An enzyme complex containing an iron-iron cluster found in species such as the photosynthetic bacterium Rhodobacter capsulatus. It is composed of two main subunits, dinitrogenase and nitrogenase reductase. Dinitrogenase, the iron-iron containing subunit, has an alpha1-beta2 or alpha2-beta2 structure, and the nitrogenase reductase subunit is a homodimer. Functions in the catalysis of the formation of oxidized ferredoxin and ammonia from reduced ferredoxin and nitrogen.