Octahedral molecular geometry

In chemistry, octahedral molecular geometry 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, 3+, which is not octahedral in the mathematical sense due to the orientation of the N-H bonds, is referred to as octahedral.Ball-and-stick model of niobium pentachloride, a bioctahedral coordination compound.Ball-and-stick model of zirconium tetrachloride, an inorganic polymer based on edge-sharing octahedra.Ball-and-stick model of molybdenum(III) bromide, an inorganic polymer based on face-sharing octahedra.View almost down the chain of titanium(III) iodide highlighting the eclipsing of the halide ligands in such face-sharing octahedra. In chemistry, octahedral molecular geometry 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, 3+, which is not octahedral in the mathematical sense due to the orientation of the N-H 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 MLa4Lb2, two isomers exist. These isomers of MLa4Lb2 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 MLa3Lb3, 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. More complicated complexes, with several different kinds of ligands or with bidentate ligands can also be chiral, with pairs of isomers which are non-superimposable mirror images or enantiomers of each other. For MLa2Lb2Lc2, a total of six isomers are possible.