Carbonyl hydrides acidic properties

Certain hydrides behave as acids in aqueous solution, in particular the simple carbonyl hydrides. This property in aqueous solution, however, does not necessarily indicate the direction of polarity of the M—H bond in the neutral hydride. For example, in some addition reactions of HCo(CO)4 to olefins the behavior of the hydrogen is sometimes protonic and sometimes hydridic (see Section III,B,3). Therefore it is unwise to classify metal hydrides as acidic or basic without reference to the particular conditions. 

General and Acid Properties. The carbonyl hydride HCo(CO)4 (3) is formed in situ from Co2(CO)g and H2 in hydroformylation mixtures or by acidification of the anion... 

The majority of homoleptic carbonyls and many other derivatives can be formed with PF3 hgands instead of CO. In many instances, CO is exchanged by PF3 under pressure. In contrast to the elusive nature of (3), its PF3 analog HCo(PF3)4 is of very high thermal stability and does not decompose below 250 °C. Its acid base properties seem to resemble those of the carbonyl hydride, however. 

HCo(CO) acts in the hydroformylation reaction more as a hydride rather than a proton donor. This behavior strongly contrasts the acidic properties of all the mentioned carbonyl hydride species, since in solution, and especially in water, they function as strong acids, with sometimes even very low pK values. The discrepancy that the very same transition metal hydrides may act as either hydride or proton sources, gave rise to controversial discussions in the literature for many years. Thus, HCo(CO) not only provides the prototypical example for the ambivalence of the TM-H bond, but also an early example of TMH complexes in homogeneous catalysis. 

Some metal carbonyl hydrides dissolved in polar solvents such as water, alcohols, nitriles, ketones, etc., are acids. The acidic character of these complexes is due not only to the polarity of the metal-hydrogen bond, but to solvation of the proton and the carbonylate anion. Acidic metal carbonyl hydrides dissolved in nonpolar solvents do not dissociate, and their chemical properties are like those of nonacidic metal hydrides. [HCo(CO)4] and [HV(CO)6] are strong acids (Table 2.28). 

Hydrides react with the alkali metals and bases to give anionic complexes. Metal carbonyl hydrides possessing strong acidic properties react in this manner most readily ... 

A particularly important derivative of the [Co(CO)4] anion is the hydrogen derivative, HCo(CO)4. Although this compound is commonly called cobalt carbonyl hydride , the term hydride is actually a misnomer since the hydrogen atom in HCo(CO)4 is not hydridic but strongly protonic in character HCo(CO)4 is as strong an acid as sulfuric acid (9S). Because of its unusual properties and relative simplicity, HCo(CO)4 has been the subject of several spectroscopic (225, 228, 229) and theoretical 230) investigations. 

The hydride complexes can be either good hydride donors (hydridic property) or acids (in which case they are still called hydrides, but do not deserve this name). This hydridic vs. acidic property depends on the charge, nature of the ligands and location of the metal in the periodic table. [VH(CO)6] and [CoH(CO)4] are strong acids. The acidity decreases as one goes down in the periodic table, or if one or several carbonyls are replaced by more donor ligands. There is also, of course, a continuum of intermediate situations where some hydrides, such as for instance [WCp(CO)3(H)], are able to behave as H+ or H donors depending on the substrate. 

Although the carbonyl group of acetaldehyde that accepts the hydride is inherently electrophilic, the enzyme enhances this property by providing a zinc ion as a Lewis acid to coordinate with the carbonyl oxygen. 

The chemical and spectroscopic properties of the acid hydride HRu(C0) I and of the ion-pairs of the [Ru(C0) I ] anion are reported and their role in the catalytic carbonylation and homologation reactions on oxygenated substrates are discussed. 

As shown in Eqs. (17) and (18), the isolated formyls 19 and 24 are capable of reducing aldehydes and ketones (37, 38, 42. 47, 66). Thus there is no doubt that hydride transfer is an intrinsic chemical property of anionic formyl complexes. One reaction of a neutral formyl complex with an aldehyde has been reported addition of benzaldehyde to (i7-C5H5)Re(NO)(CO)(CHO) (38) yields the alkoxycarbonyl complex (i7-C5H5)Re(NO)(CO)(C02CH2C6Hs) (62). This transformation, which appears to require catalysis by adventitious acid, can be viewed as occurring via attack of initially formed benzyl alcohol upon the intermediate carbonyl cation [(i -C5H5)Re(NO)(CO)2]+. 

The induction period can be explained as the result of reactions a or /3. The first, proposed by Heck 89), requires that tetracarbonylnickel have basic properties, but this could not be proved. In fact, the protonation reaction of carbonyls and carbonyl derivatives has been studied by Wilkinson and co-workers 64) Ni(CO)4 does react with acids but, contrary to Fe(CO)5, gives no NMR signal—indicating the formation of a hydride. However, the basic properties of some systems have been recently shown in complexes with phosphines and arsines. The platinum (0) 39, 40) derivatives yield stable hydrides according to the following equilibria ... 

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