Coordination Polyhedra
Visualizing the 3D spatial arrangement of ligands around a central metal ion.
1. What is a Coordination Polyhedron?
In a coordination compound, the central metal atom or ion is surrounded by electron-donating groups called ligands. The spatial arrangement of the ligand atoms directly attached to the central metal defines a geometric shape known as the coordination polyhedron.
The shape is primarily determined by the Coordination Number (CN)—the total number of ligand donor atoms bonded directly to the metal center—as well as electronic factors like hybridization and crystal field stabilization energy (CFSE).
2. Common Polyhedra by Coordination Number
Linear
Coordination Number = 2
Example: $[Ag(NH_3)_2]^+$
Tetrahedral
Coordination Number = 4
Example: $[Ni(CO)_4]$
Square Planar
Coordination Number = 4
Example: $[PtCl_4]^{2-}$
Trigonal Bipyramidal
Coordination Number = 5
Example: $[Fe(CO)_5]$
Octahedral
Coordination Number = 6
Example: $[Co(NH_3)_6]^{3+}$
3. Key Theoretical Points
- CN = 4 (Tetrahedral vs Square Planar): Compounds with CN=4 can adopt either a tetrahedral ($sp^3$) or square planar ($dsp^2$) geometry. Generally, $d^8$ metal ions (like $Pt^{2+}$, $Pd^{2+}$, $Ni^{2+}$ with strong field ligands) prefer the square planar arrangement.
- CN = 5 (Trigonal Bipyramidal vs Square Pyramidal): The energy difference between these two geometries is often very small, leading to structural fluxionality (e.g., Berry Pseudorotation in $PF_5$, though less common in pure coordination complexes).
- CN = 6 (Octahedral): This is the most common coordination polyhedron. All six positions are mathematically equivalent, with bond angles of $90^\circ$ and $180^\circ$. Distortions (like Jahn-Teller distortion) can elongate or compress the axial bonds, breaking perfect symmetry.
Test Your Knowledge
Practice MCQs on Coordination Polyhedra
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