Formation and Decomposition of Platinum–Thallium Bond, Kinetics and Mechanism. Structural Characterization of Some Metal Cyanides in the Solid State

Abstract: The kinetic and mechanistic features of a new series ofplatinum-thallium cyano compounds containing a direct andunsupported by ligands metal-metal bond have been studied insolution, using standard mix?and?measurespectrophotometric technique and stopped?flow method.These reactions are interpreted as oxidative addition of the cspecies to the square planar Pt(CN)42-complex. Each of these processes was found to befirst-order in Pt(CN)42-, the corresponding TIIIIcomplex and a cyanide ion donating species whichacts as a catalyst. Both di- and trinuclear complexes werestudied, and the kinetically significant thallium complexes intheir formation and the catalytically active cyanide sourcesare as follows: [(CN)5PtTl(CN)3]3-: Tl(CN)4?(alkaline region), Tl(CN)3(slightly acidic region) and CN?; [(CN)5Pt?Tl(CN)]?: Tl(CN)2+and Tl(CN)2+; [(CN)5Pt?Tl?Pt(CN)5]3-: [(CN)5Pt?Tl(CN)]?and HCN. Appropriatemechanisms were postulated for the overall reactions in allcases, which include i) metal?metal bond formation stepand ii) coordination of an axial cyanide ion to the platinumcenter. Two experimentally indistinguishable kinetic modelswere proposed for the formation of the dinuclear complexeswhich are different in the sequence of the two steps. In thecase of the trinuclear complex, experimental evidence isavailable to exclude one of the alternative reaction paths, andit was proven that the metal?metal bond formation precedesthe axial cyanide coordination.The cyanide ligands coordinated to TIIIIin the Pt?Tl complexes could be replacedsuccessfully with aminopolycarboxylates e.g.: mimda2-, nta3-, edta4-. The [(CN)5Pt?Tl(edta)]4-complex, with a direct metal?metal bond hasbeen prepared in solution by two different reactions: a)dissolution of [(CN)5Pt?Tl](s) in an aqueous solution of edta, b)directly from Pt(CN)42-and Tl(edta)(CN)2-. The decomposition reaction is greatlyaccelerated by cyanide and significantly inhibited by edta. Itproceeds through the [(CN)5Pt?Tl(CN)3]3-intermediate. The formation of [(CN)5Pt?Tl(edta)]4-can proceed via two different pathways dependingon the ratio of the cyanide to the edta ligand concentrations.The?direct path?at excess of edta means theformation of intermediate[(CN)4Pt···Tl(CN)(edta)]4-, followed by a release of the cyanide from theTl?centre followed by coordination of a cyanide from thebulk to the Pt?centre of the intermediate. The?indirect path?dominates in the absence of extraedta and the formation of the Pt?Tl bond occours betweenPt(CN)42-and Tl(CN)4?.Homoligand MTl(CN)4(M = TlI, K, Na) and, for the first time, Tl(CN)3species have been synthesized in the solid stateand their structures solved by single crystal X?raydiffraction method. Interesting redox processes have been foundbetween TIIIIand CN?in non?aqueous solution and in Tl2O3-CN?aqueous suspension. In the crystal structureof Tl(CN)3·H2O, the thallium(III) ion has a trigonal bypiramidalcoordination geometry with three cyanides in the trigonalplane, while an oxygen atom of the water molecule and anitrogen atom from a cyanide ligand attached to a neighboringthallium complex, form a linear O?Tl?N fragment.Cyanide ligand bridges thallium units forming an infinitezigzag chain structure. Among the thallium(III) tetracyanocompounds, the isostructural M[Tl(CN)4](M = Tl and K) and Na[Tl(CN)4]·3H2O crystallize in different crystal systems, but thethallium(III) ion has in all cases the same tetrahedralgeometry in the [Tl(CN)4]?unit.Three adducts of mercury(II) (isoelectronic with TIIII) (K2PtHg(CN)6·2H2O, Na2PdHg(CN)6·2H2O and K2NiHg(CN)6·2H2O) have been prepared from Hg(CN)2and square planar transition metal cyanides MII(CN)42-and their structure have been studied by singlecrystal X?ray diffraction, XPS and Raman spectroscopy inthe solid state. The structure of (K2PtHg(CN)6·2H2O consists of strictly linear one dimensional wireswith PtIIand HgIIcenters located alternately, dHg?Pt= 3.460 Å. The structure of Na2PdHg(CN)6·2H2O and K2NiHg(CN)6·2H2O can be considered as double salts, the lack ofhetero?metallophilic interaction between both the HgIIand PdIIatoms, dHg?Pd= 4.92 Å, and HgIIand NiIIatoms, dNi?Pd= 4.60 Å, seems obvious. Electronbinding energy values of the metallic centers measured by XPSshow that there is no electron transfer between the metal ionsin all three adducts. In solution, experimental findingsclearly indicate the lack of metal?metal bond formation inall studied HgII?CN-?MII(CN)42-systems (M = Pt, Pd and Ni). It is in contrary tothe platinum?thallium bonded cyanides.KEYWORDS:metal?metal bond, platinum, thallium,kinetics, mechanism, stopped flow, oxidative addition, cyanocomplexes, edta, redox reaction, metal cyanides, X?raydiffraction, Raman, NMR, mercury, palladium, nickel, onedimensional wire