organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...

, agostic interaction refers to the intramolecular interaction of a coordinatively-unsaturated

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 ...

with an appropriately situated C−H bond on one of its ligands. The interaction is the result of two electrons involved in the C−H bond interaction with an empty

d-orbital In quantum mechanics, an atomic orbital () is a function describing the location and wave-like behavior of an electron in an atom. This function describes an electron's charge distribution around the atom's nucleus, and can be used to calc ...

of the transition metal, resulting in a

three-center two-electron bond A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one ''non''-bonding, and one ''anti''- ...

. It is a special case of a C–H sigma complex. Historically, agostic complexes were the first examples of C–H sigma complexes to be observed spectroscopically and crystallographically, due to intramolecular interactions being particularly favorable and more often leading to robust complexes. Many catalytic transformations involving oxidative addition and reductive elimination are proposed to proceed via intermediates featuring agostic interactions. Agostic interactions are observed throughout

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

, alkylidene, and polyenyl ligands.

History

The term agostic, derived from the

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

word for "to hold close to oneself", was coined by Maurice Brookhart and Malcolm Green, on the suggestion of the

classicist Classics, also classical studies or Ancient Greek and Roman studies, is the study of classical antiquity. In the Western world, ''classics'' traditionally refers to the study of Ancient Greek literature, Ancient Greek and Roman literature and ...

Jasper Griffin Jasper Griffin (29 May 1937 – 22 November 2019) was a British classicist and academic. He was Public Orator and Professor of Classical Literature in the University of Oxford from 1992 until 2004. Early life Griffin was born on 29 May 1937. He ...

, to describe this and many other interactions between a transition metal and a C−H bond. Often such agostic interactions involve alkyl or aryl groups that are held close to the metal center through an additional σ-bond.. Short interactions between hydrocarbon substituents and coordinatively unsaturated metal complexes have been noted since the 1960s. For example, in tris( triphenylphosphine) ruthenium dichloride, a short interaction is observed between the

ruthenium Ruthenium is a chemical element; it has symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is unreactive to most chem ...

(II) center and a hydrogen atom on the ortho position of one of the nine phenyl rings. Complexes of

borohydride Borohydride refers to the anion , which is also called tetrahydroborate or more commonly tetrahydrobiopterin, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for ex ...

are described as using the

ing model. The nature of the interaction was foreshadowed in main group chemistry in the structural chemistry of

trimethylaluminium Trimethylaluminium or TMA is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula (abbreviated as , where Me stands for methyl), as it exists as a dimer. This colorless liquid is pyrophoric. It is an ...

Characteristics of agostic bonds

Agostic interactions are best demonstrated by

crystallography Crystallography is the branch of science devoted to the study of molecular and crystalline structure and properties. The word ''crystallography'' is derived from the Ancient Greek word (; "clear ice, rock-crystal"), and (; "to write"). In J ...

Neutron diffraction Neutron diffraction or elastic neutron scattering is the application of neutron scattering to the determination of the atomic and/or magnetic structure of a material. A sample to be examined is placed in a beam of Neutron temperature, thermal or ...

data have shown that C−H and M┄H bond distances are 5-20% longer than expected for isolated metal hydride and hydrocarbons. The distance between the metal and the hydrogen is typically 1.8–2.3 Å, and the M┄H−C angle is in the range of 90°–140°. The presence of a ¹H

NMR Nuclear magnetic resonance (NMR) is a physical phenomenon in which atomic nucleus, nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near and far field, near field) and respond by producing ...

signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to

hydride In chemistry, a hydride is formally the anion of hydrogen (H−), a hydrogen ion with two electrons. In modern usage, this is typically only used for ionic bonds, but it is sometimes (and has been more frequently in the past) applied to all che ...

ligands. The coupling constant ¹''J''_CH is typically lowered to 70–100 Hz versus the 125 Hz expected for a normal sp³ carbon–hydrogen bond. MLHGtiag

Strength of bond

On the basis of experimental and

computational A computation is any type of arithmetic or non-arithmetic calculation that is well-defined. Common examples of computation are mathematical equation solving and the execution of computer algorithms. Mechanical or electronic devices (or, historic ...

studies, the stabilization arising from an agostic interaction is estimated to be 10–15 kcal/mol. Recent calculations using compliance constants point to a weaker stabilisation (<10 kcal/mol). Thus, agostic interactions are stronger than most

hydrogen bond In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

s. Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. For instance, in Ziegler–Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain. This increased rigidity influences the stereoselectivity of the polymerization process.

Related bonding interactions

The term agostic is reserved to describe two-electron, three-center bonding interactions between carbon, hydrogen, and a metal. Two-electron three-center bonding is clearly implicated in the complexation of H₂, e.g., in W(CO)₃(PCy₃)₂H₂, which is closely related to the agostic complex shown in the figure.

Silane Silane (Silicane) is an inorganic compound with chemical formula . It is a colorless, pyrophoric gas with a sharp, repulsive, pungent smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental ...

binds to metal centers often via agostic-like, three-centered Si┄H−M interactions. Because these interactions do not include carbon, however, they are not classified as agostic.

Anagostic bonds

Certain M┄H−C interactions are not classified as agostic but are described by the term ''anagostic''. Anagostic interactions are more electrostatic in character. In terms of structures of anagostic interactions, the M┄H distances and M┄H−C angles fall into the ranges 2.3–2.9 Å and 110°–170°, respectively.

Function

Agostic interactions serve a key function in alkene polymerization and

stereochemistry Stereochemistry, a subdiscipline of chemistry, studies the spatial arrangement of atoms that form the structure of molecules and their manipulation. The study of stereochemistry focuses on the relationships between stereoisomers, which are defined ...

, as well as migratory insertion.

References

External links

Agostic interactions
{{chemical bonds Organometallic chemistry Chemical bonding