Hydrido complexes acidity

Despite the above similarities, many differences between the members of this triad are also to be noted. Reduction of a trivalent compound, which yields a divalent compound in the case of cobalt, rarely does so for the heavier elements where the metal, univalent compounds, or hydrido complexes are the more usual products. Rhodium forms the quite stable, yellow [Rh(H20)6] " ion when hydrous Rh203 is dissolved in mineral acid, and it occurs in the solid state in salts such as the perchlorate, sulfate and alums. [Ir(H20)6] + is less readily obtained but has been shown to occur in solutions of in cone HCIO4. 

The second method is the more general one. The choice of the base depends on the acidity of the hydrido complexes. In most cases KH, NaH or LinBu are very suitable. For the more acidic complexes, amines are also convenient. Complexes of low acidity were successfully deprotonated by KH/18-crown-6. 

The monosubstituted adduct offers the ready synthesis of a whole range of monosubstituted adducts (see Scheme 6) it is often possible to isolate in these reactions intermediates that are not readily obtained by alternative methods. Thus, in the reaction with halogen acids to yield the bridged hydrido complexes HOs3(CO)10X, it is possible to identify the intermediate HOs3(CO)uX complex in which the halogen functions as a one-electron donor bonding to only one metal center (158). 

Carbonyl Hydride Acidification of carbonylate ions often results in the formation of carbonyl hydrido Complexes complexes, which may be regarded as the conjugate acids of the carbonylates ... 

Some complexes of nickel(0) with phosphines have been found to react with several Bronsted acids in non-aqueous solvents and under an inert atmosphere to give hydrido complexes of nickel(I) and nickel(II) (equations 85-87).264,268,269... 

Vaska, L. and DiLuzio, J.W. (1961) Carbonyl and hydrido-carbonyl complexes of iridium by reaction with alcohols-hydrido complexes by reaction with acid. J. Am. Chem. Soc., 83, 2784. 

Aliphatic amines can be readily oxidized by Pd(II) to imines or iminium salts and hydrido complexes. The latter can transfer hydrogen to alkenes, leading to the formation of alkanes as byproducts of the Heck reaction (last example, Scheme 8.18). Such reactions can be avoided by using alkali carbonates as base instead of aliphatic amines [148]. Treatment of stannanes or organoboron derivatives with electron-deficient alkenes under acidic reaction conditions can also lead to formal products of Michael addition instead of the products of a Heck-type reaction [149, 150] (Scheme8.19). 

In contrast, an aprotic solvent such as tetrahydrofuran (e = 7.4) is insufficiently basic (pKa = -2.05) to level HC1, HBr, and H2S04. Thus these acids are correspondingly weaker than in the protic solvent. This, together with the lower dielectric constant which favors the formation of adducts, can, under certain very specific conditions, result in the preferential formation of hydrido complexes (i.e., attack of acid at the metal rather than dinitrogen). 

Understandably, these compounds having cis CO groups do not undergo a Hofmann acid amide degradation to give the respective hydrido complexes. The stability of these Group VIIIB carbamoyl complexes increases in the order Fe, Ru, Os. 

Generally, when dinitrogen complexes are treated with acid the strongly reduced metal is protonated, leading to oxidation of the metal and formation of a hydrido complex. This is accompanied usually by loss of dinitrogen and often by subsequent protic attack on the hydride to evolve dihydrogen (7). Of the stable mononuclear dinitrogen com-... 

The hydrido complex [Os(NO)2H(PPh3)2]+ is formed by the action of strong acids on the complex, and the reaction of CO under pressure with this gives Os(CO)4(PPh3), with C02 and N2 as by-products.249... 

The hydrido complex RhH(PPh3)3 is also a good starting point for preparing other complexes. Upon treatment with acids it eliminates hydrogen and forms an [RhX(PPh3)3] complex (equation 38). 

The action of PF3 and hydrogen at high temperatures and pressures upon mixtures of anhydrous rhodium trichloride and copper forms (equation 57) the colorless liquid [RhH(PF3)4] (b.p. 89°C/ 725 torr).32-35-208 The liquid decomposes slowly at 20°C to form [Rh(PF2)(PF3)3]2, but on heating to 140nC it decomposes to rhodium metal.32 The hydrido complex can also be formed by the action of strong acids on the potassium salt K[Rh(PF3)4] (equation 58).34... 

The complex behaves as a strong acid (Scheme 9). The deuterio complex can be prepared by treating the hydrido complex with D2S04.32 The exchange of hydrogen is much easier than in the case of [RhH(PPh3)4] which only exchanges very slowly with deuterium gas (equation 59).209... 

The preparation of the hydrido complex from tetrafluoroboric acid is difficult to reproduce. It has been reported that the preparation from HBF4 Et20 is successful only if ethanol is present. Under these conditions [RhHCl(L)(EtOH)][BF4] is formed. In THF solution the THF solvate is formed.1061... 

In deaerated methanolic solution, iron(II) hydrido complexes of the type [FeH(L)(dppe)2]+ (L = MeCN, MeCH2CN, CH,=CHCN, PhCN and p-MeC6H4CN) are reduced to [FeH(L)(dppe)2] by solvated electrons produced by y-radiolysis of the solvent.228 In pulsed radiolysis studies [FeH(L)dppe2] decays to five-coordinate [FeH(dppe)2] (half-life < 1 s). In continuous radiolysis however, [FeH(dppe)2] is not the final product and here as in its reaction with acids it is suspected that oxidative addition of a proton yields an unstable iron(III) dihydride which decom-... 

It was quickly realized that both the hydrogenation of alkenes and the reduction of the complex to the metal were brought about by hydrido species. It was therefore essential that the catalysts should be resistant to the autoreduction reaction. At the time of this discovery, many stable hydrido complexes containing 7r-acid hgands, such as tertiary phosphines, were being isolated, so the search for homogeneous catalysts concentrated on these. 

Compound [Cr(C0)g(02CCF3)] may be prepared by oxidative substitution of Ag(02CCF3) or Hg(02CCF3)2 on [Cr2(CO)io] (218). Transition metal hydrido complexes have been obtained by oxidative addition reactions of trifluoroacetic acid which often lead to the formation of trifluoroacetato complexes. Equation (3) is an example of such a... 

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