Octahedral molecular geometry

Molecular geometry

In chemistry, octahedral molecular geometry, also called square bipyramidal, describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, |[Co(NH3)6](3+), which is not octahedral in the mathematical sense due to the orientation of the |N\sH bonds, is referred to as octahedral. The concept of octahedral coordination geometry was developed by Alfred Werner to explain the stoichiometries and isomerism in coordination compounds. His insight allowed chemists to rationalize the number of isomers of coordination compounds. Octahedral transition metal complexes containing amines and simple anions are often referred to as Werner type complexes.

When two or more types of ligands (La, Lb, ) are coordinated to an octahedral metal centre (M), the complex can exist as isomers. The naming system for these isomers depends upon the number and arrangement of different ligands.

For MLL, two isomers exist. These isomers of MLL are cis, if the Lb ligands are mutually adjacent, and trans, if the Lb groups are situated 180° to each other. It was the analysis of such complexes that led Alfred Werner to the 1913 Nobel Prize–winning postulation of octahedral complexes.

For MLL, two isomers are possible a facial isomer (fac) in which each set of three identical ligands occupies one face of the octahedron surrounding the metal atom, so that any two of these three ligands are mutually cis, and a meridional isomer (mer) in which each set of three identical ligands occupies a plane passing through the metal atom.

Complexes with three bidentate ligands or two cis bidentate ligands can exist as enantiomeric pairs. Examples are shown below.

The term can also refer to octahedral influenced by the Jahn–Teller effect, which is a common phenomenon encountered in coordination chemistry. This reduces the symmetry of the molecule from Oh to D4h and is known as a tetragonal distortion.

Some molecules, such as XeF6 or IF6−, have a lone pair that distorts the symmetry of the molecule from Oh to C3v. The specific geometry is known as a monocapped octahedron, since it is derived from the octahedron by placing the lone pair over the centre of one triangular face of the octahedron as a "cap" (and shifting the positions of the other six atoms to accommodate it). These both represent a divergence from the geometry predicted by VSEPR, which for AX6E1 predicts a pentagonal pyramidal shape.

Pairs of octahedra can be fused in a way that preserves the octahedral coordination geometry by replacing terminal ligands with bridging ligands. Two motifs for fusing octahedra are common: edge sharing and face sharing. Edge and face shared bioctahedra have the formulas [M2L8(μ L)]2 and M2L6(μ L)3, respectively. Polymeric versions of the same linking pattern give the stoichiometries [ML2(μ L)2]∞ and [M(μ L)3]∞, respectively. The sharing of an edge or a face of an octahedron gives a structure called bioctahedral. Many metal pentahalide and pentaalkoxide compounds exist in solution and the solid with bioctahedral structures. One example is niobium pentachloride. Metal tetrahalides often exist as polymers with edge sharing octahedra. Zirconium tetrachloride is an example. Compounds with face sharing octahedral chains include MoBr3, RuBr3, and TlBr3.

For compounds with the formula MX6, the chief alternative to octahedral geometry is a trigonal prismatic geometry, which has symmetry D3h. In this geometry, the six ligands are also equivalent. There are also distorted trigonal prisms, with C3v symmetry; a prominent example is |W(CH3)6. The interconversion of Δ and Λ complexes, which is usually slow, is proposed to proceed via a trigonal prismatic intermediate, a process called the "Bailar twist". An alternative pathway for the racemization of these same complexes is the Ray–Dutt twist.