In the field of metal-metal multiple bonding, there are many factors that affect the metal-metal bond lengths. The most determinant cause is the structural configuration of complexes, [1] and this has been illustrated by the Group 6 metalmetal quadruple bonds. In the large number of tetragonal quadruply bonded complexes [M 2 X 8 ], [1] the increase in the internal twist angle between two MX 4 fragments leads to elongation of the metal-metal quadruple bond. [1,2] For example, those quadruply bonded dimolybdenum complexes with the torsion angles about the MoÀMo bond of 26-408 possess long MoÀMo bonds in the range of 2.18-2.19 , which is because a major part of the d bonding has been abolished.In contrast to the relatively mature metal-metal tripleand quadruple-bond chemistry, [1, 3] quintuple bonding is in its infancy. In contrast to the trigonal triply bonded and tetragonal quadruply bonded dinuclear complexes, the geometry of a quintuply bonded compound has been controversial. Theoretically, the most accepted configuration of a quintuple bonded complex is the type I trans-bent geometry (Scheme 1). [4] On the experimental side, the first quintuple bond, in [Cr 2 Ar' 2 ] (Ar' = 2,6-[2,6-(iPr 2 ) 2 C 6 H 3 ] 2 C 6 H 3 ) [5a] and derivatives, [5b] was reported by Power et al. It is worthy noting these remarkable complexes in fact adopt the type II geometry, although the central Cr 2 unit is stabilized by two monoanionic terphenyl ligands. Since then, many efforts have been dedicated to isolating dinuclear species featuring quintuple-bond character. For example, the groups of Theopold, [6] Tsai, [7] and Kempe [8] reported several type II quintuply bonded Cr 2 and Mo 2 complexes with extremely short metal-metal quintuple bonds spanned by two bidentate nitrogen-based ligands. The existence of the CrÀCr quintuple bond of the type II complex was recently corroborated by charge-density studies. [9] Preliminary reactivity studies on the type II complexes indicate these univalent and low-coordinate dinuclear species are reactive towards unsaturated organic functionalities and small molecules. [10,11] Compared to the many type II complexes, only one example of the type III complex, [Cr 2 {m-HC(N-2,6-Me 2 C 6 H 3 ) 2 } 3 ] À (1), was recently reported, although such type of species was proposed by Hoffmann et al. in 1979. [12] Unexpectedly, compound 1 possesses an ultrashort CrÀCr quintuple bond (1.74 ), [7a] which has also been supported by theoretical calculations. [7a, 13] The quintuply bonded Group 6 M 2 (M = Cr, Mo) complexes reported to date have very short M À M bond distances. Theoretical analysis on the type II Cr 2 complexes [6][7][8] implied that these short CrÀCr distances are associated with the steric bulk of the ligands. [13] Furthermore, computations on the quintuply bonded Group 6 model compounds M 2 X 2 (M = Cr, Mo, W; X = H, F, Cl, Br, CN, Me) revealed the MÀM bond distances vary with the structural conformation. [4f] Since the recognition of the structure of 1, we became interested in characterizin...