Complexes Containing Ancillary Hydride Ligands

Interestingly, the difference in ligand orientation between Bpin and Beat systems is found to be mainly steric in origin. In the case of Bpin system 9.10 the perpendicular orientation allows for interaction of boryl and hydride ligands via the formally vacant BO2 7t orbital (to which the B 2pz orbital is a major contributor) in a manner reminiscent of conventional boron-containing Lewis acids. In the case of Beat system 9.9, however, the near coplanar arrangement of ClRhB and BO2 planes means that the B- -H interaction involves the perpendicular BO2 a as the acceptor orbital (Fig. 29) [154]. 

Finally, recent work has examined in detail the kinetics of the fundamental B-H oxidative addition step which leads to the formation of rhodium(III) [and ruthenium(II)] boryl hydrides. Conversion of fac-(triphos)Rh(H)3 into fac-(triphos)Rh(H)2(Bpin) via sequential H2 reductive elimination/HBpin oxidative addition was induced by laser flash photolysis and kinetic data determined from UV measurements. Thus, an extremely high second-order rate constant was determined for the reaction of the 16-electron intermediate 

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Another class of heteroleptic alkyl complexes contains TT-donating ancillary ligands such as RU[N(Si(CH2)3)2]3 (R = 4)- The hydride... 

Another class of heteroleptic alkyl complexes contains 7t-donating ancillary ligands such as RU[N(Si(CH3)3)2]3 (R = CH3, H, BH ). The hydride species can be converted into the methyl species via reaction with BuLi and CH3Br. The methyl compound has exhibited insertion chemistry with small molecules including aldehydes, ketones, nitriles, and isocyanides (206). Stable metallacyde compounds are also known, ie,... 

The X-ray structure of the latter complex (Fig. 5) shows that it contains a [HRh(/A-H)2RhH] core. Both complexes may be considered classical rhodium hydride species, although they are of a less common variety in that the ancillary amine ligands are purely room temperature, the complex of 4 is fluctional whereas that of 3 is rigid. 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

Transition metals can contain, within their coordination field, hydrogen in molecular (H2) and/or atomic (hydride) form. These types of metal-ligand complexes are unquestionably the most dynamic molecular systems known in terms of structural variability and atom motion/exchange processes. Until about 20 years ago, metals were known to contain only atomically bound hydrogen, that is, metal hydrides and metal hydride complexes (L MH,., where L is an ancillary ligand). However the discovery by Kubas and coworkers [1] in 1983 of side-on coordination of a nearly intact dihydrogen molecule (H2) to a metal complex has led to a new paradigm in chemistry that is the subject of many review articles [2-10]. 

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