Electron Impact and Multiphoton Ionization at 351 nm,248 nm and 193 nm of the Dinuclear π-Complexes: Di-μ-Carbonyl-Biscyclopentadienyl-Di-Nickel,Di-μ-Carbonyl-Dicarbonyl-Biscyclopentadienyl-Di-Iron and Di-μ-Nitrosyl-Bis(Tetraisopropylcyclopentadienyl)-Di-Cobalt

Electron impact (EI) and nanosecond multiphoton ionization (MPI), at 351, 248 and 193 nm, of the title compounds, Cp₂Ni₂(CO)₂ [1], Cp₂Fe₂(CO)₄ [2] and (Cp-i₄)₂Co₂(NO)₂ [3], is reported (Cp = cyclopentadienyl; Cp-i₄ = tetraisopropylcyclopentadienyl). Gas-phase formation enthalpies were derived for 1 and 2 and for some fragments. Fragmentation processes were identified by metastable ion analysis. EI ionization of 1 leads to molecular ions mainly decomposing by successive losses of the CO- and Cp- ligands. With MPI at 193 nm molecular ions are not detected and Cp₂Ni₂⁺ is the ion with the highest observed mass. At 248 and 351 nm only Ni⁺ appears. At 248 nm Ni⁺ is formed after absorption of four quanta. With 2, molecular ions, which decompose mainly by successive eliminations of the four CO- ligands, are formed under EI conditions. The resulting ions of composition Cp₂Fe₂(CO) _n ⁺ (n = 3–0) eliminate Fe and the Cp- ligands and finally yield Fe⁺. At 193 nm, molecular ions and Cp₂Fe₂(CO)₃⁺ appear but only with low intensity. Ions at m/z 256 are attributed to Cp(C₄H₃)Fe₂CO⁺. In addition, Cp₂Fe₂⁺, Cp₂Fe⁺ and CpFe⁺ are seen, the latter two ions being formed after absorption of two photons. With increasing laser intensity Fe⁺ appears and finally becomes predominant; this process requires absorption of three photons. Finally, at 248 and 351 nm, only Fe⁺ is observed, formed after absorption of three photons at 248 nm. With compound 3, the EI spectrum shows mainly molecular ions and a few large fragment ions. Co⁺ and Co⁺₂are not detected. MPI at 193 nm yields molecular ions formed after absorption of one photon. Both at 248 and 351 nm only Co⁺ is observed. Its formation at248 nm requires absorption of four photons. The multiphoton excitation and fragmentation pathways of these compounds at 193 nm are characterized by ionization preceding dissociation. At 248 and 351 nm fragmentation of the neutral parent compound followed by metal atom ionization is dominant.