The octahedral molecular geometry is a part of coordination chemistry and describes a molecular geometry in which atoms or ligands are arranged around a central atom with 4 of them in the same plane as the central atom at the corners of a square (basal positions) and two more at either side of the plane forming a straight line with the central atom (the apical postions). The central atom is often a transition metal. Examples of octahedral compounds are hexafluorosulfur (SF₆) and molybdenum hexacarbonyl. The molecular geometry resembles the octahedron with eight faces (hence the prefix octa) which is the platonic solid counterpart. Compounds with an octahedral geometry have a total coordination number of 6. For example hydratation of the yellow solid nickel sulfate NiSO₄ leads to the octahedral nickel hexa-aqua sulfate Ni(H₂0)₆SO₄. Exposure to ammonia converts this compound to the blue voilet nickel hexa-ammine sulfate Ni(NH₃)₆SO₄ complex in a ligand exchange reaction. The reaction of ammonia directly with nickel sulfate also gives the blue violet compound. Many reactions with octahedral transition metal complexes in fact occur in water. When the ligand is an anion the reaction is called a anation reaction and when in the reverse reaction water replaces a ligand the reaction is called an aquation reaction. complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... Geometry of the water molecule Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. ... In chemistry, a ligand is an atom, ion or functional group that is bonded to one or more central atoms or ions, usually metals generally through co-ordinate covalent bond. ... This article is in need of attention. ... Molybdenum hexacarbonyl (also called molybdenum carbonyl or molybdenumhexacarbonyl) is an organometallic compound. ... An octahedron (plural: octahedra) is a polyhedron with eight faces. ... Prefix has meanings in linguistics, mathematics and computer science, and telecommunications. ... In solid geometry and some ancient physical theories, a Platonic solid is a convex polyhedron with all its faces being regular polygons of the same size and shape, and the same number of faces meeting at each of its vertices. ... In chemistry, the coordination number is the sum of the total number of neighbours of a central atom in a chemical compound and the number of lone pairs on it. ... Ammonia is a chemical compound with the formula NH3. ... An anion is an ion with negative charge. ...

With mixed ligands, complexes can exist as cis and trans isomers. Analysis of such complexes even lead to the postulation of octahedral complexes in the first place. One isomer exists for ML¹₅L², and two isomers exist for ML¹₄L²₂, cis and trans with the L² ligands in apical positions. For ML¹₃L²₃ there exists a facial stereoisomer with two L² ligands in a basal position and a meridional isomer in which all L² ligands are basal. Mixed bidentate ligand complexes can even be chiral. Crystal field theory is used to describe the electronic structure (color) of octahedral transition metal complexes. The ligands 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 (Delta O with O from octahedral) according to their ligand strength. Chirality refers to several phenomena, all having to do with objects that differ from their mirror image. ... Crystal field theory is used to describe the electronic structure of transition metal complexes. ... The d-block of the periodic table of elements consists of those periodic table groups that contain elements in which, in the atomic ground state, the highest-energy electron is in a d-orbital. ... Properties The electron is a subatomic particle. ...

Ligand strength has the following order for these anions:

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. In spectroscopy, the absorbance A is defined as , where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and is the intensity of the light before it enters the sample (or incident light intensity). ... The wavelength is the distance between repeating units of a wave pattern. ...

Coordination chemistry and the octahedral geometry in particular was developed by Alfred Werner. For this reason octahedral complexes are often referred to Werner-type complexes. The other molecular geometries that exist are compiled according to the AXE method. complex In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. ... Alfred Werner (December 12, 1866 - November 15, 1919) was a German Nobel prize-winning chemist. ... In chemistry, The AXE method is commonly used in formatting molecules to fit the VSEPR model that aims to explain molecular geometry. ...