Metal complexes of unsaturated tertiary phosphines and arsines. Part II. Reactivity of co-ordinated ligands

Abstract

The reactions with bromine of square planar platinum(II) complexes of the isomeric styryldimethylarsines, o-, m- and p-CH₂:CH·CH·C₆H₄·AsMe₂ have been studied. The complexes PtBr₂(m-arsine)₂ and PtBr₂(p-arsine)₂ add successively 3 equivalents of bromine. The first equivalent oxidises the metal atom giving orange, octahedral platinum(IV) complexes of general formula PtBr₄(arsine)₂ in both cases. The second and third equivalents of bromine then add to the double bonds of the ligands. The complex PtBr₂(o-arsine)₂ also adds successively 3 equivalents of bromine, but the mode of addition is different. The product of reaction with one equivalent of bromine is a yellow complex PtBr₄(o-arsine)₂, which is formulated as an octahedral platinum(IV) complex containing a metal–carbon σ-bond to the vinyl group of one of the ligands. The second equivalent of bromine adds to the remaining double bond of this complex, leaving the metal–carbon bond intact. An excess of bromine breaks this bond, giving the orange, octahedral platinum(IV) complex PtBr₄(o-arsine Br₂)₂.

Similar reaction sequences are observed in the bromination of the corresponding platinum(II) complexes of the isomeric allylphenyldimethylarsines, o-, m-, and p-CH₂:CH·CH₂·C₆H₄·AsMe₂. Steric hindrance of bromine addition at the platinum atom alone cannot account for the unusual behaviour of the complexes of the ortho-substituted ligands, because the platinum(II) complexes of o- and p-propyldimethylarsine, Prⁿ·C₆H₄·AsMe₂, are both oxidised by bromine at the metal atom, giving the usual orange octahedral platinum(IV) complexes.

By contrast, the three octahedral rhodium(III) complexes of general formula RhBr₃(arsine)₃(arsine =o, m-, and p-CH₂:CH·C₆H₄·AsMe₂) react similarly with bromine, with addition across the double bonds of the ligands. Bromine also adds to the double bonds of the methiodides of the isomeric allylphenyldimethylarsines, the order of reactivity being meta > para > ortho.

Alcoholysis and reduction of the platinum(IV) complexes of the ortho-substituted olefinic ligands are discussed, and the formation of chelate complexes of general formula PtBr₂(o-arsine) is described.