Vessally, E. “Conformational Studies on Divalent Seven-Membered Ring C6H6M (M = C, Si, Ge, Sn and Pb): DFT Calculations”. Asian Journal of Chemistry, vol. 20, no. 8, Mar. 2010, pp. 5878-82, https://asianpubs.org/index.php/ajchem/article/view/13036.