INTRODUCTION. On the basis of the ’Reactive Fragment Rypothesis’ proposed by Prof. Lu Jiaxi in 1981 for the active center of the nitrogenase, the IR spectra of some linear Mo-Fe-S clusters so-called ’fragment’ compounds, have been studied and the empirical assignments have been made INTRODUCTION. On the basis of the ’Reactive Fragment Rypothesis’ proposed by Prof. Lu Jiaxi in 1981 for the active center of the nitrogenase, the IR spectra of some linear Mo-Fe-S clusters so-called ’fragment’ compounds, have been studied and the empirical assignments have been made