Combined Fourier transform infrared and Raman spectroscopic studies of some carbonyl-diphosphine-indenyl-iron cations

The Fourier transform infrared and Raman spectra of the cations [η⁵-C₉H₇Fe(CO)_n dppa]⁺ (n = 1, 2; dppa=bisdiphenylphosphinoalkane, where alkane=methane, ethane, butane, hexane and octane) and [{η⁵-C₉H₇Fe(CO)₂}₂-μ-dppa]²⁺ indicate that the alkyl chain lengths have effects on the structures of the bidentate cations resulting in increased back-donation to carbonyl groups as the chain length increases. In contrast the alkyl chain lengths have no similar effects in the unidentate mononuclear and bridged cations.