A quadruple bond is a type of

chemical bond A chemical bond is the association of atoms or ions to form molecules, crystals, and other structures. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds or through the sharing of electrons a ...

between two

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s involving eight

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s. This bond is an extension of the more familiar types of

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

s and

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...

s. Stable quadruple bonds are most common among the

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

s in the middle of the , such as

rhenium Rhenium is a chemical element; it has symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...

tungsten Tungsten (also called wolfram) is a chemical element; it has symbol W and atomic number 74. It is a metal found naturally on Earth almost exclusively in compounds with other elements. It was identified as a distinct element in 1781 and first ...

technetium Technetium is a chemical element; it has Symbol (chemistry), symbol Tc and atomic number 43. It is the lightest element whose isotopes are all radioactive. Technetium and promethium are the only radioactive elements whose neighbours in the sense ...

molybdenum Molybdenum is a chemical element; it has Symbol (chemistry), symbol Mo (from Neo-Latin ''molybdaenum'') and atomic number 42. The name derived from Ancient Greek ', meaning lead, since its ores were confused with lead ores. Molybdenum minerals hav ...

and

chromium Chromium is a chemical element; it has Symbol (chemistry), symbol Cr and atomic number 24. It is the first element in Group 6 element, group 6. It is a steely-grey, Luster (mineralogy), lustrous, hard, and brittle transition metal. Chromium ...

. Typically the

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s that support quadruple bonds are π-donors, not π-acceptors. Quadruple bonds are rare as compared to

s and

s, but hundreds of compounds with such bonds have been prepared.

History

Chromium(II) acetate Chromium(II) acetate hydrate, also known as chromous acetate, is the coordination compound with the chemical formula, formula Cr2(CH3CO2)4(H2O)2. This formula is commonly abbreviated Cr2(OAc)4(H2O)2. This red-coloured compound features a quadruple ...

, Cr₂(''μ''-O₂CCH₃)₄(H₂O)₂, was the first

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

containing a quadruple bond to be synthesized. It was described in 1844 by E. Peligot, although its distinctive bonding was not recognized for more than a century. The first crystallographic study of a compound with a quadruple bond was provided by Soviet chemists for salts of . The very short Re–Re distance was noted. This short distance (and the salt's

diamagnetism Diamagnetism is the property of materials that are repelled by a magnetic field; an applied magnetic field creates an induced magnetic field in them in the opposite direction, causing a repulsive force. In contrast, paramagnetic and ferromagnet ...

) indicated Re–Re bonding. These researchers, however, misformulated the

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

as a derivative of Re(II), i.e., . Soon thereafter, F. Albert Cotton and Charles B. Harris reported the crystal structure of potassium octachlorodirhenate or K₂ e₂Cl₈�2H₂O. This structural analysis indicated that the previous characterization was mistaken. Cotton and Harris formulated a molecular orbital rationale for the bonding that explicitly indicated a quadruple bond. The rhenium–rhenium

bond length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...

in this compound is only 224 pm. In

molecular orbital theory In chemistry, molecular orbital theory (MO theory or MOT) is a method for describing the electronic structure of molecules using quantum mechanics. It was proposed early in the 20th century. The MOT explains the paramagnetic nature of O2, whic ...

, the bonding is described as σ²π⁴δ² with one

sigma bond In chemistry, sigma bonds (σ bonds) or sigma overlap are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals along the internuclear axis. Sigma bonding is most simply defined for diat ...

, two

pi bond In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbital ...

s and one

delta bond In chemistry, a delta bond (δ bond) is a Covalent bond, covalent chemical bond, in which four lobes of an atomic orbital on one atom orbital overlap, overlap four lobes of an atomic orbital on another atom. This overlap leads to the formation o ...

Structure and bonding

The e₂Cl₈sup>2− ion adopts an eclipsed conformation as shown at left. The delta bonding orbital is then formed by overlap of the d orbitals on each rhenium atom, which are perpendicular to the Re–Re axis and lie in between the Re–Cl bonds. The d orbitals directed along the Re–Cl bonds are stabilized by interaction with chloride ligand orbitals and do not contribute to Re–Re bonding. In contrast, the s₂Cl₈sup>2− ion with two more electrons (σ²π⁴δ²δ*²) has an Os–Os

and a staggered geometry. Many other compounds with quadruple bonds between transition metal atoms have been described, often by Cotton and his coworkers.

Isoelectronic Isoelectronicity is a phenomenon observed when two or more molecules have the same structure (positions and connectivities among atoms) and the same electronic configurations, but differ by what specific elements are at certain locations in th ...

with the dirhenium compound is the salt K₄ o₂Cl₈(

potassium octachlorodimolybdate Potassium octachlorodimolybdate (systematically named potassium bis(tetrachloromolybdate)(''Mo''–''Mo'')(4−)) is an inorganic compound with the chemical formula . It is known as a red-coloured, microcrystalline solid. The anion is of historic ...

). An example of a ditungsten compound with a quadruple bond is ditungsten tetra(hpp). Quadruple bonds between atoms of

main-group element In chemistry and atomic physics, the main group is the group (periodic table), group of chemical element, elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon ...

s are unknown. For the

diatomic carbon Diatomic carbon (systematically named dicarbon and 1λ2,2λ2-ethene), is a green, gaseous inorganic chemical with the chemical formula C=C (also written 2or C2). It is kinetically unstable at ambient temperature and pressure, being removed throug ...

(C₂) molecule as an example,

shows that there are two sets of paired electrons in the sigma system (one bonding, one antibonding), and two sets of paired electrons in a degenerate π-bonding set of orbitals. This adds up to a bond order of 2, meaning that there exists a double bond between the two

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

atoms. The

molecular orbital diagram A molecular orbital diagram, or MO diagram, is a qualitative descriptive tool explaining chemical bonding in molecules in terms of molecular orbital theory in general and the linear combination of atomic orbitals (LCAO) method in particular. A f ...

of diatomic carbon would show that there are two pi bonds and no sigma bonds. A 2012 paper by S. Shaik et al. suggests that a quadruple bond exists in dicarbon, but this is disputed.

History

Structure and bonding

See also

References

Further reading