The integrated intensities per carbonyl group («specific intensities») of the carbonyl bands of the compounds investigated are from three to seven times higher than those of saturated ketones. Cr(CO) 6 , Mo(CO) 6 and W(CO) 6 have a higher, Ni(CO) 4 has about the same specific intensity as Fe(CO) 5 . Substitution of one or two carbonyls in Fe(CO) 5 by π‐bonded hydrocarbons does not alter the specific intensity of the remaining CO‐groups, but halogen substitution lowers it by a large amount. On a frequency‐intensity diagram the points for the iron compounds lie on a straight line, indicating that influences which lower the intensity raise the frequency and vice versa . Bridging carbonyls have lower intensities than terminal ones. From the measurements on Fe 3 (CO) 12 , it is concluded that this compound does not contain bridging carbonyls and a structure in accord with previous X‐ray work is proposed.
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A 1962 study studied this question.