The reactivity with acetylene of [PtX2(Me2phen)] (X = Cl, Br, I) complexes has been investigated. Whereas the chlorido species [PtCl2(Me2phen)] exhibits negligible reactivity at short reaction times, the bromido and iodido species [PtBr2(Me2phen)] and [PtI2(Me2phen)] lead initially to formation of Pt(ii) five-coordinate complexes, [PtX2(η(2)-CH[triple bond, length as m-dash]CH)(Me2phen)], that evolve to four-coordinate alkenyl complexes of the type [PtX(η(1)-E-CH[double bond, length as m-dash]CHX)(Me2phen)]. The alkenyl complexes, in the presence of excess acetylene, establish an equilibrium with the five-coordinate alkyne-alkenyl species [PtX(η(1)-E-CH[double bond, length as m-dash]CHX)(η(2)-CH[triple bond, length as m-dash]CH)(Me2phen)] (X = Br, I). The π-bonded acetylene can be exchanged with free olefins or C[triple bond, length as m-dash]O, affording the new alkene-alkenyl or carbonyl-alkenyl complexes [PtX(η(1)-E-CH[double bond, length as m-dash]CHX)(η(2)-olefin)(Me2phen)] and [PtX(η(1)-E-CH[double bond, length as m-dash]CHX)(C[triple bond, length as m-dash]O)(Me2phen)]. The five-coordinate geometry of the alkyne-alkenyl and alkene-alkenyl complexes was assessed from NMR data and is fully consistent with that of a previously determined X-ray structure of [PtBr(η(1)-E-CH[double bond, length as m-dash]CHBr)(η(2)-CH2[double bond, length as m-dash]CH2)(Me2phen)].