Ultraviolet irradiation of a rigid 7 K argon matrix containing alkali or alkaline earth metal atoms and (NO)(2) isolated from each other by one or two layers of argon forms N2O22- dianions insulated from two M+ cations by argon atoms, and visible photolysis reverses this electron-transfer process likely involving the N2O2- anion intermediate. The isolated N2O22-dianion is identified from isotopic substitution and isotopic mixtures, which show that the new 1028.5 cm(-1) metal independent absorption involves two equivalent NO subunits. DFT calculations predict a strong 1078.1 cm(-1) fundamental for the Li(NO)(2)Li molecule and isotopic frequency ratios in excellent agreement with the observed values, which provides a model for the matrix dianion system. The spectrum of solid Na2N2O2 exhibits a 1030 cm(-1) infrared band, which strongly supports the present N2O22- dianion assignment. The electrostatic stabilization of N2O22-, which is probably unstable in the gas phase, is made possible by metal cations separated by one or two insulating layers of argon in the rigid 7 K matrix.