Octahedral MO6 units are common in oxyanions of the larger transition metals. Some compounds, such as salts of the chain-polymeric ion, even contain both tetrahedral and octahedral units. Edge-sharing is common in ions containing octahedral building blocks and the octahedra are usually distorted to reduce the strain at the bridging oxygen atoms. This results in 3-dimensional structures called polyoxometalates. Typical examples occur in the Keggin structure of the phosphomolybdate ion. Edge sharing is an effective means of reducing electrical charge density, as can be seen with the hypothetical condensation reaction involving two octahedra: