Bridging M-Cl bonds with ambiphilic phosphine-borane ligands.

A combined experimental/theoretical study provides insight into the bridging coordination of M-Cl bonds with the ambiphilic phosphine-borane ligands iPr(2)P-o(C(6)H(4))-BR(2) (PBCy(2): R = Cy; PBMes(2): R = Mes). Reaction of [PdCl(allyl)(PBCy(2))] (3) with HCl affords the related dinuclear complex [PdCl(mu-Cl)(PBCy(2))](2) (5). Subsequent cleavage of the chloride bridge by PPh(3) leads to the heteroleptic mononuclear complex trans-[PdCl(2)(PPh(3))(PBCy(2))] (6). The solid-state structures of complexes 5 and 6 substantiate the propensity of the PBCy(2) ligand to bridge Pd-Cl bonds via P-->Pd--Cl-->B interactions. DFT calculations carried out on both the model mononuclear complexes 3* and 6* reveal that in each system the energy of the linkage isomer with a Cl-->B interaction is very similar to that without. A comparison of the solution and solid-state (11)B NMR spectroscopic data for complexes 3 and 6 suggests the possible interconversion of the bridging and B-pendant forms in solution. Bridging coordination of the PBCy(2) ligand across a Rh-Cl bond is observed in the solid-state structure of the related complex [RhCl(nbd)(PBCy(2))] (7). Replacement of the Cy groups at boron by Mes substituents illustrates the role of steric factors on the participation of the Lewis acid upon coordination, no Cl-->B interaction being observed in the complex [PdCl(allyl)(PBMes(2))] (8).