Transition metal hydrides are chemical compounds 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 of them are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H−-like character (e.g., ZnH2).
Classes of metal hydrides
Binary metal hydrides
Many transition metals form compounds with hydrogen. These materials are called binary hydrides, because they contain only two elements. The hydrogenic ligand is assumed to have hydridic (H−-like) character. These compounds are invariably insoluble in all solvents, reflecting their polymeric structures. They often exhibit metal-like electrical conductivity. Many are nonstoichiometric compounds. Electropositive metals (Ti, Zr, Hf, Zn) and some other metals form hydrides with the stoichiometry MH or sometimes MH2 (M = Ti, Zr, Hf, V, Zn). The best studied are the binary hydrides of palladium, which readily forms a limiting monohydride. In fact, hydrogen gas diffuses through Pd windows via the intermediacy of PdH.
Ternary metal hydrides Ternary metal hydrides have the formula AxMHn, where A+ is an alkali or alkaline earth metal cation, e.g. K+ and Mg2+. A celebrated example is K2ReH9, a salt containing two K+ ions and the ReH92− anion. Other homoleptic metal hydrides include the anions in Mg2FeH6 and Mg2NiH4. Some of these anionic polyhydrides satisfy the 18-electron rule, many do not. Because of their high lattice energy, these salts are typically not soluble in any solvents, a well known exception being K2ReH9.
Coordination complexes The most prevalent hydrides of the transition metals are metal complexes that contain a mix of ligands in addition to hydride. The range of coligands is large. Virtually all of the metals form such derivatives. The main exceptions include the late metals silver, gold, cadmium, and mercury, which form few or unstable complexes with direct M-H bonds. Examples of industrially useful hydrides are HCo(CO)4 and HRh(CO)(PPh3)3, which are catalysts for hydroformylation.
The first molecular hydrides of the transition metals were first reported in the 1930s by Walter Hieber and coworkers. They described H2Fe(CO)4 and HCo(CO)4. 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 Geoffrey Wilkinson, HMo(C5H5)(CO)3 by E. O. Fischer, and HPtCl(PEt3)2 by Joseph Chatt. Thousands of such compounds are now known.
Cluster hydrides Like hydrido coordination complexes, many clusters feature terminal (bound by one M–H bond) hydride ligands. Hydride ligands can also bridge pairs of metals, as illustrated by [HW2(CO)10]−. The cluster H2Os3(CO)10 features both terminal and doubly bridging hydride ligands. Hydrides can also span the triangular face of a cluster as in [Ag3{(PPh2)2CH2}3(μ3-H)(μ3-Cl)]BF4. In the cluster [Co6H(CO)15]−, the hydride is "interstitial", occupying a position at the center of the Co6 octahedron. The assignment for cluster hydrides can be challenging as illustrated by studies on Stryker's reagent [Cu6(PPh3)6H6].
Synthesis
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 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.
Elimination reactions Beta-hydride elimination 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:
MLnX + LiC4H9 → C4H9MLn + LiX C4H9MLn → HMLn + H2C=CHC2H5
Oxidative additions Oxidative addition of dihydrogen to a low valent transition metal center is common. Several metals react directly with H2, though usually heat to a few hundred degrees is required. One example is titanium dihydride, which forms when titanium sponge is heated to 400-700 °C under an atmosphere of hydrogen. These reactions typically require high surface area metals. The direct reaction of metals with H2 is a step in catalytic hydrogenation. For solutions, classic example involves Vaska's complex:
IrICl(CO)(PPh3)2 + H2 ⇌ H2IrIIICl(CO)(PPh3)2 Oxidative addition also can occur to dimetallic complexes, e.g.:
Co2(CO)8 + H2 ⇌ 2 HCo(CO)4 Many acids participate in oxidative additions, as illustrated by the addition of HCl to Vaska's complex:
IrICl(CO)(PPh3)2 + HCl → HIrIIICl2(CO)(PPh3)2
Heterolytic cleavage of dihydrogen Some metal hydrides form when a metal complex is treated with hydrogen in the presence of a base. The reaction involves no changes in the oxidation state of the metal and can be viewed as splitting H2 into hydride (which binds to the metal) and proton (which binds to the base).
MLnx+ + base + H2 ⇌ HMLn(x-1)+ + Hbase+ Such reaction are assumed to involve the intermediacy of dihydrogen complexes. Bifunctional catalysts activate H2 in this way.
Thermodynamic considerations
The values shift by <6 kJ/mol upon substitution of CO by a phosphine ligand. The M-H bond can in principle cleave to produce a proton, hydrogen radical, or hydride.
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