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Article

The Many Ways To Have a Quintuple Bond

Supporting Info

Gabriel Merino,*^‡ Kelling J. Donald,^†^§ Jason S. D'Acchioli,^# and Roald Hoffmann*^§

Contribution from the Facultad de Qumica, Universidad de Guanajuato, Noria Alta s/n CP 36050, Guanajuato, Gto. Mxico, Department of Chemistry & Chemical Biology, Cornell University, Ithaca, New York 14853, and Department of Chemistry, University of WisconsinStevens Point, Stevens Point, Wisconsin 54481

J. Am. Chem. Soc., 2007, 129 (49), pp 15295–15302

DOI: 10.1021/ja075454b

Publication Date (Web): November 16, 2007

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

‡

Universidad de Guanajuato.

†

Current address: Department of Chemistry, Gottwald Center for the Sciences, University of Richmond, Richmond, VA 23173.

Cornell University.

University of Wisconsin−Stevens Point.

, gmerino@quijote.ugto.mx, rh34@cornell.edu

Abstract

The existence and persistence of five-fold (quintuple) bonding in isomers of model RMMR molecules of quite different geometry are examined theoretically. The molecules studied are RMMR, with R = H, F, Cl, Br, CN, and CH₃; M = Cr, Mo, and W. The potential energy surface of these molecules is quite complex, containing two, three, even four local minima. The structural preferences in these molecules are rationalized, and electronic factors responsible for these preferences are elucidated. The linear geometry is always a minimum, but almost never the global minimum; there is a definite preference in RMMR for either a trans-bent conformation or perturbations of the trans-bent isomer with at least one of the R groups in a bridging position about the MM bond. The potential energy surface of these RMMR molecules is relatively flat, the lowest energy conformation being that which for a given molecule attains the best compromise between maximization of the MM bonding and minimization of orbital interactions that are MR antibonding. A surprising low-symmetry C_s structure is identified, which along with the trans-bent isomer is one of the two most popular choices for the global minimum. Regardless of what isomer of the RMMR molecule is preferred, the MM quintuple bond persists.

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