Unprecedented N-N coupling of two nitrosyl ligands on dinuclear complexes was discovered, which is proposed as a critical step in the bacterial NO reductase (NOR). The N-N coupled complex (TpRu)(2)((sic)-CI)((sic)-pz){(sic)-k(2)-N(=O)-N(=O)} (2a) ((Tp = BH(pyrazol-1-yl) 3) was prepared from the reaction of TpRuCl(2)(NO) (1) with pyrazole in the presence of Et3N. The curious N-N bond distance was X-ray crystallographically determined to be 1.861(3) angstrom for the N-N coupled analogue with a 4-methylpyrazolato bridge, and was confirmed by DFT calculation. The N-N bond of 2a was cleaved by oxidation to give [{TpRu(NO)}(2)((sic)-CI)((sic)-pz)](BF4)(2) (4a center dot(BF4)(2), which on reduction reformed 2a. It is interesting to note that the N-N coupled complex 2a was also transformed into the oxo-bridged dinuclear species Sa with the evolution of N2O. These findings would provide valuable information regarding the mechanism of NO reduction to N2O by NOR.