The purpose of the experiment is to synthesize multiple cobalt complexes with various ligands and explain the ligand field transitions of these complexes using Ultraviolet-Visible (UV-Vis) spectroscopy. The experiment provides insight into specific colors of certain molecular species and respective bonding information.
Transition elements are defined by the IUPAC as atoms that have an incomplete d-subshell, most commonly referring to the d-block.1 When transition metals are not bonded to any other molecule, the d-orbitals are degenerate.2 However, when the metal starts bonding with other ligands, effects on the electrons attached to the metal are induced. Specifically, the d-orbitals split, obtain different energy …show more content…
This was combined with a solution of NaHCO3 (17.0001 g, 269 mmol), water (50.0 mL), and 30% hydrogen peroxide (0.5 mL) in a large flask. The mixture was stirred for approximately 2 minutes, until gas evolution subsided. Approximately 1 mL of the solution was diluted to 25 mL prior to obtaining the UV-Vis spectrum. The rest of the solution was saved for the preparation of [Co(OH2)6]3+. The UV-Vis spectrum was then obtained.
Synthesis of [Co(OH2)6]3+, hexaaquacobalt(III)
[Co(CO3)3]3- (20.0 mL) was slowly added to HNO3 (80.2 mL, 4.0 M), while being carefully monitored to avoid the reaction mixture from bubbling over the flask, in which CO2 gas is produced. After obtaining the UV-Vis spectrum, the solution was saved for the preparation of [Co(phen)3]3+.
Synthesis of [Co(phen)3]3+, tris(1,10-phenanthroline)cobalt(III)
A mixture of 1,10-phenanthroline (0.1152 g, 0.5 mmol) and the [Co(OH2)6]3+ solution (10.4 mL, 0.25 mmol) was prepared. Although the ratio between 1,10-phenanthroline and cobalt(III) was 2:1, the solution was expected to be 3:1. The mixture was added to 3% hydrogen peroxide (11.0 mL) and stirred for 5 minutes at room temperature. The UV-Vis spectrum was then …show more content…
The ring strain and small angles of the ligand bound to cobalt generally do not demonstrate bidentate properties. The driving force for gas evolution is Le Châtelier’s principle. The system tries to counteract and balance the effects of the reactants by releasing gas (CO2). In Figure 3, the lowest transition is at 439.2 nm, which is slightly lower than the range of 476.2-625 nm as per Maccoll et al.’s study.6 The CO32- ligand absorbs light with the longest wavelength in this experiment, so the energy jump between molecular orbitals is very small in this complex. This is consistent with the expected