Although quadruple bonding in transition-metal chemistry has been considered a thoroughly studied area, [1a] the concept of multiple bonding [1b] was reinvigorated in 2005 by the seminal discovery of the first CrÀCr quintuple bond in the isolable dimeric chromium compound Ar'CrCrAr' (Ar' = 2,6-(2,6-iPr 2 C 6 H 3 -) 2 C 6 H 3 ) by Power and co-workers. [2] Since then, the structures of several Group 6 homobimetallic compounds with very short CrÀCr (1.73-1.75 ) and MoÀMo (2.02 ) quintuple bonds have been characterized. [3] All these remarkable quintuple-bonded bimetal units are supported by either C-or N-based bridging ligands. Based on their structures, these quintuple-bonded dinuclear compounds can be simply classified into two types as illustrated in Figure 1. The existence of the type I quintuple bond was recently corroborated by experiments, [4a] and the bonding paradigms of both types were realized by theoretical investigations. [4] Preliminary reactivity studies on the type I complexes show that they are reactive towards the activation of small molecules and display interesting complexation with olefins and alkynes. [5] Up to now, both type I and II compounds have been exclusively synthesized by a procedure analogous to the Wurtz reductive coupling reaction of the corresponding chloride coordinated precursors. [2, 3] The previously reported quintuple-bonded dichromium examples were obtained by alkali metal reduction of the mononuclear [LCrCl 2 -(THF) 2 ] [3c,e] or dimeric complexes [LCr(m-Cl)] 2[3] (L = monodentate or bidentate ligand). It should be noted that all these precursors lack Cr À Cr bonding. Besides, we have recently demonstrated that the metal-metal quintuple and quadruple bond can be constructed from the corresponding quadruple and triple bond, respectively. For example, the d bonds in the quintuple-bonded species [Mo 2 {m-h 2 -RC(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] (R = H, Ph) [3h] and quadruple-bonded complex [Mo 2 {m-h 2 -Me 2 Si(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] [6] are formed by alkali metal reduction of the corresponding chloride-coordinated quadrupleand triple-bonded species, respectively. However, the formation mechanism of the metalÀmetal quintuple bonds has not been investigated. To this end, continuing our exploration in the field of quintuple-bond chemistry, we herein report the construction of a complex with a Cr À Cr quintuple bond by two subsequent one-electron-reduction steps from a halidefree homo-divalent dichromium complex to a mixed-valent intermediate (Cr I , Cr II ), and then to the final quintuplebonded product. Structural characterization of these dichromium compounds is important to shed light on the formation mechanism of the metal-metal quintuple bonds. Moreover, the metal À metal quadruple bonds can be dramatically elongated by intramolecular axial coordination, but such an interaction in the quintuple-bonding system has not been investigated. We report herein that the CrÀCr quintuple bond can be readily cleaved by disproportionation induced by intramolecular axial coo...