Metal-hydride bond, ligand insertion

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

In transfer hydrogenation with 2-propanol, the chloride ion in a Wilkinson-type catalyst (18) is rapidly replaced by an alkoxide (Scheme 20.9). / -Elimination then yields the reactive 16-electron metal monohydride species (20). The ketone substrate (10) substitutes one of the ligands and coordinates to the catalytic center to give complex 21 upon which an insertion into the metal hydride bond takes place. The formed metal alkoxide (22) can undergo a ligand exchange with the hydride donor present in the reaction mixture, liberating the product (15). 

Insertion of CSj in metal-hydride bonds gives metal dithioformates. Bonding of the ligands to the metal can take place in several ways (a) ionic in alkali and alkaline earth metal compounds (b) symmetric bidentate in most of the transition metal complexes and (c) as a positive symmetrically bonded ligand to (25), in which the HC(S)S has donated two electrons to the two electron-deficient dicarbanonaborane groups.36 Little is known about monothioformic acid and its complexes. 

However with the chemical demonstration that diazenido and hy-drazido(2-) ligands can insert into metal-hydride bonds, the formation of 1H2H as a consequence of exchange 2H2 with a metal hydride at one of these intermediate stages remains a possibility. This would explain why the exchange is accelerated in the presence of dinitrogen. 

The Ligand Insertion Reaction into the Metal Hydride Bond... 

The CO insertion reaction into the metal hydride bond is in fact a member of the class of ligand insertion reactions to which much theoretical work has been devoted (28,29-35). Some years ago we analyzed the ethylene insertion into the rhodium hydride bond of a Rh(III) hexacoordinated complex (. We later focused our attention on the CO insertion reaction into the Mn-H bond ofHMn(CO)5 (37-39) and very recently we have undertaken the study of the CO2 insertion reaction into the Cr-H bond of HCr(CO)5 (C. Bo and A. Dedieu, Inorg. Chem., in press). We will concentrate here on the CO insertion reaction and compare it to the two other insertion reactions. The study of the reaction (1) was carried out at both the SCF and... 

According to the present classification, the addition of PMe2Ph to [Os3(CO) o(/ -SCH2CH=CH2)( = H)] (569) which induces formal insertion of the organosulfur ligand into the metal hydride bond to form 612,... 

Although 187-189 were not active catalysts for polymerization process, 187 and 189 proved to be active olefin hydrosilylation catalysts, presumably 187 first reacted with a silane to form a reactive metal hydride species. They are the first examples of d° metal complexes with non-Cp ligands in the catalytic hydrosilylation of olefins. The mechanism was believed to be consistent with that of other d° metallocene-based catalysts and included two steps 1) fast olefin insertion into the metal hydride bond and 2) a slow metathesis reaction with the silane. The catalysts exhibited a high regioselective preference for terminal addition in the case of aliphatic olefins... 

Because various important industrial organic processes utilize olefins, convenient methods to convert olefins into various products are vital. Transition metal catalysts with proper ligands have proved most useful in controlling the course of these reactions. Transition metal complexes catalyze skeletal isomerization, double bond isomerization, polymerization, and other processes. Insertion of a terminal olefin into a transition metal hydride bond by 1,2-inserfion or... 

Olefin insertion is particularly facile in the case of the complexes [PtH(SnCl3)(PR3)2]. The soft ii-acceptor ligand [SnCls] stabilizes the metal-hydride bond (symbiosis of soft ligands) and hence catalyzes the insertion reaction as... 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

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