Studies in negative ion mass spectrometry. VIII—Electron capture by some monomeric and dimeric η5—cylcopentadienyl transition metal carbonyls

Abstract

The negative ion mass spectra of a series of monomeric and dimeric η5‐cyclopentadienyl transition metal carbonyls have been examined. The base peak in the case of the monomeric compounds (η5‐C5H5)V(CO)4, (η5‐C5H5)Mn(CO)3 and (η5‐CH3C5H4)Mn(CO)3 arises from a reductive decarbonylation of the parent molecule—the resulting radical anion [M–CO]− is formally isoelectronic with the molecular cations [M]− observed in the positive ion mass spectra of these compounds and subsequently undergoes successive decarbonylations to the ‘aromatic’ cyclopentadienyl anions. For the compound (η5‐C5H5)Co(CO)2, however, a molecular anion was observed as the base peak which has been formulated as [(η3‐C5H5)Co(CO)2]− in the light of considerations based on the rare gas rule. As expected, the dimeric molecules [(η5‐C5H5)M(CO)3]2 (where M = Cr or Mo) and [(η5‐C5H5)Fe(CO)2]2 (and its methyl analogue) undergo reductive cleavage of their metal‐metal bonds to give the anions [(η5‐C5H5)M(CO)3]− and [(η5‐C5H5)Fe(CO)2]− as the base peaks in their negative ion mass spectra. The dimeric nickel compound [(η5‐C5H5)Ni(CO)]2, however, reductively decarbonylates to the [M‐CO]− radical anion as its predominant fragmentation in the gas phase. Very low abundances of [(η5‐C5H5)Fe(CO)2] 2− and [(η5‐CH3C5H4)Fe(CO)2] 2− were also observed. Show more