Four-center, two-electron bonds

Schleyer, P. v. R., Bremer, M., Schotz, K., Kausch, M. Schindler, M. 1987 Four-center two-electron bonding in a tetrahedral topology. Experimental realization of three-dimensional homoaromaticity in the l,3-dehydro-5,7-adamantanediyl dication. Angew. Chem. Int. Ed. Engl. 26, 761 763. 

The energy level diagram shown in Fig. 13 along with the six electrons from the Lewis structure shown earlier shows that the lower three BMO s are filled. These MO s are four-center two-electron bonds and are therefore delocalized over the whole molecule. 

The formulas of 37-39 show the substitution of three carbonyls by a conjugated diene. Due to the fact that dienes are four-electron donors, the complexes are electron-deficient species. The electron deficiency is reduced by formation of C—H—Cr three-center, two-electron bonds, previously found for a number of other complexes (43). When 2-methyl-5-isopropyl-l,3-cyclohexadiene is used as a potential ligand, the expected complexes 37r and 39r are not obtained, but instead complexes with the isomeric l-methyl-4-isopropyl-l,3-cyclohexadiene group (37r and 39r ) are formed [Eq. (19)]. Similarly, lk and 11 form not only the corresponding complexes 39k and 391, but also the isomeric complexes with (Z)-l,3-hexadiene (39k ) and with (Z)-2-methyl-1,3-pentadiene (391 ). 

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