Test tube experiments
Easy chemical or biological reactions performed in test tubes are sometimes called test tube experiments. Solutions in test tubes are mixed at room temperature in air and the mixture is shaken to observe a color change or formation of precipitates and the results of the reactions are speculated on. University professors occasionally attempt these sorts of experiments. Although easy, these simple experiments show only the effects of visible light absorption and solubility. However, since even great discoveries can be born from such experiments, they should not be dismissed.
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.
The oxidation number of the central metal in a transition-metal compound can vary in a few steps from low to high. Namely, the oxidation state of a compound is changeable by redox reactions. 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.