In the last two decades, the chemistry of coordination compounds has made significant progress due to the increasing application of modern physical-chemical investigation methods. As an example, one can refer to the experimental and theoretical study of the light absorption properties of complex compounds, which has led to a fundamental clarification of the problem of color and constitution. Additionally, the equilibria in solutions of complex ions—particularly in aqueous solutions—have been systematically studied. The various methods for investigating complex formation in solution now represent an important specialized field of coordination chemistry. Significantly promoted by the foundational work of J. Bjerrum, especially through his dissertation "Metal ammine formation in aqueous solution," published in 1941, numerous complexes have since been investigated.