Transition metal hydride
Encyclopedia
Transition metal hydrides are chemical compound
s containing a transition metal
bonded to hydrogen
. Most transition metals form hydride
complexes and some are significant in various catalytic
and synthetic reactions. The term "hydride" is used loosely, some so-called hydrides are acidic and some are hydridic (having H--like character).
in the 1930s. After a hiatus of several years, and following the release of German war documents on the postulated role of HCo(CO)4 in hydroformylation
, several new hydrides were reported in the mid-1950s by three prominent groups in organometallic chemistry: HRe(C5H5)2 by Wilkinson
, HMo(C5H5)(CO)3 by Fischer, and HPtCl(PEt3)2 by Chatt
. Thousands of such compounds are now known.
These conversions are metathesis reactions, and the hydricity of the product is generally less than than of the hydride donor. Classical (and relatively cheap) hydride donor reagents include sodium borohydride
and lithium aluminium hydride
. In the laboratory, more control is often offered by "mixed hydrides" such as lithium triethylborohydride
and Red-Al
. Alkali metal hydrides, e.g. sodium hydride
, are not typically useful reagents.
and alpha-hydride elimination are processes that afford hydrides. The former a common termination pathway in homogeneous polymerization
. It also allows some transition metal hydride complexes to be synthesized from organolithium and Grignard reagents.
:
Oxidative addition also can occur to dimetallic complexes, e.g.:
Protonation of a complex is viewed as an oxidative process as well:
s, some of which are believed to react through a concerted mechanism
.
Although these properties are interrelated, they are not interdependent. A metal hydride can be a thermodynamically a weak acid and a weak H- donor; it could also be strong in one category but not the other or strong in both. The H- strength of a hydride also known as its hydride donor ability or hydricity corresponds to the hydride's Lewis base strength. Not all hydrides are powerful Lewis bases. The base strength of hydrides vary as much a the pKa of protons. This hydricity can be measured by heterolytic cleaving hydrogen between a metal complex and base with a known pKa then measuring the resulting equilibrium. This presupposes that the hydride doesn't heterolytically or homolytically react with itself to reform hydrogen. A complex would homolytically react with itself if the homolytic M-H bond is worth less than half of the homolytic H-H bond. Even if the homolytic bond strength above that threshold the complex is still susceptible to radical reaction pathways.
A complex will heterolytically react with itself when its simultaneously a strong acid and a strong hydride. This conversion results in disproportionation
producing a pair of complexes with oxidation state
s that differ by two electrons. Further electrochemical reactions are possible.
As noted some complexes heterolytically cleave dihydrogen in the presence of a base. A portion of these complexes result in hydride complexes acidic enough to be deprotonated a second time by the base. In this situation the starting complex can be reduced by two electrons with hydrogen and base. Even if the hydride is not acidic enough to be deprotonated it can homolytically react with itself as discussed above for an overall one electron reduction.
The H- bond strength (the hydricity) of M-H fragments have been demonstrated to be greatest for first row transition metal centers and least for second row transition metal centers with third row metal centers falling in between for corresponding complexes. These results run contrary to tenants of ligand field theory
which suggest that metal ligand bond strengths generally increase as a periodic group is descended.
s. As a result, kinetic studies
are employed to elucidate both the relevant thermodynamic parameters. Generally hydrides derived from first row transition metals display the most rapid kinetics followed by the second and third row metal complexes.
are also known. Of these bridging hydrides many are oligomaric, such as Stryker's reagent
[(Ph3P)CuH]6 and clusters such as [Rh6(PR3)6H12]2+. The final bonding motif is the non-classical dihydride
also known as sigma bond dihydrogen adducts or simple a dihydrogen complex. The [W(PR3)2(CO)3(H2)] complex was the first well characterized example of both a non-classical dihydride and sigma-bond complex in general. X-ray diffraction is generally insufficient to locate hydrides in crystal structures and thus their location must be assumed. It requires Neutron diffraction
to unambiguously cytalographically locate a hydride near a heavy atom. Non-classical hydrides have also been studied with a variety of variable temperature NMR techniques and HD Couplings.
in their proton NMR
spectra. It is common for the M-H signal to appear between δ-5 and -25 with many examples outside this range but generally all appear below 0 ppm. The large shifts arise from the influence of the excited state
s and due to strong spin-orbit coupling (in contrast, 1H NMR shifts for organic compounds typically occur in the range δ12-1). At one extreme is the 16e complex IrHCl2(PMe(t-Bu)2)2 with a shift of -50.5. The signals often exhibit spin-spin coupling to other ligands, e.g. phosphines.
