COMPARISON OF ELECTROSPRAY MASS-SPECTROMETRY WITH OTHER SOFT IONIZATION TECHNIQUES FOR THE CHARACTERIZATION OF CATIONIC PI-HYDROCARBON ORGANOMETALLIC COMPLEXES

Electrospray (ES) mass spectrometry provides a convenient method for the characterisation of a wide range of pi-hydrocarbon complex salts of the general types [FeCp(eta-arene)]BF4 [M(CO)(3)(eta C7H7)BF4 (M = Cr, Mo, W), [Fe(CO)(2)L (eta(5)-dienyl)]BF4 (L = CO or PPh(3); dienyl = C6H7, 2-MeOC(6)H(6), or C7H9), [CpFe(CO)(3)]PF6 and [CpFe(CO)(2)(eta-C2H4)]BF4. At low skimmer voltages (20 V), principal (molecular) ions are generally the only observed species. For the arene complexes, [FeCp(eta-arene)]BF4 high skimmer voltages (80-135 V) are required before fragmentation is observed. However, for the carbonyl containing cations, moderate skimmer voltages (40-55 V) also give rise to [M-CO](+), [M-2CO](+) and [M-3CO](+) ions. Very similar behaviour is observed with phosphonium and imidazolium adducts of the type [Fe(CO)(3)(eta(4)-diene.Nuc)](+) (diene = C6H7 or C7H9; Nuc = PPh(3) or Im) and [CpFe(CO)(2)(eta(1)-C2H4.PPh(3))](+). Comparison with fast atom bombardment (FAB) and field desorption (FD) mass spectra for the same complex salts indicates that fragmentation decreases along the series FAB > ES > FD, and that ES mass spectrometry is the most convenient and informative of the three soft ionization techniques.