Hydride bridged complexes

On keeping, this product converted via formal silylene extrusion reaction into the hydride bridged complex 17 ... 

Table II. Vibrational Data on Metal Hydride Bridged Complexes...

In hydride bridge complexes, three-center two-electron MO can be formed from a Is-AO of a hydrogen anion and the appropriate -orbitals of the metal atom. Those orbitals depend on the particular symmetry and can yield arrangements in the plane [29]. However, such a description is only qualitative and has to be... 

The course of these additions of lithium hydride resembles that found for the addition of borane (Nagase et al., 1980 Graham et al., 1981). With ethylene, the initial step is exothermic formation of a Jt-complex without barrier, then rate-determining transformation to the borane via a four-centre transition structure. In both the borane and lithium hydride additions, there is relatively little development of the new C—H bond with distances of 1.692 and 1.736 A respectively in the transition structures. When a carbanionic product is not formed, for example in the reaction of lithium hydride with cyclopropenyl cation yielding cyclopropene and lithium cation (Tapia et al., 1985), reaction again occurs via a hydride-bridged complex, but the C- H- -Li array remains nearly linear throughout the reaction. 

The isoelectronic group of compounds represented by 30 and 31 react differently with nucleophiles. For example, the M—Si bond in 30 is cleaved by phosphines while in 31 only CO substitution occurs (80). Incorporation of phosphines into 30 has been accomplished only when attached to the metal before addition of the silicon fragment. Conversely, 31 undergoes Fe—Si bond cleavage when treated with electrophiles, such as Cl2 and PC15, whereas hydride bridged complexes 30 remain intact (79). 

Figure 3. Above Schematic representation of the molecular orbitals calculated for the hydride bridged complex 12. Below Schematic representation of the molecular orbitals calculated for a generic diiridium(II) complex.

Hydride-bridged complexes often form even though the mononuclear hydride is known. A crucial initial step in heterolysis of complex with either an unsaturated site or one occupied by a labile ligand, for example, H2O in Equation (7) or as will be shown below. 

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