The square planar
molecular geometry in chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
describes the stereochemistry (spatial arrangement of atoms) that is adopted by certain chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s. As the name suggests, molecules of this geometry have their atoms positioned at the corners of a square on the same plane about a central atom.
The addition of two ligands to linear compounds, ML2
, can afford square planar complexes. For example, [AuCl2
adds chlorine to give square planar [AuCl4
Tetrahedral molecular geometry
In principle, square planar geometry can be achieved by flattening a tetrahedron. As such, the interconversion of tetrahedral and square planar geometries provides an intramolecular pathway for the isomerization of tetrahedral compounds. This pathway does not operate readily for hydrocarbons, but tetrahedral nickel(II) complexes, e.g.
Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...
, undergo this change reversibly.
Removal of a pair of ligands from the z-axis of an octahedron
In geometry, an octahedron is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex....
, leaving four ligands in the x-y plane. For transition metal compounds, the crystal field
Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered coordination complexes. CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes,...
splitting diagram for square planar geometry can thus be derived from the octahedral diagram. The removal of the two ligands stabilizes the dz2
level, leaving the dx2-y2
level as the most destabilized. Consequently the dx2-y2
remains unoccupied in complexes of metals with the d8
configuration. These compounds typically have 16 valence electrons
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and bonding. Many rules in chemistry rely on electron-counting:...
(eight from ligands, eight from the metal).
Numerous compounds adopt this geometry, examples being especially numerous for transition metal complexes. The noble gas compound XeF4
Xenon tetrafluoride is a chemical compound with chemical formula . It was the first discovered binary compound of a noble gas. It is produced by the chemical reaction of xenon with fluorine, , according to the chemical equation:...
adopts this structure as predicted by VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
. The geometry is prevalent for transition metal complexes with d8
configuration, which includes Rh(I), Ir(I), Pd(II), Pt(II), and Au(III). Notable examples include the anticancer drugs cisplatin
Cisplatin, cisplatinum, or cis-diamminedichloroplatinum is a chemotherapy drug. It is used to treat various types of cancers, including sarcomas, some carcinomas , lymphomas, and germ cell tumors...
] and carboplatin
Carboplatin, or cis-Diammineplatinum is a chemotherapy drug used against some forms of cancer...
. Many homogeneous catalysts are square planar in their resting state, such Wilkinson's catalyst
Wilkinson's catalyst is the common name for chlorotrisrhodium, a coordination compound with the formula RhCl3 . It is named after the late organometallic chemist and 1973 Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.-Structure and basic properties:The compound is a square planar,...
and Crabtree's catalyst
Crabtree's catalyst is the name given to a complex of iridium with 1,5-cyclooctadiene, tris-cyclohexylphosphine, and pyridine. It is a homogeneous catalyst for hydrogenation reactions, developed by Robert H. Crabtree, a professor at Yale University...
. Other examples include Vaska's complex
Vaska's complex is the trivial name for the chemical compound trans-chlorocarbonylbisiridium, which has the formula IrCl[P3]2. This square planar diamagnetic organometallic complex consists of a central iridium atom bound to two mutually trans triphenylphosphine ligands, carbon monoxide, and a...
and Zeise's salt
Zeise's salt, potassium trichloroplatinate, is the chemical compound with the formula KPtCl3]·H2O. The anion of this air-stable, yellow, coordination complex contains an η2-ethylene ligand. The anion features a platinum atom with a square planar geometry.-Preparation:This compound is commercially...
. Certain ligands (such as porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...
s) stabilize this geometry.