Quintuple bond
Encyclopedia
A quintuple bond in chemistry
is an unusual type of chemical bond
first reported in 2005 for a dichromium
compound. Single bonds, double bond
s, and triple bond
s are commonplace in chemistry. Quadruple bond
s are rarer but occur especially for Cr, Mo, W, and Re, e.g. [Mo2Cl8]4− and [Re2Cl8]2−. In a quintuple bond, ten electrons participate in bonding between the two metal centers, allocated as σ2π4δ4.
In some cases of high-order bonds between metal atoms, the metal-metal bonding is facilitated by ligands that link the two metal centers and reduce the interatomic distance. By contrast, the chromium dimer with quintuple bonding is stabilized by bulky terphenyl
(2,6-[(2,6-diisopropyl)phenyl]phenyl) ligand
s. The species is stable up to 200 °C. The chromium-chromium quintuple bond has been analyzed with multireference ab-initio and DFT methods, which were also used to elucidate the role of the terphenyl ligand, in which the flanking aryls were shown to interact very weakly with the chromium atoms, causing only a small weakening of the quintuple bond. A 2007 theoretical study identified two global minima for quintuple bonded RMMR compounds: a trans-bent molecular geometry
and surprisingly another trans-bent geometry with the R substituent in a bridging position.
In 2005, a quintuple bond was postulated to exist in the hypothetical uranium
molecule U2 based on computational chemistry
. Diuranium compounds are rare, but do exist; for example, the U2Cl anion.
In 2007 the shortest ever metal to metal bond (180.28 pm) was reported to exist also in a compound containing a quintuple chromium-chromium bond with diazadiene bridging ligands. Other metal-metal quintuple bond containing complexes that have been reported include quintuply bonded dichromium with [6-(2,4,6-triisopropylphenyl)pyridine-2-yl](2,4,6-trimethylphenyl)amine bridging ligands and a dichromium complex with amidinate bridging ligands.
Synthesis of quintuple bonds is usually achieved through reduction of a dimetal species using potassium graphite. This adds valence electron
s to the metal centers, giving them the needed number of electrons to participate in quintuple bonding. Below is a figure of a typical quintuple bond synthesis.
(already contains a Mo2 quadruple bond) and a lithium amidinate, followed by reduction with potassium graphite:
, two are pi bond
s, and two are delta bond
s. the σ-bond is the result of mixing between the dz2 orbital on each metal center. The first π-bond comes from mixing of the dyz orbitals from each metal while the other π-bond comes from the dxz orbitals on each metal mixing. Finally the δ-bonds come from mixing of the dxy orbitals as well as mixing between the dx2-y2 orbitals from each metal. A graphical representation of this mixing can be seen below.
Molecular orbital calculations have elucidated the relative energies of the orbitals created by these bonding interactions. As shown in the figure below, the lowest energy orbitals are the pi bonding orbitals followed by the sigma bonding orbital. The next highest are the delta bonding orbitals which represent the HOMO
. Because the 10 valence electrons of the metals are used to fill these first 5 orbitals, the next highest orbital becomes the LUMO which is the delta antibonding
orbital. Though the pi and delta orbitals are represented as being degenerate
, they in fact are not. This is because the model shown here is a simplification and that hybridization of s, p, and d orbitals is believed to take place, causing a change in the orbital energy levels.
-carbon interactions.
The bidentate ligand can act as a sort of tweezer in that in order for chelation
to occur the metal atoms must move closer together, thereby shortening the quintuple bond length. The two ways in which to obtain shorter metal-metal distances is to either reduce the distance between the chelating atoms in the ligand by changing the structure, or by using steric effects
to force a conformational change in the ligand that bends the molecule in a way that forces the chelating atoms closer together. An example of the latter is shown below:
The above example shows the ligand used in the dimolybdenum complex shown in section 2. When the carbon between the two nitrogens in the ligand has a hydrogen bound to it, the steric repulsion is small. However, when the hydrogen is replaced with a much more bulky phenyl ring the steric repulsion increases dramatically and the ligand "bows" which causes a change in the orientation of the lone pairs of electrons on the nitrogen atoms. These lone pairs are what is responsible for forming bonds with the metal centers so forcing them to move closer together also forces the metal centers to be positioned closer together. Thus, decreasing the length of the quintuple bond. In the case where this ligand is bound to quintuply bonded dimolybdenum the quintuple bond length goes from 201.87 pm to 201.57 pm when the hydrogen in replaced with a phenyl group. Similar results have also been demonstrated in dichromium quintuple bond complexes as well.
