The coordination chemistry of hydrogen

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). 

Even the H atom itself can form compounds in which its coordination number (CN) is not just 1 (as expected) but also 2, 3, 4, 5 or even 6. A rich and unexpectedly varied coordination chemistry is thus emerging. We shall deal with the H atom first and then with the H2 molecule. 

By far the most common CN of hydrogen is 1, as in HCl, H2S, PH3, CH4 and most other covalent hydrides and organic compounds. Bridging modes in which the H atom has a higher CN are shown schematically in the next column — in these structures M is typically a transition metal but, particularly in the Mi-tnode and to some extent in the x3-mode, one or more of the M can represent a main-group element such as B, Al C, Si N etc. Typical examples are in Table 3.3. Fuller discussion and references, when appropriate, will be found in later chapters dealing with the individual elements concerned. 

5 -Mg2NiH4(d4) (1 covalent Ni-D 149pm plus 4 ionic Mg-D 230pm) 

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The 

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The coordination chemistry of hydrogen is still being intensively studied and new developments are continually being reported. 

The coordination chemistry of hydrogen cyanide, cyanogen and cyanogen halides. B. Corain, Coord. Chem. Rev., 1982, 47,165-200 (111). 

The coordination chemistry of the large, electropositive Ln ions is complicated, especially in solution, by ill-defined stereochemistries and uncertain coordination numbers. This is well illustrated by the aquo ions themselves.These are known for all the lanthanides, providing the solutions are moderately acidic to prevent hydrolysis, with hydration numbers probably about 8 or 9 but with reported values depending on the methods used to measure them. It is likely that the primary hydration number decreases as the cationic radius falls across the series. However, confusion arises because the polarization of the H2O molecules attached directly to the cation facilitates hydrogen bonding to other H2O molecules. As this tendency will be the greater, the smaller the cation, it is quite reasonable that the secondary hydration number increases across the series. 

RhCl(PPh3)3 has been used for the homogeneous hydrogenation of various diene-based polymers, and its catalytic mechanism is understood to a considerable extent. Parent et al. [81] proposed a mechanism which has been found to be consistent with the kinetic data for various diene-based polymer hydrogenation systems and an understanding of the coordination chemistry of RhCl(PPh3)3 in solution. The main points comprising the mechanism are outlined as follows ... 

The coordination chemistry of the simplest of these, methanediamine, NH2CH2NH2 (a fully hydrogenated form of the aminidinato (13) ligand), is very poorly investigated. Of more recent interest are poly(methylene) amines, for example, (68) formed by reaction of formaldehyde and amines in the presence of a metal ion. The stability of such ligands in the absence of metal ions has yet to be established. 

Particularly since the initial discovery of dihydrogen complexes by Kubas [48], the coordination chemistry of H2 has revealed rich complexity [49]. There is little information on the precise pathway by which hydrogen adds to coordinative-... 

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