Polyhydride compounds

Although it is clear that alkane activation is possible, the work done so far using soft metals as catalyst has not produced a reaction that converts alkanes into useful products. Nor does it point very clearly in the direction where progress will be made. It may be that studies on dimeric and cluster compounds, on polyhydrides, or on transition metal alkyls will indicate the paths to be explored. The work using hard catalysts is at present very limited and can be expected to be developed much further in future years. 

X-ray structural data also exist for two seven-coordinate polyhydrides, H3Re(DPPE)(PPh3)2 and H3Re(DPPE)2 (30) (DPPE is bisdiphenylphosphi-noethane, also known as diphos). Analysis of the data for the latter compound... 

The reaction of an alkene (in excess) with a polyhydride can strip hydrogen from the metal. The open sites formed in this process can now react, whether by cyclometalation or by coordinating the alkenes. If the alkene has accessible C—H bonds (e.g. propene) these may be broken by the metal. Indeed dehydrogenation may proceed until a stable 16-electron compound has been formed. In most cases this hydrogen stripped from the ligand is passed to the excess of alkene. Examples of these types of reaction have been described by Wilkinson (Scheme 5).131... 

Polyhydrides can be obtained in different oxidation states up to the highest for a given element many, according to H nmr spectra, are non-rigid. Reductive elimination of H2 can give lower oxidation state compounds of greater reactivity due to coordinative unsaturation. 

Turning first to the binuclear octahydrido compounds, [(H)2(PR3)2Re]2(/u.-H)4, we note that they are now most efficiently made by the action of NaBH4 on Re2CLt(PR3)4.53 It is found that both eclipsed (18-D-XIV) and staggered structures (18-D-XV) are adopted, depending on the phosphine and the crystallization conditions.54 There are also many other less symmetrical dirhenium polyhydrides.55 For example, the following reaction sequence occurs ... 

True hydride-like reactivity is often seen for the early metals, for anionic complexes, and for some polyhydrides. These metals are closer in behavior to the main group hydrides, which are often hydridic in character. One problem pointed out by Labinger is that if L MH is an 18e compound, hydride loss will give a 16e L M+ species that is rarely, if ever, stable. Coordinative unsaturation encourages the reduced substrate to bind to the metal (equation 17). This makes it difficult to define a scale of H donor power independent of the nature... 

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