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....
is an unusual type of chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
first reported in 2005 for a dichromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
compound. Single bonds, double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s, and triple bond
Triple bond
A triple bond in chemistry is a chemical bond between two chemical elements involving six bonding electrons instead of the usual two in a covalent single bond. The most common triple bond, that between two carbon atoms, can be found in alkynes. Other functional groups containing a triple bond are...
s are commonplace in chemistry. Quadruple bond
Quadruple bond
A quadruple bond is a type of chemical bond between two atoms involving eight electrons. This bond is an extension of the more familiar types double bonds and triple bonds. Stable quadruple bonds are most common among the middle members transition metal elements such rhenium, tungsten, molybdenum...
s are rarer but occur especially for Cr, Mo, W, and Re, e.g. [Mo2Cl8]4− and [Re2Cl8]2−. In a quintuple bond, ten electrons participate in bonding between the two metal centers, allocated as σ2π4δ4.
In some cases of high-order bonds between metal atoms, the metal-metal bonding is facilitated by ligands that link the two metal centers and reduce the interatomic distance. By contrast, the chromium dimer with quintuple bonding is stabilized by bulky terphenyl
Terphenyl
Terphenyls are a group of closely related aromatic hydrocarbons. Also known as diphenylbenzenes or triphenyls, they consist of a central benzene ring substituted with two phenyl groups. The three isomers are ortho-terphenyl, meta-terphenyl, and para-terphenyl. Commercial grade terphenyl is...
(2,6-[(2,6-diisopropyl)phenyl]phenyl) ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. The species is stable up to 200 °C. The chromium-chromium quintuple bond has been analyzed with multireference ab-initio and DFT methods, which were also used to elucidate the role of the terphenyl ligand, in which the flanking aryls were shown to interact very weakly with the chromium atoms, causing only a small weakening of the quintuple bond. A 2007 theoretical study identified two global minima for quintuple bonded RMMR compounds: a trans-bent molecular geometry
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
and surprisingly another trans-bent geometry with the R substituent in a bridging position.
In 2005, a quintuple bond was postulated to exist in the hypothetical uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
molecule U2 based on computational chemistry
Computational chemistry
Computational chemistry is a branch of chemistry that uses principles of computer science to assist in solving chemical problems. It uses the results of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids...
. Diuranium compounds are rare, but do exist; for example, the U2Cl anion.
In 2007 the shortest ever metal to metal bond (180.28 pm) was reported to exist also in a compound containing a quintuple chromium-chromium bond with diazadiene bridging ligands. Other metal-metal quintuple bond containing complexes that have been reported include quintuply bonded dichromium with [6-(2,4,6-triisopropylphenyl)pyridine-2-yl](2,4,6-trimethylphenyl)amine bridging ligands and a dichromium complex with amidinate bridging ligands.
Synthesis of quintuple bonds is usually achieved through reduction of a dimetal species using potassium graphite. This adds valence electron
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s to the metal centers, giving them the needed number of electrons to participate in quintuple bonding. Below is a figure of a typical quintuple bond synthesis.
Dimolybdenum quintuple bonds
In 2009 a dimolybdenum compound with a quintuple bond and two diamido bridging ligands was reported with a Mo-Mo bond length of 202 pm. The compound was synthesised starting from potassium octachlorodimolybdatePotassium octachlorodimolybdate
Potassium octachlorodimolybdate is the inorganic compound with the formula K4Mo2Cl8. This red-coloured salt consists of potassium cations and the octachlorodimolybdate anion, Mo2Cl84−. The anion is of historic interest because it was one of the earliest illustrations of a quadruple bonding...
