Question

Give the wavelengths of maximum absorption for each solution as shown in the spectra provided. Use the absorption wavelengths to explain why the [Ni(H2O)6] 2+ solution appears yellow-green but the [Co(H2O)6] 2+ solution appears pink.

[Ni(H2O)6] 2+ maximum absorbance at 720 nm

[Co(H2O)6] 2+ maximum absorbance at 510 nm

Answer 1

When six ligands approach a metal center to form an octahedral complex, the five degenerate d-orbitals split into three lower-energy degenerate t2g orbitals and two higher-energy degenerate eg orbitals. The distance of the splitting between the t2g and eg orbitals is dictated by the strength of the ligand according to the spectrochemical series.

Hund's rule still applies and electrons fill orbitals one at a time, but they fill in accordance to size of the splitting of the t2g and eg orbitals. If the split is small, electrons will fill all the orbitals singly before pairing. This maximizes the number of unpaired electrons and is called high-spin. Likewise, a strong-field causes a large t2g-eg split: electrons pair in the t2g set before filling the higher-energy eg orbitals. This minimizes the number of unpaired electrons and is called low-spin. The drive for electrons to pair is governed by the energy (or size) of the orbital splitting compared to the energy of electron pairing. If the energy of pairing is high compared to the energy of moving into the eg orbitals, then electrons are high-spin. If the energy of pairing is low compared to the energy of moving into the higher eg orbitals, then electrons are low-spin.

The distance that electrons have to move from the lower t2g state to the higher eg state in the metal center dictates the energy of electromagnetic radiation that the complex absorbs. If that energy is in the visible region (400-700 nm, 1.77 eV - 3.1 eV), the complex generally has a color. Weak-field ligands (I-→ OH-) cause small splittings and complexes absorb low-energy light (i.e. red) which appear green in color. Strong-field ligands (EDTA → CN-) absorb high-energy light (i.e. blue-violet) and appear red-yellow in color. Complexes with ligands that are between strong and weak on the spectrochemical series, like ammonia, can adopt either a weak- or strong-field geometry.

The color-ligand relationship is the rationale for the name "spectrochemical series".

Because the distance in orbital splitting varies with ligand strength, a coordination complex with the same metal center can have a variety of colors based upon the coordinating ligand. For example, an aqueous solution of Ni shows green colour.

Where as in the aqua complex of the Co the transition is the d-d- transion and shows pink colour.