Preparation of Hydrido Complexes

There are several methods for the preparation of metal carbonyl hydrides  

Reactions of complexes with reducing agents and metals. 

In a similar manner, NaBH4 reacts with almost all transition metal carbonyls to give anionic, polynuclear hydrides. Analogous reactions involving carbonyl complexes possessing other ligands are represented by the following  

Various other reducing agents may also be employed, such as H3PO2, NaH2P02, Na2S2 04, formic acid, etc. 

Reactions involving a hydrogen atom transfer from a solvent or a coordinated group. 

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It might be thought that reductive methods would be suitable for the preparation of hydrido-complexes. In fact, most such compounds are prepared by substitution reactions, using a source of H such as NaBH4 the formation of, e.g., NaBCl4 as a product assists the thermodynamics (cf. the use of LiAlH4 in the preparation of SiH4 in Section 10.3 above). 

Preparation of Hydrido(hydroxo), Hydrido(alkoxo), and Hydrido(carboxylato) Complexes by Metathesis... 

Before discussing the preparation of late transition metal complexes resulting from the activation of O-H bonds by late transition metal complexes, we wbl describe metathesis methods for the preparation of hydrido(hydroxo), hydrido(alkoxo), and hydrido(carboxylato) complexes. Though many methods of preparation of transition metal hydroxides, alkoxides, etc. by a metathesis reaction have been reported [1], only a limited number of examples of the preparation of hydrido(hydroxo), hydri-do(alkoxo) complexes etc. by metathesis are available. 

Scheme 6-2 Preparation of hydrido(hydroxo) and hydrido (hydroxide) complexes by oxidative addition of water...

Merola reported the preparation of hydrido(carboxylato)iridium(lll) complexes, mer-[lrCl(0C(0)R)(H)(PMe3)3] (90) (R = Ph, Me), by oxidative addition of acetic acid or benzoic acid to [Ir(cod)(PMe3)3]Cl (67) [46]. The structure of 90 (R = Ph) in which the carboxylato ligand coordinates as an T -ligand, was confirmed by X-ray analysis. The reaction of 67 with salicylic acid yielded the product 91, which resulted from activation of the O-H bond of the carboxylato but not of the hydroxo group (Scheme 6-13). 

Hydrido-(l-pyrazolyl)borates [H4 B(C3H3N2) ]-, in short [H4 B(pz) ], often with more or less bulky substituents in 3- and/or 5-positions of the pz ring, have been used for the preparation of mononuclear complexes with controlled coordination sphere about a metal.195 With Cd as a metal center different coordination numbers and geometries can be realized, depending on the pyrazolylborate used, but also on the co-ligands. 

J. Chatt, and J. M. Davidson, The Tautomerism of Arene and Ditertiary Phosphine Complexes of Ruthenium(O), and the Preparation of New Types of Hydrido-Complexes of Ruthenium(II), J. Chem. Soc. 1965, 843-855. 

The hydrido species ReH(N2)(dppe)2 results from the reaction of (Et4N)2ReH9 with dppe in propan-2-ol under an N2 atmosphere.36 An alternative preparation of this complex involves the reaction of N2 with photogenerated coordinatively unsaturated ReH(dppe)2. This reaction can be reversed through the photo induced loss of N2. Reaction of this complex with MeX (X = Br or I) gives ReX(N2)(dppe)2, while displacement of the N, ligand occurs upon its reaction with C2H4 and CO.36 The properties of the hydride and phenylimido complexes resemble those of the other derivatives (Table 3). 

Reaction with Co(CN)5H . General Considerations. Except for methyl iodide, primary, secondary, and tertiary alkyl iodides formed the corresponding alkanes, accompanied by minor quantities of alkenes, in good yield when added to well-stirred solutions of hydrido complex in a hydrogen atmosphere. Only about 35% of methyl iodide was converted to methane, the remainder forming the stable methylcobalt complex. This apparent preferential competition by pentacyanocobaltate(II) for the methyl radical has been used to prepare the methylcobalt complex (22). 

In addition to the work described here, experiments with styrenes and a,)8-unsaturated acids, esters and ketones, as well as related halides (24), suggest certain generalizations concerning the effect of structure on organocobalt complex stability and catalysis. Table IX lists several organocobalt complexes, whose preparations, either by reaction of the halide with pentacyanocobaltate(II) or by addition of hydrido complex to the corresponding unsaturated substrate, have been attempted. In many cases, both methods were used. 

Methods of preparing selected hydrido complexes were given in equations 23.2 and 23.46-23.50. Selected properties of the mononuclear complexes HMn(CO)5, H2pe(CO)4 and HCo(CO)4 are given in Table 23.6. Metal hydrides play an important role in organometallic chemistry, and scheme 23.57 illustrates some ligand transformations involving M—H bonds HCo(CO)4 is an industrial catalyst (see Section 26.4). 

The KOH-alcohol reduction is a general one for the preparation of hydrido or hydridocarbonyl complexes, but detailed mechanistic studies are lacking. 

Chatt J, Davidson JM (1965) The tautomerism of arene and ditertiary phosphine complexes of Ruthenium(O), and the preparation of new types of hydrido-complexes of ruthenium(II). J Chem Soc 843-855... 

Oxidative addition of G-H bonds to Pt(0) is utilized in the preparation of arylplatinum complexes. Furan, benzoxazole, and benzothiazole react with Pt(PEt3)4 to cause selective oxidative addition of a G-H bond, giving hydrido(aryl)platinum complexes 208-210. ... 

Scheme 6-1 Preparation of Hydrido(acetato) complexes by metathesis reactions 6.2.2...

First hydrido complex of a transition metal prepared by W. Hieber and F. Leutert. 

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