IR spectra for metal hydrides are exhibit bands near 2000 cm−1 for νM-H, although the intensities are variable. These signals can be identified by deuterium labeling.
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 containing a transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
bonded to hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. Most transition metals form hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
complexes and some are significant in various catalytic
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
and synthetic reactions. The term "hydride" is used loosely, some so-called hydrides are acidic and some are hydridic (having H--like character).
History
The first metal hydrides to be characterized were H2Fe(CO)4 and HCo(CO)4, in work by Walter HieberWalter Hieber
Walter Hieber was an inorganic chemist, known as the father of metal carbonyl chemistry. He was born 18 December, 1895 and died 29 November, 1976. Hieber's father was Johannes Hieber, an influential evangelical minister and politician....
in the 1930s. After a hiatus of several years, and following the release of German war documents on the postulated role of HCo(CO)4 in hydroformylation
Hydroformylation
Hydroformylation, also known as oxo synthesis or oxo process, is an important industrial process for the production of aldehydes from alkenes. This chemical reaction entails the addition of a formyl group and a hydrogen atom to a carbon-carbon double bond...
, several new hydrides were reported in the mid-1950s by three prominent groups in organometallic chemistry: HRe(C5H5)2 by Wilkinson
Geoffrey Wilkinson
Sir Geoffrey Wilkinson FRS was a Nobel laureate English chemist who pioneered inorganic chemistry and homogeneous transition metal catalysis.-Biography:...
, HMo(C5H5)(CO)3 by Fischer, and HPtCl(PEt3)2 by Chatt
Joseph Chatt
Joseph Chatt, CBE FRS was a renowned researcher in the area of inorganic and organometallic chemistry. His name is associated with the description of the pi-bond between transition metals and alkenes, the so-called Dewar-Chatt-Duncanson model.Chatt received his Ph.D. at the University of...
. Thousands of such compounds are now known.
Hydride transfer
Nucleophilic main group hydrides convert many transition metal halides and cations into the corresponding hydrides:- MLnX + LiBHEt3 → HMLn + BEt3 + LiX
These conversions are metathesis reactions, and the hydricity of the product is generally less than than of the hydride donor. Classical (and relatively cheap) hydride donor reagents include sodium borohydride
Sodium borohydride
Sodium borohydride, also known as sodium tetrahydridoborate, is an inorganic compound with the formula NaBH4. This white solid, usually encountered as a powder, is a versatile reducing agent that finds wide application in chemistry, both in the laboratory and on a technical scale. Large amounts are...
and lithium aluminium hydride
Lithium aluminium hydride
Lithium aluminium hydride, commonly abbreviated to LAH or known as LithAl, is an inorganic compound with the chemical formula LiAlH4. It was discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic synthesis, especially for the reduction of esters,...
. In the laboratory, more control is often offered by "mixed hydrides" such as lithium triethylborohydride
Lithium triethylborohydride
Lithium triethylborohydride , commonly abbreviated to LiTEBH or Superhydride, is a powerful and selective reducing agent used in inorganic and organic chemistry. LiTEBH is far more powerful than lithium borohydride and more powerful than lithium aluminium hydride in many cases...
and Red-Al
Red-Al
Red-Al, or sodium bisaluminumhydride, is an organoaluminium compound with the formula NaAlH22. The name which is a trademark of Sigma-Aldrich refers to its being a Reducing aluminium compound. It is used almost exclusively as a solution in toluene, which is used as a reagent in organic synthesis...
