Carbonyl, carboxylate, Fe-C, Fe-N and Fe-S stretching bands of [Fe(cysteine) 2 (CO) 2 ]Y 2 (Y = Li + , Na + , K + , Bu 4 N + ) have been assigned on solid samples. Analysis of the C-O stretching region (2100-1900 cm - 1 ) is indicative of the presence of four geometrical isomers, as confirmed by molecular mechanics calculations. Trans isomers appear to be more stable. The isomeric composition of the solids is strongly dependent on the nature of the counter cations in the precipitate. The specific influence of Li + is described.