The Arsenic-Arsenlc Double Bond Revisited Approximately 10 years ago several well-known organic arsenicals were commonly depicted as possessing arsenicarsenic double hpnds (the srsena group) in spite of firm structural evidence to the contrary. This led to items appearing in print such as "Mythical Bond Lives" (I) and "Textbook Errors, 127: The Structure of Salvarsan and the Arsenic-Arsenic Double Bond" (2). These attempts to clarify the situation emphasized that, in cases where the arseno group had been depicted, the actual structures were either cyclopolyarsines or polyarsines. In the last few years the status of the arsenogroup has undergone a complete change. Reports of the syntheses of and symmetrical (4) (R"-As=As-R") examples appeared virtually simultaneousunsymmetrical (3) (R-As=As-R') ly. The key to the stability of these arseno groups apparently lies in the size of substituents, with bulky suhstituents preventing cyclization or polymerization. Bulky substituents that have been used are R = 2,4.6-(t-Bu)&H~. R' = CH(SiMe3)2,and R" = C(SiMe&. Interesting features of the arsenic-arsenic double bond include a bond length (2.224-2.245 A) considerably shorter than any previously reported As-As value and several absorption hands in the UV-visible region of the spectrum (5). The subject of double bonding between elements beyond the first row of the periodic table has been reviewed recently. (6,
n.
Literature Cited I. Lsyton,A. J. Chrm. Br. 1974.10,269. 2. Levinmn. A. S. J. Chcm. Educ. 1977,54,98. 3. Cowley, A. H.; Lmeh, J. G.; Narmsn. N. C.: Pakulski, M. J. Am. Chem. Soc. 198L105.5506. 4. Courot, C.: Elcudie,J.; Madauie, Y.; Ranaivonjatovo, H.;WoU,J-G. Telmhed. Left. 1983,24,2769. 6. C0wicy.A. H.; Kilduff, J. E.;Laxh. J. G.: Mehrotra. S. K.; Norman. N. C.: Pakuiski, M.; Whittlesey, B.R.: Atwood, J. L.:Hunter, W. F. Inorg. Chem. 1984.23, "m" 6. Cuwiey, A. H.Acc. Chem.Res. 1984,17,386. 7. Cowley. A. H. Poiykdddn 1384.3.389.
Alfred S. Levlnson Portland State University Ponland. OR 97207
Volume 64
Number 5
May 1987
407
