Studies in negative ion mass spectrometry. VIII —Electron capture by some monomeric and dimeric η 5 —cylcopentadienyl transition metal carbonyls

The negative ion mass spectra of a series of monomeric and dimeric η 5 ‐cyclopentadienyl transition metal carbonyls have been examined. The base peak in the case of the monomeric compounds (η 5 ‐C 5 H 5 )V(CO) 4 , (η 5 ‐C 5 H 5 )Mn(CO) 3 and (η 5 ‐CH 3 C 5 H 4 )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 (η 5 ‐C 5 H 5 )Co(CO) 2 , however, a molecular anion was observed as the base peak which has been formulated as [(η 3 ‐C 5 H 5 )Co(CO) 2 ] − in the light of considerations based on the rare gas rule. As expected, the dimeric molecules [(η 5 ‐C 5 H 5 )M(CO) 3 ] 2 (where M = Cr or Mo) and [(η 5 ‐C 5 H 5 )Fe(CO) 2 ] 2 (and its methyl analogue) undergo reductive cleavage of their metal‐metal bonds to give the anions [(η 5 ‐C 5 H 5 )M(CO) 3 ] − and [(η 5 ‐C 5 H 5 )Fe(CO) 2 ] − as the base peaks in their negative ion mass spectra. The dimeric nickel compound [(η 5 ‐C 5 H 5 )Ni(CO)] 2 , however, reductively decarbonylates to the [M‐CO] − radical anion as its predominant fragmentation in the gas phase. Very low abundances of [(η 5 ‐C 5 H 5 )Fe(CO) 2 ]   2 −and [(η 5 ‐CH 3 C 5 H 4 )Fe(CO) 2 ]   2 −were also observed.