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 CoF63- and the strong field complex Co(NH3)63+.
Co3+ 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 dz2 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 dx2-y2 orbital in changing something along the z-axis.
There is also no effect on the dxy orbital energy of changing the charge on the z-axis.
Both the dxz and dyz 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 d8 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
Co3+ 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 Co3+ 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.