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Strong Pi Donors and Acceptors

I need help with this question:
1. Six-coordinate CR(3) complexes of the type trans-(CrL4A2)n+, generally have magnetic moments consistent with three unpaired electrons, which suggests occupancy of the d orbitals as shown to the right. In principle, a complex with only one unpaired electron could be generated by a suitable choice of L and A such that the separation between the dxy and the dxz, dyz orbitals becomes larger than the spin pairing energy.

Assume that L is a sigma donor only ligand. Would strong pi donors or strong pi acceptors be the best choice for axial ligand A in order to increase the separation of the dxy and the degenerate dxz, dyz orbitals? Explain the basis for your choice being careful to:
1. explicitly indicate the relative energies of the metal and ligand pi orbitals
2. explicitly indicate the specific ligand orbital-metal orbital interactions that will influence the relative energies of the dxy and the degenerate dxz, dyz orbitals.

There is a diagram in the attachment.

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This is a case of tetragonal distortion (to be specific, elongation) of octahedral complex. So Jahn-Teller effect is operative.

When an octahedral complex exhibits elongation, the axial bonds are longer than the equatorial bonds. For a compression, it is the reverse; the equatorial bonds are longer than the axial bonds. Elongation and compression effects are dictated by the amount of overlap between the metal and ligand orbitals. Thus, this distortion varies greatly depending on the type of metal and ligands. In general, the stronger the metal-ligand orbital interactions are, the greater the chance for a Jahn-Teller effect to be observed.

Elongation Jahn-Teller distortions occur when the degeneracy is broken by the stabilization (lowering in energy) of the d orbitals with a z component, while the orbitals without a z component are destabilized (higher in energy) as ...

Solution Summary

Strong Pi donors and acceptors are examined.

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