The occurrence and representation of three-centre two-electron bonds in covalent inorganic compounds

Abstract

Although compounds that feature 3-centre 2-electron (3c-2e) bonds are well known, there has been no previous effort to classify the interactions according to the number of electrons that each atom contributes to the bond, in a manner analogous to the classification of 2-centre 2-electron (2c-2e) bonds as either normal covalent or dative covalent. This article provides an extension to the Covalent Bond Classification (CBC) method by categorizing 3c-2e interactions according to whether (i) the two electrons are provided by one or by two atoms and (ii) the central bridging atom provides two, one, or zero electrons. Class I 3c-2e bonds are defined as those in which two atoms each contribute one electron to the 3-centre orbital, while Class II 3c-2e bonds are defined as systems in which the pair of electrons are provided by a single atom. Class I and Class II 3c-2e interactions can be denoted by structure-bonding representations that employ the "half-arrow" notation, which also provides a convenient means to determine the electron count at a metal centre. In contrast to other methods of electron counting, this approach provides a means to predict metal-metal bond orders that are in accord with theory. For example, compounds that feature symmetrically bridging carbonyl ligands do not necessarily have to be described as "ketone derivatives" because carbon monoxide can also serve as an electron pair donor to two metal centres. This bonding description also provides a simple means to rationalize the theoretical predictions of the absence of M-M bonds in molecules such as Fe(2)(CO)(9) and [CpFe(CO)(2)](2), which are widely misrepresented in textbooks as possessing M-M bonds.

Methyl Complexes of the Transition Metals

“…Some excellent reviews and book chapters have focused on this topic, 143 some considering the metal-metal bonding of the bridging methyl compounds and the nature of the bridging ligands as threecentre two-electron bonds. 9,144 A search in the Cambridge Structural Database for bimetallic transition metal complexes with bridging methyl ligands produced only 75 results, a very small number compared to the more than five thousand entries retrieved for metal methyl (TM-CH3) compounds. Most compounds feature early transition metals, for example, there are at least 19 di-yttrium species, 7 compound containing scandium and with 11 zirconium, whereas there are no examples with silver and only one example with gold which has already been described.…”

“…8b-h Relevant theoretical and structural aspects have also been highlighted within severalhomobimetallic species with bridging methyl ligands synthesized recently. 9,10 Aside from main group methyl compounds, including LiMe, Mg(Me)X, ZnMe2, or AlMe3, of great fundamental and practical relevance, [1][2][3][4][5][6] methyl complexes of transition metals can be arguably considered the most important family of compounds with σ-M-C bonds. 7 They have led to major breakthroughs in organometallic chemistry, from the isolation of the first methylidene complex 11 to the low temperature characterization of the first σ-CH4 complex.…”

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

“…Thebonding in anti-3C + resembles anti-2C + ( Figure S8) with slightly less bond order caused by the steric and electronic effects.C alculations indicate an absence of Co-Co bond in trans-3C + ( Figure S9) despite spin density is largely localized on two cobalt atoms. [16] We have described the isolation, characterization and crystal structures of ac lass of dicobalt metalloradicals (1C…”

Didáctica para la Enseñanza del Concepto de Valencia Química

“…En este sentido, este estudio propone el mentefacto como una didáctica de enseñanza en clases que permita integrar el concepto de valencia secundaria al concepto tradicionalmente enseñado. Algunas investigaciones ya se han realizado vinculado instrumentos didácticos para la enseñanza de conceptos de la ciencias básicas (Carneiro y Dal Farra, 2011;Latasaet al, 2012;Lorenzo et al, 2011) y, particularmente, en la enseñanza de la química (Green et al, 2012;Simõeset al, 2013;Vega et al, 2014;Green y Parkin, 2014).…”