Hydride ligands structural features

While the last elass of eomplexes considered in this section, the compounds 145, closely resemble the usual silane a-complexes, other multicenter H Si interactions discussed above have spectroscopic and structural features common to both the IHI and a-complexes. This enigmatic situation can be explained well by the structure 132 in terms of a a-coordination of the Si-H bonds of the hypervalent ligand (H +iSiX3)" 1) to metal, which thus includes both the hypervalent interaction of the silicon with the hydride atoms and the a-complexation of the Si-H bonds to metals. The key features of complexes with multicenter H Si interactions are summarized in Table VIII, where a comparison with the IHI and the residual H-Si interactions in silane a-complexes is given. 

The work on biomimetic models for [NiFe] and [FeFe] hydrogenase has been described in several review articles [15b 158]. In this work, many of the structural features important for proper function found in the native systems have been successfully incorporated for example, the bimetallic Ni-Fe or Fe-Fe core with rather short metal-metal distances and an open coordination site at one metal, the sulfur-rich environment (terminal and bridging thiolate ligands), CO/CN ligation of the iron(s), and the incorporation of a base for acceptance of the proton and, more recently, of hydride bridges. Still-existing problems of many model systems are the O2 sensitivity, the high overpotentials, and lack of activity (low turnover rates). 

The salient structural features of this molecule include (i) approximate c. symmetry (ii) a strong trans influence exerted by the hydride ligand... 

A comprehensive review of early transition metal hydrides including those of Nb and Ta is available.447 Hydrido-metallocene systems are discussed in detail there. The present account concentrates on other ligand systems, although mono- or polynuclear metallocene systems with interesting structural features are covered. Polyhydride systems have been reviewed from a theoretical standpoint.448... 

The solution and solid state structural features of the optically pure [Rh(TpMenth)(CO)2] have been reported, TpMenth being found in both k2- and k3-coordination modes. The optically active TpMenth and XpMementh are able to control the stereoselectivity of the C-H bond activation by their coordinated rhodium center. Irradiation of [Rh(TpMenth)(CO)2] under N2 resulted in the generation of an 85 15 mixture of diastereomeric alkyl hydrides due to intramolecular cyclometalation reaction involving the methyl substituents on the ligand isopropyl group (Fig. 3.51).240... 

The structure of the rhodium complex has no features attributable to a high hydride ligand. For the salt K2[ReH9], there are three formula weights per unit cell 192), and the rhenium atoms are separated by large distances (minimum of 5.5 A) so that discrete enneahydridorhenate anions must exist in the crystal. The anion has Da, symmetry with six hydrogens at the comers of a trigonal prism and three beyond the centers of each equatorial face. The Re—H distance varies between 1.6 and 1.72 A... 

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