Exocyclic disilenes

Among a number of review articles on stable disilenes published until now,6 9 the most comprehensive would be those written by Okazaki and West in 1996, which is hereafter abbreviated to review OW ,7 and by Weidenbruch in 2001.9 At the time when review OW was written, the number of known stable disilenes was 27 and the type was limited only to acyclic disilenes with rather simple substitution modes. During less than 10 years since then, the number of stable disilenes increased dramatically up to more than 70, and the types include not only acyclic disilenes but also endo- and exocyclic disilenes, bicyclic disilenes, and tetrasiladienes. Very recently, the syntheses of the first stable trisilaallene and disilyne were achieved. All these recent experimental achievements have stimulated renewed theoretical studies, and the interplay between theory and experiments has contributed to the innovation of the understanding of the bonding, structure, and reactivity of disilenes and to the development of the potentials of silicon chemistry. Since review OW appeared in a volume of Adv. Organomet. Chem. series, the present article is intended to be a supplemental review covering the studies performed during 1996-2004. 

After review OW, the molecular structures in the solid state of 13 acyclic di-silenes, 13 endo- and exocyclic disilenes, 4 silicon-based dienes, 6 disilenes with radical, cation, and anion center, and one disilyne have been determined by X-ray analysis. Although the structural characteristics of a number of these new disilenes are summarized in a recent review by Weidenbruch,9 the structural parameters of all new disilenes reported after review OW are listed in Tables I-IV. 

A (Z)-isomer of exocyclic ABSi = SiAB-type disilene, disilyldiaminodisilene 62, is obtained by a tetramerization of stable diaminosilylene 102 [Eq. (27)].33,34 Quite interestingly, 62 dissociates in solution rapidly into the corresponding silylene 103 and then slowly into the diaminosilylene 102. 

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