Ionization and appearance potential measurements in arene chromium tricarbonyls

The ionisation potentials and the appearance potentials for ions arising from the loss of carbonyl groups as well as certain other low-energy fragmentation ions have been measured for a series of substituted arene chromium tricarbonyls. The values obtained are found to correlate very well, for the loss of one and two carbonyl groups, with both the CO force constant k and the Hammett σ_p function and substituent on the benzene ring. The appearance potentials of the ions arising by the loss of three carbonyl groups do not exhibit a high correlation. Arene⁺ ions are found to occur at energies intermediate between those for the loss of two and three carbonyl groups.The various effects observed are discussed in terms of a model which involves definite contributions from both σ- and π- bonding of carbonyl groups in the gaseous cations.