A New Class of Oligonuclear Platinum-Thallium Compounds with a Direct Metal-Metal Bond

摘要

A new series of four binuclear platinum-thallium cyano compounds containing a direct and unsupported by ligands metal-metal bond has been prepared in aqueous solution. These compounds are represented by the formula [(NC)_5Pt-Tl(CN)_(n - 1)]~((n - 1)-) (n = 1 - 4 for compounds I, II, III IV, respectively) and [(NC)_5Pt-Tl-Pt(CN)_5]~(3-) (for compound V). The oligonuclear complexes are synthesised according to the reaction mPt(CN)_4~(2-) + Tl~(3+) + nCN~- <-> [Pt_mTl(CN)_(4m + n)]~(3 - 2m - n). Thus, there occurs a change of the coordination number of the Pt center from four (square planar) to six (octahedral). Consequently, the formation of binuclear platinum-thallium cyano compounds involves at least two step: (i) formation of metal-metal bond and (ii) formation of (NC)_5Pt- unit by a cyanide transfer process. - The complexes exist in an equilibrium, which also includes the parent complexes Pt(CN)_4~(2-) and Tl(CN)_n~(3 - n) (n = 0 - 4), and can be controlled by varying the cyanide concentration and/or pH of the solution. The stability constants of the compounds β_N = [Pt_mTl(CN)_(4m + n)~(3 - 2m - n)] / {[Pt(CN)_4~(2-)]~m·[Tl~(3+]·[CN~-]~n} have been determined by means of multinuclear NMR (~(195)Pt, ~(205)Tl): logβ_N = 19.9 ± 0.4, 307 ± 0.3, 38.6 ± 0.3, and 44.8 ± 0.2 for I, II, III, and IV (m = 1, n = 1 - 4), and 32.1 ± 0.3 for V (m = 2, n = 2), respectively, (in 1 M NaClO_4 as ionic medium, at 25 ℃). To our knowledge, the present work constitutes the first detailed equilibrium study of metal-metal bonded compounds; it indicates that also other cluster formation reactions described in the literature may represent real equilibria.