Fluxional hydride ligands

The hydride complexes [MoH(CO)2(chel)2]+ (chel = variety of chelating diphosphines and arsines) are prepared by protonation of the parent Mo° dicarbonyls and appear to have a monocapped octahedral structure in solution, with the fluxional hydride moving around the possible capping positions. This process has been extensively studied by NMR.65 A number of other cationic seven-coordinate analogues [MoX(CO)2(chel)2]+ (X = halide or pseudohalide chel = chelating ligand usually phosphine or arsine) are also known (Table 5).la... 

A common feature of metal clusters is their stereochemical nonrigidity, in which carbonyl and hydride ligands exchange their coordination sites. Mixed-metal clusters are ideally suited for studies of the fluxional processes in clusters because of the low symmetry inherent in their metal framework. In such clusters, the majority of the ligands are in chemically nonequivalent positions and should thus be distinguishable by NMR... 

The hydride iron carbonyl anion, [HFe(CO)4] , has a structure that is best described as a distorted tbp with the hydride ligand occupying an axial site. The anion is fluxional both in solution and in the solid state.113 The main use is found in organic synthesis and catalysis.114 A typical reaction is the reduction of olefins... 

NMR is also useful for detecting and analyzing M H-C interactions. The signal for agostic H is usually upheld of tetramethylsilane as in a hydride ligand but can be difficult to locate because of high fluxionality. In Eq. (12.5), each proton of a... 

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