H. Taube wrote that he found a hint of the inner-sphere electron transfer mechanism from test tube experiments. He mixed Cr2+(aq) and I2 in a test tube in order to clarify the oxidation of Cr2+(aq) and observed the change of color to the one characteristic of [Cr(H2O)6]3+ via green. The green color is due to [(H2O)5CrI]2+ which is unstable and changes to [Cr(H2O)6]3+ + I-. He assumed that this was due to the formation of a Cr-I bond before Cr(II) was oxidized by I2. Subsequently, he performed another test tube experiment using [(NH3)5CoCl]2+ as an oxidant and found that Cr2+(aq) was converted into [Cr(H2O)6]3+ via green [(H2O)5CrCl]2+. This reaction established the inner-sphere electron transfer mechanism in which a Co-Cl-Cr bridge forms between Co3+ and Cr2+ and led to the Nobel Prize in a later year.