Ligand exchange of thermochromic, LETC, systems exhibiting a reversible change in absorbance of electromagnetic radiation as the temperature of the system is reversibly changed are described. The described LETC systems include one or more than one transition metal ion, which experiences thermally induced changes in the nature of the complexation or coordination around the transition metal ion(s) and, thereby, the system changes its ability to absorb electromagnetic radiation as the temperature changes.In accordance with one aspect of the present invention, a thermochromic system is disclosed comprising a first metal ion and a second metal ion and at least one ligand that complexes with the first metal ion to form a L.epsilon.MLC and complexes with the second metal ion to form a H.epsilon.MLC wherein an increase in temperature of the system results in a reversible net increase in its ability to absorb light energy in the 400 nm to 1400 nm range due to the ligand transferring from the first metal ion to the second metal ion.