The reactions with bromine of platinum(II) complexes containing a co-ordinated double bond have been studied. The chelate complex PtBr₂,o-arsine formed by o-styryldimethylarsine, o-CH₂:CH·C₆H₄·AsMe₂, reacts with one equivalent of bromine to give an orange, insoluble compound PtBr₄,o-arsine which contains neither free nor co-ordinated double bonds. On heating this compound in methanol, hydrogen bromide is eliminated and a methoxy-derivative of the initial chelate complex, of formula PtBr₂,o-MeOCH:CH·C₆H₄·AsMe₂, is formed.

The chelate complex PtBr₂,o-arsine formed by o-allylphenyldimethylarsine, o-CH₂:CH·CH₂·C₆H₄·AsMe₂, also reacts with bromine to form an orange compound PtBr₄,o-arsine. This readily loses bromine in organic solvents, re-forming the initial chelate complex, but on heating in a number of alcohols, hydrogen bromide is eliminated. The alkoxy-substituted chelate complexes so formed are assigned the structure PtBr₂,o-RO·CH₂·CH:CH·C₆H₄·AsMe₂(R = Me, Et, Prⁿ, or Prⁱ) on the basis of the n.m.r. spectrum of the methoxy-derivative, showing that the double bond has shifted from its position in the initial chelate complex.

A dimeric, halogen-bridged structure with metal–carbon σ-bonds is suggested for the complexes of formula PtBr₄,o-arsine, and possible mechanisms for the reactions are discussed.