Metal-hydride bond, ligand insertion reaction

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). 

AUcyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles (see Electrophilic Reaction). Hydrogenation (see Hydrogenation) of niobium or tantalum M R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta bonds are common. The unsaturated reagents include aUcenes, aUcynes, CO, NO, RN=C=NR, CNR, and others. 

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... 

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... 

Olefins can insert into metal-alkyl bonds as well as into metal-hydride bonds provided that a vacant coordination site is available on the metal center. This reaction can thus be repeated even when the alkyl ligand lengthens, which makes the proposed mechanism a Ziegler-Natta olefin polymerization. This type of polymerization is detailed in Chap. 15.1 ... 

Thus, the isonitrile ligand easily undergoes insertion reactions, even into metal-hydride bonds. The nucleophilic additions onto the ligand carbon atom are also facile. For instance, amines readily add to yield diaminocarbene complexes ... 

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