Orginally represented as 3,5,6-triphenyl-2,3,5,6-tetraazabicyclo[2.1.1]hex-1-ene. Prepn: DE 159692 (1904 to E. Merck); M. Busch, Ber. 38, 856 (1905). Corrected structure determn as meso-ionic compound: W. Baker, W. D. Ollis, Q. Rev. Chem. Soc. 11, 15 (1957); G. A. Olah, J. Inorg. Nucl. Chem. 16, 225 (1961). Mass spectrum: W. D. Ollis, C. A. Ramsden, J. Chem. Soc. Perkin Trans. 1 1974, 645. NMR study: W. Bocian et al., Magn. Reson. Chem. 33, 134 (1995). Use in determn of metals: N. Gantchev, A. Dimitrova, Mikrochim. Acta 1971, 476; T. J. Koralewski, G. A. Parker, Anal. Chim. Acta 113, 389 (1980); H. Chen et al. Acta Chim. Sin. 1989, 412. In determn of nitrate: A. Hulanicki, M. Maj, Talanta 22, 767 (1975); A. Hioki et al. Anal. Sci. 6, 757 (1990). Chemical sensor of stratospheric nitric acid: K. Meadows, V. R. Morris, Proc. NOBCChE 25, 1 (1998).