1. For three-membered rings with one Si, Ge, Sn, or Pb, see: E. Roskamp, C. Roskamp, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven (Eds.), Comprehensive Heterocyclic Chemistry II, Vol. 1A, Pergamon, Oxford, 1996, pp. 305–331. For three-membered rings with two heteroatoms including Se or Te, see: N. Tokitoh, N. Choi, W. Ando, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven (Eds.), Comprehensive Heterocyclic Chemistry II, Vol. 1A, Pergamon, Oxford, 1996, pp. 457–468. For four-membered rings with one Si, Ge, Sn, or Pb, see: E. Lukevics, O. Pudova, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven (Eds.), Comprehensive Heterocyclic Chemistry II, Vol. 1B, Pergamon, Oxford, 1996, pp. 867–886. For four-membered rings with two heteroatoms including Si to Pb, see: R.T. Conlin, T.P. Meagher, in: A.R. Katritzky, C.W. Rees, E.F.V. Scriven (Eds.), Comprehensive Heterocyclic Chemistry II, Vol. 1B, Pergamon, Oxford, 1996, pp. 1175–1187.
2. For cyclotrisilenes, see: (a) T. Iwamoto, C. Kabuto, M. Kira, J. Am. Chem. Soc. 121 (1999) 886.
3. (b) M. Ichinohe, T. Matsuno, A. Sekiguchi, Angew. Chem. Int. Ed. 38 (1999) 2194. For cyclotrigermenes, see: A. Sekiguchi, N. Fukaya, M. Ichinohe, N. Takagi, S. Nagase, J. Am. Chem. Soc. 121 (1999) 11587. For cyclotetrasilene, see: M. Kira, T. Iwamoto, C. Kabuto, J. Am. Chem. Soc. 118 (1996) 10303.
4. For hypercoordinate silicon species, see: C. Chuit, R.J.P. Corriu, C. Reyé, in: K.-y. Akiba (Ed.), Chemistry of Hypervalent Compounds, Wiley-VCH, New York, 1999 (Chapter 4).
5. (a) M.L.P. Silva, J.M. Riveros, J. Mass Spectrom. 30 (1995) 733.