Metal hydrido complexes reactivity

Organometallics such as Grignard and lithium reagents give rise to deinsertion in hydrido complexes (cf Sect. 3.1). However, their reactivity is quite different toward normal sigma bonded transition metal-group IVg metal complexes. 

This 6-hydrogen elimination in 2-rhoda oxetanes is apparently favored over reductive elimination to an epoxide. Moreover, the reverse step, i.e., the oxidative-addition of epoxides to Rh and Ir results in 2-rhoda oxetanes [85] and/or hydrido formylmethyl complexes [86]. Therefore, assuming that 2-metalla oxetanes are intermediates in the oxygenation of alkenes by group VIII transition metals, the reported reactivity would account for selectivity to ketones in the catalytic reactions based on these metals. 

The general reactivity of hydrido complexes is high, and it is related mainly to the coordination number of the central metal atom. Thus, four- and five-coordinate complexes usually are more reactive than related six-coordinate species. The latter, however, ordinarily are more reactive than the corresponding complexes which do not contain M-H bonds. Such reactivity may be induced either by operation of the trans effect or, where possible, by the reductive elimination of a molecule of hydrogen halide with or without assistance of a base. 

Halpern has utilized the principles of reactivity or coordination compounds to explain reaction mechanisms, some examples of which involve metal 7r-complexes. His concept of hydrogenation of olefins is based upon the formation and cleavage of a hydrido transition metal complex. 

Complexes of Cu(II), Cu(I), Ag(I), Hg(II), Hg(I), Co(I), Co(II), Pd(II), Pt(II), Rh(I), Rh(II), Ru(II), Ru(III), and Ir(I) have catalyzed homogeneous hydrogenation reactions in solution. In each case H2 is split by the catalyst with the formation of a reactive transition-metal hydride (or hydrido) complex as an intermediate. Three distinct mechanisms have been advanced, as given below. The first mechanism involves heterolytic splitting ... 

Treatment of methoxynitrido cluster [Ru3(CO)9(//3-CO)(//3-NOMe)] with stoichiometric amounts of the hydrido complex [CpMo(CO)3H] in THF afforded two trinuclear Ru-Mo clusters 160 and 161, in which metal exchange has occurred. However, the same methoxynitrido cluster when reacted with the organomercurial [ CpMo(CO)3 2Hg] resulted in the formation of a pentanuclear trimetallic species 162. Although [CpMo(CO)3H] and [ CpMo(CO)3 2Hg] possess the same isolobal CpMo(CO)3 fragment, their reactivity is rather different. 

The reactivities of hydrido(phenoxo) complexes of trons-[MH(OPh)L2] (6 M = Ni 7 M = Pt) (L = phosphine) were examined (Eqs. 6.29, 6.30 Scheme 6-16), and a high nucleophiUdty for the metal-bound phenoxide was suggested [9, 10]. Reaction with methyl iodide produced anisole and trans-[MH(I)L2] for both Ni and Pt complexes. Phenyl isocyanate also provided the insertion products into the metal-phenoxo... 

From a historic point of view, metal-catalyzed or metal-promoted hydroamina-tions were first achieved with alkali metals [4]. The use of soluble transition-metal complexes as catalysts for the OHA reaction was pioneered by DuPont workers during the 1970s, the best results being obtained with Rh and Ir salts [5], Later, the finding that electron-rich Ir(I) species cleanly activated N—H bonds to form Ir-amido-hydrido species [6] opened the way to study the reactivity of these amides... 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

Section 3 is devoted to the complexes of hydride, alkyl, alkenyl, and silyl ligands. Hydrido see Hydride Complexes of the Transition Metals) and alkyl complexes of most transition metals, and in particular those of nickel, are often involved in many catalytic processes of conunercial importance therefore, an in-depth understanding of the fundamental reactivities of these complexes is crucial to expanding their practical apphcations. Silyl complexes are involved in the transformations of organosilicon compounds, and for this reason their basic reactivities and structural properties are of interest. 

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