Leccture #33 November 22, 1996 | ||

The weak field splitting and strong field splitting of the d-orbitals illustrated. | ||

The spectroscopic series is a ranking of various ligands according to the strength of the "crystal field" they produce about a transition metal ion. | ||

The electron configuration in the weak field complex CoF_{6}^{3-} and the strong field complex Co(NH_{3})_{6}^{3+}. | ||

Co^{3+} complex ions, their crystal field splitting energies, the color of light absorbed, and the color that the complex appears. | ||

Leaving the discussion of coordination number = 6 (octahedral geometries) and proceeding to coordination number four. | ||

Changing an octahedral geometry into a square planar geometry distribution of charges | ||

Removing the ligands (charges) along the z-axis to an infinite distance from the central species converts the octahedral geometry to a square planar geometry. | ||

What kind of change to we expect the removal of z-axis charges to have on the 3d valence orbitals? | ||

Removing the z-axis ligands stabilizes the d_{z2} orbital relative to its previous energy. | ||

How are the remaining d-orbitals affected by removing the z-axis charges? | ||

There is no effect on the d_{x2-y2} orbital in changing something along the z-axis. | ||

There is also no effect on the d_{xy} orbital energy of changing the charge on the z-axis. | ||

Both the d_{xz} and d_{yz} are lowered in energy (become stabilized) when charge along the z-axis is removed. | ||

Summary of the switch to a square planar geometry | ||

The valence electron energy diagram for d-electrons in a square planar geometry (crystal field). | ||

Now it becomes clear that the square planar geometry correlates with the observation of extra stability of a transition metal comlex ion with a d^{8} configuration on the central atom | ||

Returning to an octahedral geometry, we look at the effect of having a weak ligand plus five stronger ligands. The weak ligand is assumed to be on the z-axis which stabilizes transition metal ion valence orbitals directed that way | ||

Co^{3+} transition metal complex ions with one of the ligands varying shifts the wavelength of light absorbed to longer wavelengths as that one ligand becomes "weaker" in the field it produces. | ||

Measured optical properties (colors) of Co^{3+} transition metal complex ions. What do you estimate the color of the last complex to be? This last complex is meant to be just the trans geometric isomer. | ||

The tetrahedral crystal field geometry (without demonstrating how to generate the result) turns out to be the exact inverse orbital order of that seen in the octahedral geometry. |