Tetrahedral molecular geometry

In a tetrahedral molecular geometry a central atom is located at the center with four substituents located at the corners of a tetrahedron. The bond angles are cos^-1(-1/3) ≈ 109.5°. When all four substituents are the same and it is a perfect tetrahedron then it belongs to point group T_d. This molecular geometry is found for saturated compounds of carbon and silicon. Some other molecules and ions with this particular geometry include the xenon tetroxide molecule XeO₄, the perchlorate ion ClO₄^-, the sulfate ion SO₄^2-, the phosphate ion PO₄^3- and tetrakis(triphenylphosphine)palladium(0).

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Inverted tetrahedral geometry

Geometrical constraints in a molecule may cause a severe distortion of a tetrahedral geometry towards an inverted one. In inverted carbon for instance all 4 substituents are now on the same side ^[1].

organic molecules displaying inverted carbon are tetrahedranes and propellanes. The penalty usually is increase in strain energy for the molecule resulting in increased reactivity.

Note that inversion also takes place in so-called Walden inversion and nitrogen inversion but with different meanings.

Planarization

A tetrahedron can also be distorted by increasing the angle between the two opposite bonds (again by force) resulting in the extreme case in complete flattening. For carbon this phenomenon can be observed in a class of compounds called the fenestranes.

See also

• The other molecular geometries are collected according to the AXE method.

References

1. ^ Inverted geometries at carbon Kenneth B. Wiberg Acc. Chem. Res.; 1984; 17(11) pp 379 - 386; doi:10.1021/ar00107a001