# 7.1.2: Summary of the three most common geometries

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## A brief summary of the factors for predicting geometry

### The three most important factors for determining metal complex geometry in biological metal binding sites:

1. Stabilization Energy (including LFSE)
2. Steric interactions between ligands
3. Protein folding or constraints caused by limitations in bond angle (geometric constraints)

### Splitting of d-orbitals in the three most-common geometries: OCTAHEDRAL (most common)

• Is the preferred geometry for most metals because 6 ligands contribute to stabilizing electrophilic metal center.
• Will always have more negative (stable) LFSE than analogous tetrahedral case.
• Is more sterically crowded

Tetrahedral (2nd most common)

• Small $$\Delta$$ means less negative LFSE (less stable in terms of LFSE)
• Always high spin
• Best case in terms of steric crowding around the metal center

Square Plane ($$d^8$$ and $$d^9$$)

• The bigger the $$\Delta$$, the more likely it will be square planar due to huge LFSE benefit.
• Pt and Pd are almost always square planar while Ni is often tetrahedral.
• In the case of Ni, strong filed ligands favor square plane.
• $$d^9$$ metals prefer square plane (or something similar) as a result of Jahn-Teller distortion of octahedral geometry.
• Highest-energy orbital is usually empty in $$d^8$$.