Tetrahedral molecular geometry

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In a Tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos⁻¹(−1/3) ≈ 109.5° when all four substituents are the same, as in CH₄. This molecular geometry is common throughout the first half of the periodic table. The perfectly symmetrical tetrahedron belongs to point group T_d, but most tetrahedral molecules are not of such high symmetry. Tetrahedral molecules can be chiral.

Examples

Main group chemistry

Aside virtually all saturated organic compounds, most compounds of Si, Ge, and Sn are tetrahedral. Often tetrahedral molecules feature multiple bonding to the outer ligands, as in xenon tetroxide (XeO₄), the perchlorate ion (ClO₄⁻), the sulfate ion (SO₄²⁻), the phosphate ion (PO₄³⁻]). Thiazyl trifluoride, SNF₃ is tetrahedral, featuring a sulfur-to-nitrogen triple bond.^[1]

Ammonia can be classified as tetrahedral, if one considers the lone pair as a ligand as in the language of VSEPR theory. The H-N-H angles are 107°, being contracted from 109.4°, a difference attributed to the influence of the lone pair.

Transition metal chemistry

Again the geometry is widespread, particularly so for complexes where the metal has d⁰ or d¹⁰ configuration. Illustrative examples include tetrakis(triphenylphosphine)palladium(0), nickel carbonyl, and titanium tetrachloride. Many complexes with incompletely filled d-shells are often tetrahedral, e.g. the tetrahalides of iron(II), cobalt(II), and nickel(II).

Exceptions and distortions

Inversion of tetrahedral occurs widely in organic and main group chemistry. The so-called Walden inversion illustrates the stereochemical consequences of inversion at carbon. Nitrogen inversion in ammonia also entails transient formation of planar NH₃.

Inverted tetrahedral geometry

Geometrical constraints in a molecule can cause a severe distortion of idealized tetrahedral geometry. In compounds featuring "inverted carbon," for instance, the carbon is pyramidal.^[1].

Image:Invertedcarbon.png

Organic molecules displaying inverted carbon are tetrahedranes and propellanes. Such molecules are typically strained, resulting in increased reactivity.

Planarization

A tetrahedron can also be distorted by increasing the angle between the two opposite bonds. In the extreme case, flattening results. For carbon this phenomenon can be observed in a class of compounds called the fenestranes.

See also

• AXE method
• Orbital hybridisation

References

External links

• Examples of Tetrahedral molecules
• Animated Tetrahedral Visual
• Elmhurst College
• Point group Symmetries
• 3D Chem - Chemistry, Structures, and 3D Molecules
• IUMSC - Indiana University Molecular Structure Center]
• [1]
• Molecular ModelingTemplate:MolecularGeometry

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