Hydrogen hydrido anions

In covalent hydrides, hydrogen and the metal are linked by a covalent bond. Aluminum, silicon, germaninm, arsenic, and tin are some of the metals whose covalent hydride structures have been stndied extensively. Some ionic hydrides, snch as LiH or MgH2, exhibit partial covalent character. The complex hydrides, such as lithium aluminum hydride and sodium borohydride, contain two different metal atoms, usually an alkali metal cation bound to a complex hydrido anion. The general formula for these compounds is M(M H4) , where the tetrahedral M H4 contains a group IIIA metal such as boron. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

The hydrido complexes are diamagnetic, and are square planar or five-coordinate. Their stability, in general, increases with the number of coordinated phosphines. In the complex [Ni(BH4)(H)(PCy3)2] (153) the nickel atom is coordinated in the equatorial positions by two hydrogens of the borohydride and by one hydride anion. 

It is deduced from infrared data (206) that iron pentacarbonyl reacts with bromine at — 80° C to form the cationic halogeno carbonyl [Fe(CO)sBr]+Br (206), and the covalent Br—CO—Fe(CO)4Br. From similar infrared data, however, it is concluded that the product is a seven-coordinate non-ionic complex Fe(CO)5Br2 (76) The reaction of Os(CO)3(PPh3)2 with hydrogen chloride produces a cationic hydrido carbonyl, which may be isolated, although it readily reverts to a neutral dichloride by attack of the associated chloride anion (52). 

These studies have Indicated that simple rhodium carbonyl complexes, e.g., mono- and binuclear species are Involved in the fragmentation and aggregation reactions of rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen. They indicate that it is possible to write formal equations for such reactions in the case of rhodium carbonyl anionic hydrido clusters (equation 25) and for the more particular situation when there are not hydrides present (equation 26)... 

Many of the carbonyl cluster species are anions (cations being virtually unknown), hydrido species, or both. The relationships of these to each other and to neutral clusters in terms of electron count are the same as in simpler metal carbonyls, namely, one CO can be replaced by two hydrogen atoms, one H and one negative charge, or two negative charges. Protonation or deprotonation reactions are usually... 

In its complexes, except for those of molecular hydrogen, hydrogen is always treated as anionic. Hydrido is used for hydrogen coordinating to all elements including boron.3... 

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