Hydride bridged complexes characterization

The Sm complex [254] is characterized by a formal twenty electron configuration. The Sm atom is coordinated by four hydrides. The heterometallic complex of Y and Re has an open triangular structure as shown in Fig. 6.26. The Re(2)-Y distance is 4.186 A and the other Re(l)-Y distance of 3.090 A. This suggests the presence of a hydride bridge for Re(l)-Y. Short Re-Re distance of 2.576 is also accounted for by the earlier explanation. The bridging hydride structure is also supported by NMR data [255]. 

Solutions of 35 and 36 are diamagnetic and the complexes could be characterized by H and 27A1 NMR.48 It is not clear whether the apparent antiferromagnetic coupling of the two zirconium centers is mediated by Zr-Zr, Zr-Al-Zr interactions, or by the hydride bridges. 

Recently, a proposal has been put forth that a /raor-addition process may be possible through dinuclear ruthenium intermediates.34 As shown in Scheme 5, reaction of tetraruthenium aggregate A with phenylacetylene results in the fully characterized bridging dinuclear alkenyl complex B. The authors propose a direct /ra .r-dclivcry of hydride through a dinuclear intermediate may be active in the hydrosilylation catalyzed by A, though compound B itself is unreactive to Et3SiH. 

Since the hydroformylation reaction for most substrates shows a first order dependence on the concentration of rhodium hydride, the reaction becomes slower when considerable amounts of rhodium are tied up in dimers. This will occur at low pressures of hydrogen and high rhodium concentrations. Dimer formation has mainly been reported for phosphine ligands [17, 42, 45], but similar dimeric rhodium complexes from monophosphites [47] and diphosphites [33, 39] have been reported. The orange side product obtained from HRh(15)(CO)2 was characterized as the carbonyl bridged, dimeric rhodium species Rh2(15)2(CO)2 [39]. 

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