Osmium boryl systems

Fig. 14 Widely investigated half-sandwich iron and five-coordinate osmium boryl systems (showing definition of the torsion angle 0 for the former)...

A range of five- and six-coordinate osmium boryl complexes has been synthesized making use of similar approaches to those reported above for ruthenium. In particular, the reaction of phenylosmium(II) precursors with boranes, which proceed via elimination of benzene, has been shown to be a useful entry point into octahedral and square pyramidal osmium boryl complexes. Significant further chemistry has been reported on these systems, including substitution at both boron and metal centres, which has shed light on fundamental issues of structure/bonding and reactivity [12]. 

Scheme 12 Osmium boryl complexes generated from catecholboryl system 8.104...

A number of tethered osmium boryl complexes have been developed recently in which one of the boryl substituents features an additional pendant donor which coordinates to the metal centre as a tether . For systems resulting from reactions with bifunctional donors such as 2-aminopyridine two possibilities exist, depending on which donor atom interacts with the boron centre and which with the metal. Thus, tethered boryl complexes result from coordination of the anionic donor at boron, with the neutral donor tether coordinated at osmium the reverse coordination possibility leads to the formation of intramolecular base-stabilized borylene complexes. 

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