Polyhydrides

Polyhydrides are complexes such as FeH4(PR3)3, with a H M ratio exceeding 3. Hydrogen is not as electronegative as carbon, and so the metal in a polyhydride is not as oxidized as in a polyalkyl. Polyhydrides therefore retain more of the properties of low-valent complexes than do polyalkyls. For example, many of them are 18e, and relatively soft ligands (in the vast majority of cases a phosphine or a cyclopentadienyl) are required to stabilize them. Rare examples of N-donor-stabilized polyhydrides are [TpReH ] and [BpReH7] (Tp = tris-pyrazolylborate (5J7) Bp = bis-pyrazolylmethane).  

ReH7(P p-tolyl)3)2 has the structure ReHsCHa) with a stretched H—H distance (1.357 instead of 0.8-1.0 A in normal or unstretched Ha complexes) and so the oxidation state is difficult to define because the structure is half way between the Re(V) and Re(VII) extreme formulations. Re H7(dH e) is classical, however. A related Re tetrahydride exists in a tautomeric equilibrium (Eq. 15.19).23  

There is still doubt about the structures of some other polyhydrides, and this is an area in which X-ray crystallography is of limited use bet use of the small X-ray scattering factor for H. Crystals of the size appropriate for neutron work can be difficult to grow (Section 10.10), and NMR spectroscopic data (Section 10.7) are not always definitive. 

Polyhydrides often have coordination numbers in excess of 6, a consequence of the small size 6f the hydride ligand. Nine is the norma) limit on the number of ligands imposed by the availability of nine orbitals, but if a polyhydride can adopt a nonclassical structure with an H2 molecule bound via a single metal orbital, this limit can be exceeded. A rare example of such a complex is [WH7(PPh(CH2CH2PPh2)2)]+ (Eq. 15.20), which is stable up to -20°C in solution. Since 15.8 is classical with terminal M—H bonds, and therefore efi, there are no metal lone pairs and so protonation must occur at the M—H bond to give an H2 complex directly. If it were classical, 15.9 would exceed the maximum allowed oxidation state and coordination number for a transition metal. 

Almost all polyhydrides are fluxiona) in the NMR and the hydrides show coupling to any phosphines present. The number of hydrides present (n) can be pre cted with some confidence ftx m the 18e rule, but a useful experimental method involves counting the multiplicity (n -I-1) of the P NMR peak, after the phosphine ligand protons have been selectively decoupled (Section 10.4). 

Photochemical substitution is useful because it usually expels H2 to generate one or more 2e sites at the metal (Eq. 15.25).  

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Perhaps the most exciting recent development in the chemistry of hydrogen is the discovery that, in transition metal polyhydrides, the molecule Hj can act as a dihapto ligand, (see below). 

There is even a rare example involving a d polyhydride ... 

Bayse, C.A. and Hall, M.B. (1999) Prediction of the Geometries of Simple Transition Metal Polyhydride Complexes hy Symmetry Analysis. Journal of the American Chemical Society, 121, 1348-1358. 

A series of Irv boryl polyhydrides, cp IrH2XBR2 (X = H and BR2 = Bpin (1) (pin = 02C2Me4), Beat (2) (cat = 02C6H4), B02C6H2-3,4-Me2 (3) Bey (4), X = Bpin and BR2 = Bpin (5)) have been prepared and isolated by Hartwig and Kawamura according to Reaction Scheme 1. The X-ray crystal characterization of (1) is reported. (1) to (5) react regiospecifically with alkanes to produce free functionalized products. 

Polynuclear complexes associated with transition metal polyhydrides and silver cations have been described those which present silver—metal bonds are more numerous and they will be... 

Path C (Scheme 17.5), which involves reductive elimination of formic acid before reaction with H2, is only possible for dihydride and polyhydride catalysts. This pathway was proposed for the catalyst precursor [Rh(nbd)(PMe2Ph)3]+ by Tsai and Nicholas [19], who observed the intermediates [RhH2(PMe2Ph)3L]+ and [RhH( /2-02CH)(PMe2Ph)2L]+ (L=HzO or solvent). 

Generation of Reactive Intermediates via Photolysis of Transition-Metal Polyhydride Complexes... 

The photochemical properties of several transition metal polyhydride complexes are described. Irradiation of [MoH Cdppe) ] (dppe =... 

A number of studies have demonstrated that photolysis of transition metal di- and polyhydride complexes can lead to the generation of very reactive intermediates, generally via photo-induced loss of H9 Cl, 2). Two such examples are shown in eqs 1 (3) and 2 (4-8). Z... 

Photoinduced reductive elimination of from polyhydride complexes, HxMLn (x > 3) can, in principle, lead to the generation... 

Since photolysis of monomeric di- and polyhydride transition metal complexes often leads to elimination of H2 as the... 

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