In chemistry, octahedral molecular geometry commonly describes a molecular geometry in which 6 ligands are symmetrically arranged around a central atom. All six ligands are chemically equivalent. The central atom is often a transition metal. The octahedron has eight faces (hence the prefix octa) and thus is one of the Platonic solids, the octahedron (although octahedral molecules typically have an atom in their centre). A perfect octahedron belongs to the point group O_h. Examples of octahedral compounds are sulfur hexafluoride (SF₆) and molybdenum hexacarbonyl. The term octahedral is used somewhat loosely: ^*3+, which is not octahedral in the mathematical sense due to the orientations of the N-H bonds, is referred to as octahedral.

The concept of octahedral transition metal compounds was developed by Alfred Werner. His insight allowed chemists to understand much prior coordination chemistry. Octahedral transition-metal complexes containing amines and simple anions are often referred to Werner-type complexes.

Isomerism

When two or more types of ligands are coordinated to an octahedral metal centre, the complex can exist as isomers. Two isomers exist for ML^a₄L^b₂. These isomers of ML^a₄L^b₂ are called cis, in which the two L^b ligands are mutually adjacent, and trans, with the L^b groups are situated at 180°. It was the analysis of such complexes that lead Werner to postulate octahedral complexes in the first place. For ML^a₃L^b₃, two isomers are again possible - a facial isomer (often abbreviated to fac) wherein the three identical ligands are mutually cis, and a meridional isomer (mer) in which the three ligands are coplanar. More complicated complexes, with several different kinds of ligands or with bidentate ligands can also be chiral.

Splitting of d-orbitals in octahedral complexes

In an octahedral complex, the five d-orbitals do not have the same energy. In contrast, the "free ion", e.g. gaseous Ni²⁺, the 3d orbitals are equi-energetic, also called degenerate. The reason for the splitting of the d-orbitals is that the lobes of the five orbitals interact with ligands differently. The d_z2 and d_x2_-y2, the so-called e_g set, are aimed directly at the ligands. On the other hand, the lobes of the d_xz, d_xy, and d_yz orbitals, the so-called t_2g set, are aimed toward the space between the six ligands. The labels t_2g and e_g refer to irreducible representations," which describe the symmetry properties of these orbitals. Thus, when a metal is an in octahedral environment, the d-orbitals lose their degeneracy by splitting into these e_g and t_2g sets. The energy gap separating these two sets is the basis of Crystal Field Theory. If the symmetry of the complex is lower, more splitting occurs. For example, the t_2g and e_g sets split further in trans-ML^a₄L^b₂.

Reactions

Given that virtually uncountable variety of octahedral complexes exist, it is not surprising that a wide variety of reactions have been described. These reactions can be classified as follows:

• Ligand substitution reactions (via a variety of mechanisms)
• Ligand addition reactions (e.g. protonation).
• Redox reactions (where electrons are gained or lost)
• Rearrangements where the ligand migrate within the coordination sphere.

Many reactions of octahedral transition metal complexes occur in water. When an anionic ligand replaces a coordinated water molecule the reaction is called a anation. The reverse reaction, water replacing an anionic ligand, is called an aquation reaction. For example, the slowly aquates to give [Co(NH₃)₅(H₂O)³⁺ in water, especially in the presence of acid or base. Addition of concentrated HCl converts the aquo complex back to the chloride, via an anation process.

Ligand Field Theory

Ligand Field Theory (LFT) is used to describe the electronic structure of transition metal complexes. LFT is related to Crystal Field Theory but recognizes both the sigma and pi-bonding components of the metal-ligand interactions. The ligands again cause the d-orbital electrons to split into higher and lower energy groups in a process called crystal field splitting or ligand field splitting Δ_o according to their ligand strength.

Ligand strength has the following order for these electron donors:

Poor: iodine < bromine < fluorine < acetate < oxalate < water < pyridine < cyanide :strong

So called weak field ligands have a small Δ_o and absorb light at longer wavelengths.

See also

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• octahedral clusters
• Ligand field theory
• ligand

molecular geometry