Henry Taube created a solution of \(ce{Cr^3+_{aq}}\), which is blue in color. He added \(\ce{I2}\) and observed a color change to the hallmark green color of Cr(III). The green color was a result of oxidation to form \( \ce{[Cr(H2O)5I]^2+}\). Taube figured that for this transformation to occur, that a Cr-I bond must form prior to Cr(II) being oxidized by I 2 . Subsequently, he performed another experiment using [(NH 3 ) 5 CoCl] 2+ as an oxidant and found that Cr 2 + (aq) was converted into [Cr(H 2 O) 6 ] 3+ via a green [(H 2 O) 5 CrCl] 2+ . This reaction established the inner-sphere electron transfer mechanism in which a Co-Cl-Cr bridge forms between Co 3 + and Cr 2 + and led to the Nobel Prize in a later year. Redox reactions The oxidation number of the central metal in a transition-metal compound can vary. Namely, the oxidation state of a compound is changeable by red uction/ ox idation (redox) reactions in which electrons are transfered to one species to another. As a consequence of this, the bond distance and the bond angle between the metal and coordinating elements, or between metals, change, and at times the whole structure of a complex can be distorted remarkably or the compound may even decompose. The reactions of a metal compound with various reducing or oxidizing agents are also very important from the viewpoint of synthetic chemistry. Especially, reduction reactions are used in the preparation of organometallic compounds, such as metal carbonyls or cluster compounds. Meanwhile, the study of electron transfer between complexes, especially the redox reactions of transition metal complexes, has developed. Taube won the Nobel Prize (1983) for the study of electron transfer reactions in transition metal complexes, classifying such reactions into two mechanisms. The mechanism of electron transfer in which a bridging ligand is shared between two metals is called the inner-sphere mechanism , and the one involving a direct transfer of electrons between two metals without a bridging ligand is called the outer-sphere mechanism .