The conceptually simple mixed-valent diiron compound (NEt(4))(5)[(NC)(5)Fe(mu-tz)Fe(CN)(5)] with the 1,2,4,5-tetrazine (tz) bridging ligand was obtained as a thermally and air-stable material that displays large and highly variable electrochemical comproportionation constants between about 10(8) (in water) and 10(19.0) (in acetonitrile). Strong metal-metal interaction is also evident from spectroscopic results obtained for the solid and for the dissolved species. The rather intense intervalence charge-transfer band occurs around 2400 nm; infrared and Mössbauer spectra reveal the high spectroscopic symmetry of the system according to an (Fe(2.5))(2) formulation. DFT calculations on the [(NC)(5)Fe(mu-tz)Fe(CN)(5)](6-) ion confirm the presence of very low-lying pi(tz) and high-lying d(Fe) orbitals.