New classes of alkynylplatinum(IV) complexes are described, and the first examples of alkynylpalladium(IV) complexes have been detected, including a triorganopalladium(IV) diphosphine complex. Alkynyliodine(III) triflate reagents IPh(CtCR(2))(OTf) (R-2 = SiMe3, Ph) are able to transfer alkynyl groups to diorganoplatinum(II) and "pincer" complexes Pt(O2CAr)(NCN) (NCN = [2,6-(dimethylaminomethyl)-phenyl- N,C,N](-), Ar = Ph, 4-trifluoromethylphenyl (Ar-F)); on addition of iodide ion, complexes with new alkynylplatinum(IV) kernels may be isolated: PtIMe2(C equivalent to CR2)(bpy) (1, 2, 3a) (bpy) 2,2'-bipyridine), PtI(C4H8)(C equivalent to CSiMe3)(bpy) (4a), PtIPh2(C equivalent to CSiMe3)(Bu(2)(t)bpy) (5a), and PtI(O2CAr)(C equivalent to CR2)(NCN) [Ar = Ph (14, 15), Ar-F (16, 17)]. For the reagent PtMe2(dmpe) [dmpe = 1,2-bis(dimethylphosphino)ethane], the unstable complexes PtIMe2(C equivalent to CR2)(dmpe) (6a, 8a) are detected prior to reductive elimination of ethane and isolation of PtI(C equivalent to CR2)(dmpe) (7, 9). Isomerism is exhibited by the octahedral fac-(PtR2)-R-IV- CtCR2) complexes, where complexes 1 and 2 form as a mixture of complexes with the alkynyl group opposite the bidentate ligand and cis to the iodo ligand (1a-6a, 8a), and cis to the bidentate ligand and trans to the iodo ligand (1b, 2b), complexes 3a-5a, 6a, and 8a having R groups mutually cis. An X-ray structural analysis for 16 shows distorted octahedral geometry about the metal with the iodo ligand trans to the carbon donor of the mer-pincer ligand. Studies of the reactivity of related palladium(II) substrates at low temperature allow detection of the pincer complex Pd(O2CPh)(OTf)(C equivalent to CSiMe3)(NCN) (19) and PdIMe2(C equivalent to CSiMe3)(dmpe) (18) as a model for frequently proposed "(PdXR3P2)-X-IV" species in catalytic and stoichiometric reactions.