Question

A The complex ion \(\left[\mathrm{Co}\left(\mathrm{CO}_{3}\right)_{3}\right]^{3-},\) an octahedral complex with bidentate carbonate ions as ligands, has one absorption in the visible region of the spectrum at \(640 \mathrm{nm} .\) From this information, (a) Predict the color of this complex, and explain your reasoning. (b) Is the carbonate ion a weak- or strong-field ligand? (c) Predict whether \(\left[\mathrm{Co}\left(\mathrm{CO}_{3}\right)_{3}\right]^{3-}\) will be paramagnetic or diamagnetic.

Short answer

(a) The complex appears green. (b) Carbonate is a weak-field ligand. (c) Complex is paramagnetic.

Step-by-step solution

Understanding Ligand Color Absorption

The color observed in a complex is the complementary color to the color light it absorbs. Here, the ligand absorbs light at 640 nm, which corresponds to the red region of the spectrum.

Determine the Complex Ion Color

Since the complex absorbs red light, the color of the complex will be the complementary color to red, which is green.

Evaluate Ligand Field Strength

The strength of a ligand field is related to the wavelength of light absorbed. Longer wavelengths (like 640 nm) correspond to weaker field ligands. Therefore, the carbonate ion is likely a weak-field ligand.

Assess the Magnetic Properties

Co(III) in an octahedral complex with weak-field ligands will have unpaired electrons due to a high-spin configuration. This leads to the complex being paramagnetic.

Key concepts

Color of Complexes

In coordination chemistry, the beautiful array of colors observed in complex ions derives from the absorption of specific wavelengths of light. Complexes, like \([\mathrm{Co}(\mathrm{CO}_3)_3]^{3-}\), absorb particular colors in the visible spectrum, and we perceive the complementary color. For example, this complex absorbs light at 640 nm, which is in the red region of the spectrum. Consequently, its complementary color is green, explaining why \([\mathrm{Co}(\mathrm{CO}_3)_3]^{3-}\) appears green.

• When a complex absorbs red light (640 nm), it will appear green.
• The color wheel concept helps in predicting the observed color by compensating for the absorbed spectrum.
• The exact color depends on factors like the nature of the metal center and ligands involved.

Ligand Field Theory

Ligand Field Theory (LFT) helps understand how ligands affect the distribution of electrons around a metal ion, and subsequently, their absorption of light. In our context, the carbonate ion influences the strength of the ligand field in \[\mathrm{Co}(\mathrm{CO}_3)_3\]^{3-}\ by its position as a weak-field ligand. Since it absorbs relatively long wavelength light (640 nm), it indicates a weaker ligand field and a smaller energy gap \((\Delta)\) between split d-orbitals.

• Weak-field ligands, like carbonate, cause smaller splitting and energy difference in d-orbitals.
• The energy difference \(\Delta\) correlates with absorbed light; longer wavelength means weaker field.
• Ligand strength affects not only color but also magnetic properties of the complex.

Paramagnetism and Diamagnetism

Magnetic properties in coordination complexes arise from the presence or absence of unpaired electrons. In \[\mathrm{Co}(\mathrm{CO}_3)_3\]^{3-}\, the cobalt ion is in +3 oxidation state. Being octahedral with weak-field ligands like carbonate, the d-orbitals have a high-spin configuration, resulting in unpaired electrons that lead to paramagnetism.

• Paramagnetic compounds contain unpaired electrons and are attracted to magnetic fields.
• Diamagnetic compounds have all paired electrons and are slightly repelled by magnetic fields.
• The magnetic property impacts how complexes behave in applied magnetic fields, useful for identification and characterization.