DOI: 10.1002/chem.201201291 X-Ray Absorption Spectroscopic, Crystallographic, Theoretical (DFT) and Chemical Evidence for a Chalcogen–Chalcogen Two Center/Three Electron Half Bond In an Unprecedented “Subselenide” Se 2 3 Ligand Shu A. Yao, [a] Kyle M. Lancaster, [b] Andreas W. Gçtz, [c] Serena DeBeer, [b, d] and John F. Berry* [a] Bond orders and oxidation states play an essential role in chemistry to help understand reactivity and physical proper- ties. [1] With advances in synthetic chemistry, some complexes with ambiguous electronic structures have been reported that challenge the concepts of oxidation states and bond orders. For example, the electronic structure of metal dithio- lene complexes was fiercely debated decades ago but these compounds are now better understood to contain redox active ligands. [2] More recently, an unusual S···S distance in a Cu 3 S 2 compound [3] has led to an animated debate about whether the compound contains a singly-bonded S 2 2 unit or two non-bonded S 2 ligands. [4] One of us proposed that the Cu 3 S 2 compound may contain a half-bonded S 2 3 , or “subsul- fide”, ligand, [5] though to date experimental support for this assignment is lacking, despite extensive investigations. [6] The possibility that dichalcogen E 2 n species could be redox non- innocent ligands suggested that a potentially large number of reported compounds may be misassigned or are poorly understood. We therefore felt that it was of the utmost im- portance to provide a definitive test of whether an E 2 3 ligand exists. Herein we present compelling experimental/theoretical evidence that the known neutral compound (Cp’Ni) 2 ACHTUNGTRENNUNG(m 2 -Se 2 ) (1, Cp’ = 1,2,3,4 tetraisopropylcyclopentadiene) [7] may be for- mulated electronically to have a half-bonded “subselenide” Se 2 3 ligand. This unprecedented assignment necessitates formulation as a mixed-valent Ni II /Ni III compound that is de- localized (average oxidation state, + 2.5); DFT calculations accord with this formulation and the compounds diamag- netism. Noteworthy is that the half-bonded (2c/3e) Se 2 3 ligand may be viewed as possessing a “net” one-electron chalcogen–chalcogen bonding interaction involving two bonding and one antibonding electrons (s 2 s* 1 ), similar to H 2  or He 2 + . Based upon the comprehensive electrochemi- cal analysis of 1 by Sitzmann and co-workers, [7] who synthe- sized and determined room-temperature X-ray crystal struc- tures of the Se and corresponding S and Te dimers, [7] we report chemical oxidation and reduction of 1, which led to the isolation and crystallographic characterization of the new one-electron oxidized [(Cp’Ni) 3 ACHTUNGTRENNUNG(Se 2 ) 2 ] + BF 4  (2) and re- duced [Cp* 2 Co]ACHTUNGTRENNUNG[(Cp 2 ’Ni) 2 ACHTUNGTRENNUNG(m 2 -Se 2 )] (3). Compound 1 was first synthesized by Sitzmann and co- workers in 2001, [7] and attracted our interest for several rea- sons. First, the reported Se···Se distance in 1, 2.85  (Scheme 1, B) is intermediate with respect to the distances expected for Ni 2 Se 2 compounds containing no Se  Se bond (e.g., 3.29  in (CpNiSePh) 2 , [8] C in Scheme 1) or a Se  Se single bond (e.g., 2.33  in (LNiSe) 2 with L = b-diketimi- nate, [9] A in Scheme 1). Second, the sulfur and tellurium an- alogues of 1 were also reported to have unusual E···E bond distances. [7] Sitzmann and co-workers were unsatisfied with their ability to describe the electronic structure of their com- pounds and concluded that, “the complexes appear to be nei- ther Ni II complexes with an E 2 2 bridge nor Ni III complexes [a] Dr. S. A. Yao, Prof. J. F. Berry Department of Chemistry, University of Wisconsin–Madison 1101 University Ave., Madison, WI 53706 (USA) E-mail : berry@chem.wisc.edu [b] Prof. K. M. Lancaster, Prof. S. DeBeer Department of Chemistry and Chemical Biology Baker Laboratory, Cornell University, Ithaca, NY 14853 (USA) [c] Dr. A. W. Gçtz San Diego Supercomputer Center University of California San Diego, La Jolla, CA 92093 (USA) [d] Prof. S. DeBeer Max-Planck-Institut für Bioanorganische Chemie Stiftstrasse 34–36, 45470 Mülheim an der Ruhr (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201201291. Supporting Informa- tion includes: synthesis procedures of 2 and 3, computational, crystal- lographic information, Ni-K edge data and full MO diagram of 1, UV/Vis-NIR spectra of 1 and 4, and EPR spectrum of 3. Scheme 1. Diagram of dichalcogen redox chemistry and structures of three Ni 2 Se 2 complexes with different Se···Se distances. Chem. Eur. J. 2012, 00,0–0  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& COMMUNICATION