Covalent bonded bridging ligands hydrogenations

Primary phosphanes, like primary amines, possess two hydrogen atoms and thus can be doubly deprotonated. A doubly deprotonated primary phosphane is known as a phosphinidine. It can realize a variety of different bonding modes with metals (see Fig. 7.18). It can form two covalent bonds and one donor bond. It can act as a two electron or a four electron donor ligand, either as a terminal or a bridging ligand. 

Hydrogen also exhibits two less common forms of bonding. Boron hydrides form a wide variety of compounds in which the hydrogen acts as a bridging species involving three-center two-electron bonds. Such species are said to be electron deficient because they do not have sufficient electrons for conventional two-electron covalent bonds. The second, less common, form is that of the coordinated hydrides in which the H" ion acts as a ligand bound to a transition metal atom. 

On the other hand, since a hydrogen atom in an M-H moiety does not possess a lone pair, a compound that features a bridging hydride ligand cannot be represented in terms of two 2-center-2-electron M-H bonds. Rather, the [M(p,-H)M] interaction must be described as a 3-center-2-electron bond. However, while simple M-X line and M-e-L arrow representations of normal covalent and dative covalent bonds provide a convenient means to evaluate the electron count of a metal center, there are many varied representations for 3-center-2-electron [M( x-H)M] interactions in the literature (Fig. 4). As a consequence, the derived electron count and resulting M-M bond order depends upon one s interpretation of the 3-center-2-electron [M(jjt-H)M] interaction. In such cases, the derived M-M bond order may be controversial. 

Mode 3). With regard to the linker-bridged chiral ligand, the linker moiety may be composed by either covalent bonds or noncovalent interactions such as hydrogen bonding or ligand-to-metal coordination [7, 48]. 

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