M-H complex

The chelate effect also favors oxidative addition of the C2—H bonds of imidazo-lium salts because it provides stabilized complexes. The reaction of a pyridine-imidazolium salt with [lrCl(cod)]2 yields the oxidative addition product, even in the absence of a base (Scheme 3.9), thus confirming that the oxidative addition of an imidazolium salt should be considered as a vahd process for the preparation of NHC—M—H complexes [24]. 

The isolation of the first NHC - M - H complexes obtained by oxidative addition of an imidazolium salt to a low valent group 10 metal was achieved by Cavell and coworkers in 2003 [156]. A NHC-Pt(O) complex with two monoalkene ligands reacted with an imidazolium salt to provide an isolable NHC-PtH complex (Scheme 41). Carbene metal hydrides of Ni and Pd were obtained one year later by C - H oxidative addition of the corresponding imidazolium salts to bis-NHC Ni(0) and Pd(0) complexes (Scheme 41) [157]. 

Formation of M-H complexes by refluxing metal halides, or complex halides, with alcohols in the presence of stablizing hands occurs via formation of a metal alkoxide followed by j -hydride elimination . This reaction represents the reverse of insertion of an aldehyde or ketone into an M-H bond. 

Earner, L. Barner-Kowollik, C. Davis, T.P. Stenzel, M.H. Complex molecular architecture polymers via RAFT. Aust. J. Chem. 2004,57 (1), 19-24. 

Figure 2. Spin-restricted metal fragment orbital energies, and overlap values for L M-H complexes. (Calculated M-H distances (in A) are also reported.)...

The main bond characteristics of dihydrogen bonding is anticipated to be of electrostatic nature. Thus, the ability of L M-H complexes to interact with protic compounds is expected to be directly related to the basicity of transition metal hydrides, as we have defined them above. This in our mind provides an excellent additional method to quantify the basicity of TMH complexes by the strength of the L MH—HX bonding. 

Yet additional possibilities arise from proton capture by the unoxidized complex M-H. This species has basic properties by virtue of metal lone pairs (only for d" configurations with n > 2), other ligands lone pairs, or even the hydride itself (leading to the formation of a H2 o-complex). It has been shown in many cases, in fact, that a M-H complex has a greater kinetic basicity at the hydride ligand, even when the thermodynamically stable protonation product is a classical polyhydride [106-110]. The proton transfer step to MH is shown in equation 22. The fate of M is in principle the same as above (equations 15, 16 or 17-18), leading to the possible overall stoichiometries of equations 23,24 and 25, respectively. 

Scheme 32 Catalysts designed to avoid formation of unreactive H-M-H complexes...

The key stage of the catalytic reaction is the formation of the hydride M— H complex with the simultaneous coordination of the unsaturated X=Y bond of the substrate to the metal atom. Then the unsaturated bond (for example, CO) reacts with the M— H bond followed by the addition of the second hydrogen atom. 

Gregory, A., Stenzel, M.H. Complex polymer architectures via RAFT polymerization from fundamental process to extending the scope using click chemistry and nature s building blocks. Prog. Polym. Sci. 37(1), 38-105 (2012)... 

Conjugated olefins apparently add to M—H complexes with greater ease than do ethylene or alkyl sul ituted ethylenes. Thus butadiene gives but-2-enyl complexes. 

---