Chemical bonding is at the very heart of chemistry. Although main-group-element E-E' bond orders range up to triple bonds, higher formal bond orders are known between transition metals. Here we review recent developments related to the synthesis of formally quintuply bonded transition metals in coordination compounds, and their theoretical description. The quadruple bond fascinated chemists for about 40 years. Recently, a stable molecule containing a formal quintuple bond initiated a renaissance in synthesizing and understanding bonds with high bond orders. Ultrashort metal-metal distances as low as 1.73 Å are one of the outcomes. First results indicate that the relevance of these bimetallic platforms to synthetic chemistry can be addressed through quintuple-bond reactivity studies. The theoretical description of the bonding situation in molecules with extreme bond orders has only just begun.

The nature of the chemical bond is of fundamental importance, and has always fascinated scientists.[1] Metal-metal bonds are of particular interest, as bond orders greater than four are known [2,3] and are of considerable current interest.[4]The quest for the shortest metal-metal bond is strongly linked with the element chromium [2,5] and has very recently been reinitiated after the first observation of a bond order greater than four for this metal in a stable compound.[3] Soon afterwards, the shortest metal-metal bond with a chromium-chromium distance of 1.80 was observed in a dimeric chromium complex with such a high bond order.[6] Detailed studies on ArCrCrAr complexes (Ar = aryl) performed at the same time showed that such small values can be obtained for this class of compounds as well.[7] Some years ago, we started working with aminopyridinato complexes of chromium [8] and herein report the synthesis and the (electronic) structure of a bimetallic Cr I 2 complex with a drastically shortened metalmetal distance. The very short metal-metal bond of only 1.75 results from a combination of Powers concept for the stabilization of bond orders higher than four, [3,7] HeinCottons principles on the realization of extremely short metal-metal bonds with bridging anionic ligands of type XYZ, [2,9] and a minimization of additional metal-ligand interactions by optimal steric shielding (Scheme 1).The deprotonation of 1 with potassiumhydride leads to potassium [6-(2,4,6-triisopropylphenyl)pyridin-2-yl](2,4,6-trimethylphenyl)amide, which readily reacts with [CrCl 3 (thf) 3 ] affording complex 2 (Scheme 2). Compound 2 can be isolated as a green crystalline material in good yield. In the 1 H NMR spectrum, only broad signals can be observed, and magnetic susceptibility experiments show a magnetic moment m eff (300 K) = 3.2 m B . When 1 is deprotonated with BuLi and allowed to react with CrCl 2 in THF, the Cr II 2 complex 3 is obtained in good yield as a green crystalline material after removal of the solvent and subsequent extraction with toluene. The molecular structure of 3 is shown in Figure 1.[10] Compound 3 is the first Cr II complex in which the deprotonated aminopyridine has a strained bidentate coordination mode and does not act as a bridging ligand.[11]The chromium-nitrogen bond lengths clearly distinguish this compound as an amidopyridine; i.e., the anionic function of the ligand is localized on the N amido atom (N2).[12] Reduction of 4 with potassium graphite (KC 8 ) in THF, followed by Scheme 1. Shortening of the metal-metal bond by high bond order, bridging coordination of anionic ligands of type XYZ, and minimizing the additional metal-ligand interactions by steric shielding.Scheme 2. Synthesis of 2, 3, and 4 (TIP = 2,4,6-triisopropylphenyl, Mes = 2,4,6-trimethylphenyl).

Bonds are at the very heart of chemistry. Although the order of carbon–carbon bonds only extends to triple bonds, metal–metal bond orders of up to five are known for stable compounds, particularly between chromium atoms. Carbometallation and especially carboalumination reactions of carbon–carbon double and triple bonds are a well established synthetic protocol in organometallic chemistry and organic synthesis. We now extend these reactions to compounds containing chromium–chromium quintuple bonds. Analogous reactivity patterns indicate that such quintuple bonds are not as exotic as previously assumed. Yet the particularities of these reactions reflect the specific nature of the high metal–metal bond orders.

The synthesis and structure of a homobimetallic chromium complex is reported. The ligand used to stabilise the quintuply bonded metals is a sterically fine‐tuned guanidinate. A chromium–chromium bond length of 1.7293(12) Å was observed. It is the shortest metal–metal distance reported for a stable compound yet.

Herein, the ligand-based concept of shortening quintuple bonds and some of its limitations are reported. In dichromium-diguanidinato complexes, the length of the quintuple bond can be influenced by the substituent at the central carbon atom of the used ligand. The guanidinato ligand with a 2,6-dimethylpiperidine backbone was found to be the optimal ligand. The reduction of its chromium(II) chloride-ate complex gave a quintuply bonded bimetallic complex with a Cr-Cr distance of 1.7056 (12) Å. Its metal-metal distance, the shortest observed in any stable compound yet, is of essentially the same length as that of the longest alkane C-C bond (1.704 (4) Å). Both molecules, the alkane and the Cr complex, are of remarkable stability. Furthermore, an unsupported Cr(I) dimer with an effective bond order (EBO) of 1.25 between the two metal atoms, indicated by CASSCF/CASPT2 calculations, was isolated as a by-product. The formation of this by-product indicates that with a certain bulk of the guanidinato ligand, other coordination isomers become relevant. Over-reduction takes place, and a chromium-arene sandwich complex structurally related to the classic dibenzene chromium complex was observed, even when bulkier substituents are introduced at the central carbon atom of the used guanidinato ligand.