A brief summary of the factors for predicting geometry
The three most important factors for determining metal complex geometry in biological metal binding sites:
- Stabilization Energy (including LFSE)
- Steric interactions between ligands
- 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 \(d^8\).
Modified or created by Kathryn Haas (khaaslab.com)