. Alkali metal hydrides, e.g. sodium hydride
Sodium hydride
Sodium hydride is the chemical compound with the empirical formula NaH. It is primarily used as a strong base in organic synthesis. NaH is representative of the saline hydrides, meaning it is a salt-like hydride, composed of Na+ and H− ions, in contrast to the more molecular hydrides such as...
, are not typically useful reagents.
Elimination reactions
Beta-hydride eliminationBeta-hydride elimination
Beta-hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the beta carbon. For instance butyl groups can undergo this reaction but methyl groups cannot...
and alpha-hydride elimination are processes that afford hydrides. The former a common termination pathway in homogeneous polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
. It also allows some transition metal hydride complexes to be synthesized from organolithium and Grignard reagents.
- MLnX + LiC4H9 → C4H9MLn + LiX
- C4H9MLn → HMLn + H2C=CHC2H5
Oxidative additions
Addition of dihydrogen to a transition metal center is perhaps the most classic example of oxidative additionOxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre...
:
- MLn + H2 ⇌ H2MLn
Oxidative addition also can occur to dimetallic complexes, e.g.:
- [Co2(CO)8]2 + H2 ⇌ 2 HCo(CO)4
Protonation of a complex is viewed as an oxidative process as well:
- MLn + H+ ⇌ HMLn+
Heterolytic cleavage of dihydrogen
A hydride can be formed without changing the oxidation state of the metal by splitting H2 into hydride and proton. Typically such syntheses involve the use of a metal cation and a base. The mechanism for this reaction may involve the intermediacy of dihydride or hydrogen complexes. This pathway is also an important mechanism for bifunctional catalystBifunctional catalyst
A bifunctional catalyst is term coined to refer to a specific set of hydrogenation catalyst containing Lewis acid and Lewis base....
s, some of which are believed to react through a concerted mechanism
Concerted reaction
In chemistry, a concerted reaction is a chemical reaction in which all bond breaking and bond making occurs in a single step. Reactive intermediates or other unstable high energy intermediates are not involved. Concerted reaction rates tend not to depend on solvent polarity ruling out large buildup...
.
- MLnx+ + Base + H2 ⇌ HMLn(x-1)+ + HBase+
Thermodynamic considerations
The M-H bond can in principle cleave to produce a proton, hydrogen radical, or hydride.- HMLn ⇌ MLn- + H+
- HMLn ⇌ MLn + H
- HMLn ⇌ MLn+ + H-
Although these properties are interrelated, they are not interdependent. A metal hydride can be a thermodynamically a weak acid and a weak H- donor; it could also be strong in one category but not the other or strong in both. The H- strength of a hydride also known as its hydride donor ability or hydricity corresponds to the hydride's Lewis base strength. Not all hydrides are powerful Lewis bases. The base strength of hydrides vary as much a the pKa of protons. This hydricity can be measured by heterolytic cleaving hydrogen between a metal complex and base with a known pKa then measuring the resulting equilibrium. This presupposes that the hydride doesn't heterolytically or homolytically react with itself to reform hydrogen. A complex would homolytically react with itself if the homolytic M-H bond is worth less than half of the homolytic H-H bond. Even if the homolytic bond strength above that threshold the complex is still susceptible to radical reaction pathways.
- 2 HMLnz ⇌ 2 MLnz + H2
A complex will heterolytically react with itself when its simultaneously a strong acid and a strong hydride. This conversion results in disproportionation
Disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
producing a pair of complexes with oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s that differ by two electrons. Further electrochemical reactions are possible.
- 2HMLnz ⇌ MLnz+1 + MLnz-1 + H2
As noted some complexes heterolytically cleave dihydrogen in the presence of a base. A portion of these complexes result in hydride complexes acidic enough to be deprotonated a second time by the base. In this situation the starting complex can be reduced by two electrons with hydrogen and base. Even if the hydride is not acidic enough to be deprotonated it can homolytically react with itself as discussed above for an overall one electron reduction.
