Iodine (I 2 ) as a Janus-Faced Ligand in Organometallics Andrey Yu. Rogachev and Roald Hoffmann* Baker Laboratory, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States * S Supporting Information ABSTRACT: The four known diiodine complexes have distinct geometries. These turn out, as we demonstrate by a bonding analysis, to be a direct consequence of diiodine acting as an acceptor in one set, the van Koten complexes, and as a donor in the Cotton, Dikarev, and Petrukhina extended structure. The primary analytical tool utilized is perturbation theory within the natural bond orbital (NBO) framework, supported by an energy decomposition analysis. The study begins by delineating the difference between canonical molecular orbitals (MOs) and NBOs. When iodine acts as an acceptor, bonding collinearly in the axial position of a square-planar d 8 Pt(II) complex, the dominant contributor to the bonding is a σ*(I-I) orbital as the acceptor orbital, while a mainly d z 2 orbital centered on the metal center is the corresponding donor. That this kind of bonding is characteristic of axial bonding in d 8 complexes was supported by model calculations with incoming donors and acceptors, NH 3 and BH 3 . In contrast, the distinct “bent” coordination of the I 2 bound at the axial position of the [Rh 2 (O 2 CCF 3 ) 4 ] paddle-wheel complex is associated with a dominant donation from a p-type lone pair localized on one of two iodine atoms, the σ*(Rh-Rh) antibonding orbital of the metal complex acting as an acceptor orbital. We check the donor capabilities of I 2 in some hypothetical complexes with Lewis acids, H + , AlCl 3 , B(CF 3 ) 3 . Also, we look at the weakly bound donor-acceptor couple [(I 2 )·(I 2 )]. We explore the reasons for the paucity of I 2 complexes and propose candidates for synthesis. ■ INTRODUCTION Bristling with Lewis base functionality, their lone pairs, one would expect that dihalogens (X 2 , X = F, Cl, Br, I) would be excellent ligands in transition metal complexes. Yet such compounds are most rare. 1 Also rare, but somewhat more abundant, are metal complexes in which a ligand acts as an acceptor, a Lewis acid, and not a donor. One might think these two kinds of oddity would have no intersection, but they do. We tell here the remarkable story of I 2 , a ligand that bridges these two rare categories. In two of the few well-characterized transition metal complexes of dihalogens, 1 and 2 (Figure 1a,b), the bonding is, as we will show, completely different. In one of them, the van Koten compounds, 2 I 2 bonds predominantly as an acceptor, while in others, as a donor, 3 with well-delineated stereochemical consequences. An extensive literature search revealed only two further crystallographically characterized organometallic complexes of I 2 (Figure 1c,d), 4,5 one a very recent example. In both of them, iodine bonds as an acceptor, as in the van Koten compounds. 2 The description of a ligand as a donor or an acceptor has always had attached to it a degree of ambiguity. Consider the protonation of a transition metal complex. There are two extreme perspectives of what happens on diprotonation of, for example, Fe(CO) 4 2- : one is that a Lewis acid, H + , protonates the metal-based lone pair of a tetrahedral d 10 complex; the other is that two electrons are transferred on bonding (time and space unspecified) to H + , oxidizing the metal, making the ligand H - , so that one has an octahedral d 6 Fe(II) complex. The truth is in between, of course, and the geometry of Fe(CO) 4 H 2 is partway between a tetrahedral and octahedral one. 6 Putting such clear ambiguities of oxidation and reduction aside, they would exist for super Lewis acids (BR 3 ) and Lewis bases (ER 3 - , E = group 14) alike, one is left with a handful of cases of ligands that bind in a primarily acceptor fashion to a transition metal. The work of the Eisenstein group on SnH 3 coordination 7 as well as recent studies of the Gabbai ̈ group, 8 point to this kind of bonding, as does an older organometallic story, that of SO 2 complexes. 9 This small molecule can bond as a donor or an acceptor, with different geometrical con- sequences. The SO 2 complexes form a distinct parallel to the I 2 bonding we describe. The van Koten compound is a relatively uncommon 5- coordinate Pt(II) complex. While the 16-electron nature of the Pt(II) complex in principle allows coordination of a fifth ligand, the linear end-on coordination of diatomic iodine, occupying the fifth axial position (Scheme 1A), does not “follow” in its directionality the frontier orbital lone pair(s) of I atoms, were one to assume I 2 is a donor here. The HOMO of I 2 is a π* Received: December 25, 2012 Published: February 5, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 3262 dx.doi.org/10.1021/ja312584u | J. Am. Chem. Soc. 2013, 135, 3262-3275