STRUCTURAL LESSONS FROM MAIN-GROUP METALLOCENES

The metallocenes of the Group 2 and Group 14 elements display a higher degree of structural and chemical similarity than would be expected based on the differences in their electronic configurations alone. A comparison of their structures suggests that the interplanar ring angles in both are primarily determined by the size and shape of the cyclopentadienyl ligands; the two additional valence electrons in the Group 14 compounds exert little stereochemical influence. Although the two metallocene families display differences in reactivity, the presence or absence of metal valence electrons is not always a useful criterion for distinguishing between them. In Group 14 metallocenes, the electrons are not efficient donors to electrophiles, and sufficiently bulky cyclopentadienyl rings can interfere with the extent of redox reactions. Examination of main-group metallocene melting points reveals a trend based on the symmetry and flexibility of the cyclopentadienyl ring. In rigorously monomeric species, compounds with longer metal-carbon bonds have lower melting points.