The shortest metal–metal bond so far is found in complex anion 2 (CrCr 1.7397(9) Å), which is the one‐electron reduction product of thermally stable mixed‐valent complex 1 (see scheme). Experimental data unambiguously show that the electronic configuration of 1 is σ2π4δ3, and theoretical studies confirm the presence of a CrCr quintuple bond in 2. Ar=2,6‐C6H3(CH3)2.

Bubbling O into a THF solution of Co(BDPP) (1) at -90 °C generates an O adduct, Co(BDPP)(O) (3). The resonance Raman and EPR investigations reveal that 3 contains a low spin cobalt(III) ion bound to a superoxo ligand. Significantly, at -90 °C, 3 can react with 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPOH) to form a structurally characterized cobalt(III)-hydroperoxo complex, Co(BDPP)(OOH) (4) and TEMPO. Our findings show that cobalt(III)-superoxo species are capable of performing hydrogen atom abstraction processes. Such a stepwise O-activating process helps to rationalize cobalt-catalyzed aerobic oxidations and sheds light on the possible mechanism of action for Co-bleomycin.

Heating K(4)Mo(2)Cl(8) and 2 equiv of Li[RC(NDipp)(2)] (R = H, Ph; Dipp = 2,6-i-Pr(2)C(6)H(3)) in tetrahydrofuran (THF) at 60 degrees C gives two paddlewheel type quadruply bonded dimolybdenum complexes, Mo(2)(mu-Cl)[Cl(2)Li(OEt(2))][mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (1), Ph (2)). The Mo-Mo bond lengths of 1 and 2 are 2.0875(4) and 2.0756(8) A, respectively, indicating typical Mo-Mo quadruple bonds. Reduction of 1 and 2 by two electrons results in the isolation of their corresponding Mo-Mo quintuple bonded complexes, Mo(2)[mu-eta(2)-RC(N-2,6-i-Pr(2)C(6)H(3))(2)](2) (R = H (3), Ph (4)), and the Mo-Mo bond lengths dramatically decrease to 2.0187(9) A (3) and 2.0157(4) A (4), a consequence of the formation of the second delta bond and representing the shortest metal-metal bonds beyond the first row elements. The Mo-Mo quintuple bonding characters are corroborated by DFT calculations at the level of BP86/def2-TZVP and BP86/def2-TZVPP.