Several general conclusions can be drawn from the information now available on polyhydride complexes. Perhaps their most striking property is simply their existence in such great numbers. Their relative stability to loss of H 2 seems to be thermodynamic, not simply kinetic, because many synthetic routes involve H 2 addition, and H 2 addition/elimination rarely shows large activation energies. This implies that MH bond energies do not fall much below their usual values (ca. 60 kcal/mol −1) [288] even for polyhydrides, since only this can prevent H 2 (bond energy: 103 kcal mol −1) loss from being thermodynamically favorable. The first-row elements do not form so extensive a series of polyhydrides, perhaps because the MH bond strengths are lower. Similarly the general stability trends—Group VIII (but not Pd, Pt) ⋍ Group VII > Group VI > General V and 3rd row > 2nd row—can probably also be understood in the same way. The polyalkyls do not form an extensive analogous series, perhaps because MC bonds are generally weaker than MM bonds [288], and alkyl groups are also more bulkly than hydride ligands. However, the recent preparation of (C 5Me 5)IrME 4 [289], analogous to (C 5Me 5)IrH 4, may point the way towards the synthesis of further polyalkyls It was once thought that the highly electronegative elements F and O are best able to stabilize the highest oxidation states of metals. We now know that other small anionic ligands such as Me and H are also effective. As the least electronegative of all the formally anionic ligands (H, Me, O, and the halides) capable of giving polyligated species, the polyhydrides are expected to be the most electron rich, and to have the most extensive chemistry with π-bonding co-ligands. This aspect also has been illustrated in this review. Very few cluster polyhydrides have been prepared. The few known examples suggest that hydride ligands bond just as strongly to interstitial, bridging, and terminal sites in clusters as in classical polyhydrides. For example, [(IrH 2LL′) 3(μ 3-H)] 2+ [290] does not lose H 2 readily, even on heating. This in turn suggests that even “pure” or homoleptic cluster hydrides of the type [M x H y ] n− should be capable of existence. The corresponding polyalkyls may also be stable; [Re 3Me 9] n is known [291]. Both cationic and anionic polyhydrides are relatively rare, but it is likely that this represents prejudices of synthetic chemists rather than any unusual instability of these species, which may well have interesting properties. The upper limit for n in polyhydride complexes MH n L m has up to now been the maximum valency of the metal (e.g. Re, 7; W, 6; Ta, 5), but the recent isolation by Kubas and co-workers [292] of a complex of molecular hydrogen opens the way for the synthesis of polyhydrides of the type M(H 2) n ) n/2 L m where n > maximum valence. One feature of the reaction chemistry of polyhydrides that is very promising but is only just beginning to be exploited is thermal, photochemical, or protolytic H 2 loss to give species containing multiple vacant coordination sites. Their importance to the newly-emerging field of alkane activation has led to the extensive application of polyhydrides. Both catalytic alkane dehydrogenation [80,105,283,286] and alkane CC bond activation [284] also seem to require multiple sites. The center of interest in transition metal polyhydrides is beginning to move from the more traditional areas of synthesis and structural studies to the newer areas of reaction chemistry and catalysis. We hope that this review reflects the recent developments and will stimulate further research in this area.