Complex hydrides nomenclature

Although the lUPAC has recommended the names tetrahydroborate, tetrahydroaluminate, etc, this nomenclature is not yet ia general use. Borohydrides. The alkaU metal borohydrides are the most important complex hydrides. They are ionic, white, crystalline, high melting soHds that are sensitive to moisture but not to oxygen. Group 13 (IIIA) and transition-metal borohydrides, on the other hand, are covalendy bonded and are either Hquids or sublimable soHds. The alkaline-earth borohydrides are iatermediate between these two extremes, and display some covalent character. 

In more recent publications a new, more systematic nomenclature for hydrides and complex hydrides has been adopted. Examples of both nomenclatures are shown below ... 

Any pattern of nomenclature for the complex hydrides should be capable of extension to the complex alkyl and aryl derivatives. The last two examples indicate how this is accomplished. In strict analogy the alkyl groups should be designated methylo, ethylo, etc., but this may depart too much from custom and imply too definite a conviction as to the nature of the carbon-metal bond. 

The fact that metal hydrides can be acidic may seem paradoxical in view of the nomenclature that insists that all complexes with a M-H bond be referred to as hydrides regardless of whether their reactivity is hydridic or not. Not only can some metal hydrides donate a proton, but some can be remarkably acidic. Some cationic dihydrogen complexes are sufficiently acidic to protonate Et20 [8], and some dicationic ruthenium complexes have an acidity comparable to or exceeding that of HOTf [9],... 

This is an extremely complex field, requiring major modifications to organic nomenclature and the use of terms and techniqnes from inorganic nomenclature. Even so, there are a significant number of boron componnds whose names are not satisfactory to members of either the organic or inorganic community (see Fox and Powell, loc. cit.. Chapter 27, for details). Because the number of hydrogen atoms in neutral and anionic boron hydrides often bears no simple relationship to the number of boron atoms, borane names must express the number of both. 

Even though conventions of nomenclature designate all metal complexes with a bond to hydrogen as metal hydrides, many metal hydrides can exhibit acidic as well as hydridic behavior. Norton and coworkers studied extensively the thermodynamic and kinetic acidities of a wide series of metal carbonyl hydrides in acetonitrile [7]. Table 3.1 shows a few of the plQ values reported to document the range of pK, values and to illustrate some of the trends more extensive compilations are available [7]. The acidity of (CO)4CoH, the metal hydride involved in catalytic hydroformylations, is 8.3, which is similar to the acidity of HCl in CH3CN. Clearly, some metal hydrides can exhibit quite acidic behavior Morris... 

Regarding H, the oxidation number for nomenclature shall be I and -I when H is connected with nonmetals and metals, respectively, except in hydride complexes when it shall be -I. While these recommendations are useful for their purpose, they have some consequences that are conceptually curious. For example, the following two reactions and others of a similar kind become redox reactions ... 

Scheme 7 The tuck-in cerium complex and cerium hydride used in the experiment and models used in the calculations with labelling nomenclature...

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