMS). Whereas the NR mass spectrum of the carbonyl complex exhibits a signal corresponding to the reionized neutral molecule Fe(CO), for the neutral Fe(C 2H 4) complex no recovery signal is observed; rather, in the course of the experiment the complex dissociates to Fe and C 2H 4. An explanation for this seemingly contradictory behaviour of structurally related metal complexes in a NR process is provided by ab initio MO calculations, in which the geometries of Fe(C 2H 4) + ( 4B 2), Fe(C 2H 4) ( 5B 2), Fe(CO) + ( 4Σ −, Fe(CO) ( 3Σ −, and Fe(CO) ( 5Σ −) have been fully optimized at the QCISD(T) level of theory. From the theoretical results, a neutralization-reionization scheme for organometallic ions MX + emerges which considers the effects caused by curve-crossing from a bound state to a repulsive ground-state asymptote of the neutral building blocks M and X. Thus, even for bound organometallic complexes MX, recovery signals in the NR mass spectrum can only be detected if the internal energy deposited in MX in the vertical electron-transfer reaction MX + → MX is too small to permit this curve-crossing. If dissociation occurs on the time scale of the NR experiments, the spectrum exhibits features of both the metal M and the ligand X, thus revealing the structural properties of the (organic) ligand X bound to the metal ion.