STEREOCHEMISTRY OF ORGANOMETALLIC COMPOUNDS .6. ELECTRONIC AND STERIC EFFECTS IN TRICARBONYL(ALKYLBENZENE)CHROMIUMS

The n.m.r. spectra are reported for a series of tricarbonyl (alkylbenzene) chromiums, in which the alkyl groups are of variable size. The spectra of tricarbonyl(toluene-, ethylbenzene-, isopropylbenzene-, 2,4,5- trideuterioisopropylbenzene-, 2,3,4,5-tetradeuterioisopropylbenzene-, and m-di-isopropylbenzene-)chromiums are temperature variable and this variation is attributed to an equilibrium between two conformations, one preferred for electronic reasons, and the other for steric reasons. An approximate quantitative treatment is reported which leads to acceptable values for the entropy changes associated with the loss of free rotation in ethyl and isopropyl groups. The spectra of tricarbonyl-(t-butylbenzene-, 1,3-di-t-butylbenzene-, 1,4-t-butylbenzene-, and 1,3,5-tri-t-butylbenzene-)chromiums are each interpreted in terms of a single conformation, preferred for steric reasons, while the spectrum of tricarbonyl-(m-xylene)chromium is interpreted in terms of a single, electronically preferred, conformation. The spectra of tricarbonyl-(t-butylbenzene- and isopropylbenzene-) molybdenum are discussed.