Synthesis, Reactions, and Rearrangement of X(PR′₃)₂M[C(=PR)X] (M = Pt, Pd; X = Cl, Br; R′ = Et, Ph; R = 2,4,6-Tri-tert-butylphenyl): Mechanism of the Transition Metal Promoted Conversion of X₂C=PR to R—C≡P

Oxidative addition reactions of X₂C=PR (X = Cl, Br; R = 2,4,6-tri-tert-butylphenyl) with M(PEt₃)₄ (M = Pt, Pd) or (C₂H₄)Pt(PPh₃)₂ initially yield the cis isomer of square planar (X)-(PR′₃)₂M[C(=PR)X] (II); these complexes (IIa-IId), where PR′₃ is PEt₃, rearrange rapidly in the presence of free PEt₃ to give the trans isomers (Ia‒Id). In contrast, the cis isomers (IIe and IIf), where PR′₃ is PPh₃ and M is Pt, react further to give R—C≡P and cis-X₂Pt(PPh₃)₂. In polar solvents (CH₂Cl₂ and CHCl₃), all the addition products (I and II) convert to R—C≡P and cis- or trans-X₂M(PR′₃)₂ via the surprising phosphabicyclo intermediate (X)(PR′₃)₂Pt(X—PBC) (III and IV); the structure of IIIa was established crystallographically. In the presence of H₂O, (X)(PEt₃)₂Pt[C(=PR)X] (Ia and Ib where X = Cl, Br) give the oxophosphabicyclo complex (X) (PEt₃)₂Pt[(H) O=PBC] (Va and Vb) which was characterized by X-ray diffraction. A mechanism for the conversion of (X)(PR′₃)₂M[C(=PR)X] to R—C≡P and X₂M-(PR′₃)₂ is proposed.