- Two deprotonations: MLnz + H2 + 2Base ⇌ MLnz-2 + 2H+Base
- Deprotonation followed by homolysis: 2MLnz + H2 + 2Base ⇌ 2MLnz-1 + 2H+Base
The H- bond strength (the hydricity) of M-H fragments have been demonstrated to be greatest for first row transition metal centers and least for second row transition metal centers with third row metal centers falling in between for corresponding complexes. These results run contrary to tenants of ligand field theory
Ligand field theory
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...
which suggest that metal ligand bond strengths generally increase as a periodic group is descended.
Kinetics and mechanism
The rates of proton-transfer to and between metal complexes are often slow. Many hydrides are inaccessible to study through Bordwell thermodynamic cycleBordwell thermodynamic cycle
A Bordwell thermodynamic cycle use experimentally determined and reasonable estimates of Gibbs free energy values to determine unknown and experimentally inaccessible values.- Overview :...
s. As a result, kinetic studies
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
are employed to elucidate both the relevant thermodynamic parameters. Generally hydrides derived from first row transition metals display the most rapid kinetics followed by the second and third row metal complexes.
Bonding motifs
Metal complexes containing terminal hydrides are common. Complexes featuring bridgingBridging ligand
A bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...
are also known. Of these bridging hydrides many are oligomaric, such as Stryker's reagent
Stryker's reagent
Stryker's reagent Stryker's reagent Stryker's reagent ([(PPh3)CuH]6, also known as the Osborn complex, is a hexameric copper hydride ligated with triphenylphosphine. It is a brick red crystalline solid that is very sensitive to air...
[(Ph3P)CuH]6 and clusters such as [Rh6(PR3)6H12]2+. The final bonding motif is the non-classical dihydride
Dihydrogen complex
Dihydrogen complexes are coordination complexes containing intact H2 as a ligand. The prototypical complex is W32. This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported...
also known as sigma bond dihydrogen adducts or simple a dihydrogen complex. The [W(PR3)2(CO)3(H2)] complex was the first well characterized example of both a non-classical dihydride and sigma-bond complex in general. X-ray diffraction is generally insufficient to locate hydrides in crystal structures and thus their location must be assumed. It requires Neutron diffraction
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 thermal or cold neutrons to obtain a diffraction pattern that provides information of...
to unambiguously cytalographically locate a hydride near a heavy atom. Non-classical hydrides have also been studied with a variety of variable temperature NMR techniques and HD Couplings.
- Classical Terminal: M-H
- Classical BridgingBridging ligandA bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually restricted to small ligands such as pseudohalides or to ligands that are...
: M-H-M
- Non-ClassicalDihydrogen complexDihydrogen complexes are coordination complexes containing intact H2 as a ligand. The prototypical complex is W32. This class of compounds represent intermediates in metal-catalyzed reactions involving hydrogen. Hundreds of dihydrogen complexes have been reported...
: M-H2
Spectroscopy
Late transition metal hydrides characteristically show up-field shiftsChemical shift
In nuclear magnetic resonance spectroscopy, the chemical shift is the resonant frequency of a nucleus relative to a standard. Often the position and number of chemical shifts are diagnostic of the structure of a molecule...
in their proton NMR
Proton NMR
Proton NMR is the application of nuclear magnetic resonance in NMR spectroscopy with respect to hydrogen-1 nuclei within the molecules of a substance, in order to determine the structure of its molecules. In samples where natural hydrogen is used, practically all of the hydrogen consists of the...
spectra. It is common for the M-H signal to appear between δ-5 and -25 with many examples outside this range but generally all appear below 0 ppm. The large shifts arise from the influence of the excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s and due to strong spin-orbit coupling (in contrast, 1H NMR shifts for organic compounds typically occur in the range δ12-1). At one extreme is the 16e complex IrHCl2(PMe(t-Bu)2)2 with a shift of -50.5. The signals often exhibit spin-spin coupling to other ligands, e.g. phosphines.
IR spectra for metal hydrides are exhibit bands near 2000 cm−1 for νM-H, although the intensities are variable. These signals can be identified by deuterium labeling.