X- or L-Lewis acid interactions

The ability of ambiphilic ligands to bridge M-X or M-L fragments (coordination mode F resulting from the concomitant coordination of the donor site to the metal and interaction of the Lewis acid with a coligand) has also been demonstrated experimentally. Such bridging coordination remains rare, and most of the known examples involve PB ligands.74 

The first example was reported by Baker and Marder in 1995.75 The monohydride [MH(PMe3)(r/2-CH2PMe2)] complexes 83 (M = Ru) 

According to X-ray diffraction data, in the solid-state complex 87 adopts a six-membered metallacyclic structure, as the result of P-Pd-Cl-B bridging coordination. The interaction of the borane moiety with the chlorine atom is apparent from the short Cl-B distance (2.165(2) A) and the pyramidalization of the boron environment (EBa = 349.1°). The bridging coordination of the Pd-Cl bond by the PB 7b contrasts with the B-pendant coordination mode adopted by complex 72, deriving from the related ligand 7e featuring mesityl groups at boron (see Section IV.A). This illustrates the role of steric factors in the participation, or not, of the Lewis acid in the coordination assembly. 

The ability of Cl-B interactions to persist upon chemical transformations in the coordination sphere of the metal was then explored.22 

In order to gain more insight into the magnitude of Cl-B interactions, DFT calculations were carried out on model complexes 87 and 89 bearing Me instead of z-Pr substituents at the phosphorus atoms.22 

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