The extremely air-sensitive 19-electron [FeICp hexaalkylbenzene)] complexes (alkyl = n-propyl, n-butyl, n-pentl) have been synthesized; variable-temperature Mössbauer data indicate a drastic decrease of the quadrupole splitting upon increasing temperature which does not correspond to a Boltzmann population of the upper Kramer's doublet due to the 'chain effect'. It is proposed that an increasing proportion of the 17-electron Fe(I) state is involved as the chain length increases. © 1995.