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The  13 C NMR parameters of several cyclobutabenzenes ( 1a - 1e ) and their corresponding complexes (cyclobutabenzene)M(CO) 3  (M = Cr, Mo, W, 2 - 4 ) are presented and discussed. Two cyclobutabenzenes 1b and 1d and their chromium tricarbonyl complexes 2b and 2d have been examined in greater detail: the signal assignments have been confirmed by 2 D-INADEQUATE and the one bond coupling constants  1 J( 13 C , 13 C) measured at natural abundance using 1 D-INADEQUATE . Although the results provide no insight into why the chromium complex of 1,2 - dioxocyclobutabenzene (1e) is much more reactive at the keto groups than 1e itself, they do reveal a weak but not negligible alternation of the carbon s-character in the C - C bonds of the 6-membered rings both in the free cyclobutabenzenes and in the complexes, thus providing evidence for slight bond fixation (Mills-Nixon effect).

Bonding in Cyclobutabenzene Chromium Tricarbonyl Complexes Studied by 13 C NMR. Evidence of π-Bond Localization from 1 J( 13 C, 13 C)