Synthesis and reactivity of semibridging borylene complexes

Reaction of the terminal borylene complexes [(OC)(5)MdBdN(SiMe(3))(2)] (M = Cr, W) with [M '(PCy(3))] (M ' = Pd, Pt) at room temperature led to spontaneous formation of the heterodinuclear complexes [(OC)(4)M-(mu-CO){mu-BN(SiMe(3))(2)}M '(PCy(3))(2)] (M = Cr, M ') Pd; M = W, M ' = Pd; M = Cr, M ' = Pt) with concomitant liberation of PCy(3). X-ray diffraction studies revealed that the borylene ligand adopts a semibridging position between the two different metal centers, thus establishing a new coordination mode for borylenes and providing further experimental evidence for the theoretically predicted close relationship between BR and CO. After prolonged reaction times the liberated phosphine substitutes the carbonyl group in trans-position to the borylene moiety, forming complexes of the type [(Cy(3)P)(OC)(3)M(mu-CO)-{mu-BN(SiMe(3))(2)}M '(PCy(3))] (M = Cr, M ' = Pt; M = W, M ' = Pt; M = W, M ' = Pd). Heating or photolytic activation of [(OC)(4)M(mu-CO){mu-BN(SiMe(3))(2)}Pd(PCy(3))] afforded the terminal borylene complexes trans[(Cy(3)P)(OC)(4)M=BN(SiMe(3))(2)], which were fully characterized in the case of M) Cr. Structural data of trans-[(Cy(3)P)(OC)(4)Cr=BN(SiMe(3))(2)] confirm the presence of an enhanced Cr-B pi-back-donation imposed by the phosphine ligand in trans-position.