(already contains a Mo2 quadruple bond) and a lithium amidinate, followed by reduction with potassium graphite:
Bonding
As stated above metal-metal quintuple bonds have a σ2π4δ4 configuration. This means the of the five bonds present between the metal centers one is a sigma bondSigma bond
In chemistry, sigma bonds are the strongest type of covalent chemical bond. They are formed by head-on overlapping between atomic orbitals. Sigma bonding is most clearly defined for diatomic molecules using the language and tools of symmetry groups. In this formal approach, a σ-bond is...
, two are pi bond
Pi bond
In chemistry, pi bonds are covalent chemical bonds where two lobes of one involved atomic orbital overlap two lobes of the other involved atomic orbital...
s, and two are delta bond
Delta bond
In chemistry, delta bonds are chemical bonds of the covalent type, where four lobes of one involved atomic orbital overlap four lobes of the other involved atomic orbital...
s. the σ-bond is the result of mixing between the dz2 orbital on each metal center. The first π-bond comes from mixing of the dyz orbitals from each metal while the other π-bond comes from the dxz orbitals on each metal mixing. Finally the δ-bonds come from mixing of the dxy orbitals as well as mixing between the dx2-y2 orbitals from each metal. A graphical representation of this mixing can be seen below.
Molecular orbital calculations have elucidated the relative energies of the orbitals created by these bonding interactions. As shown in the figure below, the lowest energy orbitals are the pi bonding orbitals followed by the sigma bonding orbital. The next highest are the delta bonding orbitals which represent the HOMO
Homo
Homo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
. Because the 10 valence electrons of the metals are used to fill these first 5 orbitals, the next highest orbital becomes the LUMO which is the delta antibonding
Antibonding
Antibonding is a type of chemical bonding. An antibonding orbital is a form of molecular orbital that is located outside the region of two distinct nuclei...
orbital. Though the pi and delta orbitals are represented as being degenerate
Degenerate energy levels
In physics, two or more different quantum states are said to be degenerate if they are all at the same energy level. Statistically this means that they are all equally probable of being filled, and in Quantum Mechanics it is represented mathematically by the Hamiltonian for the system having more...
, they in fact are not. This is because the model shown here is a simplification and that hybridization of s, p, and d orbitals is believed to take place, causing a change in the orbital energy levels.
Ligand role in metal-metal quintuple bond length
Quintuple bond lengths vary from complex to complex and are heavily dependent on the ligands bound to the metal centers. Nearly all complexes containing a metal-metal quintuple bond have bidentate bridging ligands, and even those that do not like the terphenyl complex mentioned earlier, have some bridging characteristic to it through metal-ipsoArene substitution patterns
Arene substitution patterns are part of organic chemistry IUPAC nomenclature and pinpoint the position of substituents other than hydrogen in relation to each other on an aromatic hydrocarbon.- Ortho, meta, and para substitution :...
-carbon interactions.
The bidentate ligand can act as a sort of tweezer in that in order for chelation
Chelation
Chelation is the formation or presence of two or more separate coordinate bonds between apolydentate ligand and a single central atom....
to occur the metal atoms must move closer together, thereby shortening the quintuple bond length. The two ways in which to obtain shorter metal-metal distances is to either reduce the distance between the chelating atoms in the ligand by changing the structure, or by using steric effects
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
to force a conformational change in the ligand that bends the molecule in a way that forces the chelating atoms closer together. An example of the latter is shown below:
The above example shows the ligand used in the dimolybdenum complex shown in section 2. When the carbon between the two nitrogens in the ligand has a hydrogen bound to it, the steric repulsion is small. However, when the hydrogen is replaced with a much more bulky phenyl ring the steric repulsion increases dramatically and the ligand "bows" which causes a change in the orientation of the lone pairs of electrons on the nitrogen atoms. These lone pairs are what is responsible for forming bonds with the metal centers so forcing them to move closer together also forces the metal centers to be positioned closer together. Thus, decreasing the length of the quintuple bond. In the case where this ligand is bound to quintuply bonded dimolybdenum the quintuple bond length goes from 201.87 pm to 201.57 pm when the hydrogen in replaced with a phenyl group. Similar results have also been demonstrated in dichromium quintuple bond complexes as well.