Transition metal hydride

Binary metal hydrides

Many transition metals form compounds with hydrogen, called binary hydrides because they only contain two elements. These compounds are invariably insoluble in all solvents. 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 A_xMH_n, 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 other 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 an industrially useful hydrides are HCo(CO)₄ and HRh(CO)₂(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 Wilkinson, HMo(C₅H₅)(CO)₃ by Fischer, and HPtCl(PEt₃)₂ by 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)₁₁ 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].

Hydride transfer

Nucleophilic main group hydrides convert many transition metal halides and cations into the corresponding hydrides:

ML_nX + LiBHEt₃ → HML_n + 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:

ML_nX + LiC₄H₉ → C₄H₉ML_n + LiX C₄H₉ML_n → HML_n + H₂C=CHC₂H₅

Oxidative additions

Addition of dihydrogen to a transition metal center is perhaps the most classic example of oxidative addition:

ML_n + H₂ ⇌ H₂ML_n

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:

ML_n + H⁺ ⇌ HML_n⁺

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).

ML_n^x+ + Base + H₂ ⇌ HML_n^(x-1)+ + HBase⁺

Such reaction are assumed to involve the intermediacy of dihydrogen complexes. Bifunctional catalysts activate H2 in this way.

Thermodynamic considerations

The M-H bond can in principle cleave to produce a proton, hydrogen radical, or hydride.

HML_n ⇌ ML_n⁻ + H⁺ HML_n ⇌ ML_n + H HML_n ⇌ ML_n⁺ + 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 as the pKa of protons. This hydricity can be measured by heterolytic cleaving hydrogen between a metal complex and base with a known pK_a 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 HML_n^z ⇌ 2 ML_n^z + H₂

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 states that differ by two electrons. Further electrochemical reactions are possible.

2HML_n^z ⇌ ML_n^z+1 + ML_n^z-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: ML_n^z + H₂ + 2Base ⇌ ML_n^z-2 + 2H⁺Base Deprotonation followed by homolysis: 2ML_n^z + H₂ + 2Base ⇌ 2ML_n^z-1 + 2H⁺Base

Hydricity

For a Lewis acid, its affinity for a hydride ligand is called its hydricity:

ML_nHⁿ⁻ ⇌ ML_n⁽ⁿ⁺¹⁾⁻ + H⁻

Since hydride does not exist as a stable anion in solution, this equilibrium constant (and its associated free energy) are calculated from measurable equilibria. The reference point is the hydricity of a proton, which in acetonitrile solution is calculated at -76 kcal/mol:

H⁺ + H⁻ ⇌ H₂

Relative to a proton, most cations exhibit a lower affinity for H-. Some examples include:

[Ni(dppe)₂]²⁺ + H⁻ ⇌ [HNi(dppe)₂]⁺ ΔG(298) = -63 kcal/mol [Ni(dmpe)₂]²⁺ + H⁻ ⇌ [HNi(dmpe)₂]⁺ ΔG(298) = -50.7 [Pt(dppe)₂]²⁺ + H⁻ ⇌ [HPt(dppe)₂]⁺ ΔG(298) = -53 [Pt(dmpe)₂]²⁺ + H⁻ ⇌ [HPt(dmpe)₂]⁺ ΔG(298) = -42.6

These data suggest that [HPt(dmpe)₂]⁺ would be a strong hydride donor, reflecting the relatively high stability of [Pt(dmpe)₂]²⁺. The ionic bond character in transition metal hydrides can be measured for D ligands via deuterium quadrupole coupling constants.

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 cycles. 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.

Bonding motifs

Metal complexes containing terminal hydrides are common. Complexes featuring bridging ligands are also known. Of these bridging hydrides many are oligomeric, 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 simply dihydrogen complexes. 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.

Classical Terminal: M-H Classical Bridging: M-H-M Non-Classical: M-H₂

Spectroscopy

Late transition metal hydrides characteristically show up-field shifts 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 states 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.

Metal hydrides exhibit IR bands near 2000 cm⁻¹ for ν_M-H, although the intensities are variable. These signals can be identified by deuterium labeling.

History

The first reported metal hydrido complex was H₂Fe(CO)₄, obtained by the low temperature protonation of an iron carbonyl anion. The next reported hydride complex was (C₅H₅)₂ReH. In 1957, Chatt and Shaw described trans-PtHCl(PEt₃)₂ the first non-organometallic hydride (i.e., lacking a metal-carbon bond).