New Class of Oligonuclear Platinum-Thallium Compounds with a Direct Metal-Metal Bond. 5. Structure Determination of Heterodimetallic Cyano Complexes in Aqueous Solution by EXAFS and Vibrational Spectroscopy

摘要

The structures of three closely related heterodimetallic cyano complexes, [(NC)_5Pt-Tl(CN)_n]~(n-) (n = 1-3), formed in reactions between [Pt~(II)(CN)_4]~(2-) and Tl~(III) cyano complexes, have been studied in aqueous solution. Multinuclear NMR data (~(205)Tl, ~(195)Pt, and ~(13)C) were used for identification and quantitative analysis. X-ray absorption spectra were recorded at the Pt and Tl L_(III) edges. The EXAFS data show, after developing a model describing the extensive multiple scattering within the linearly coordinated cyano ligands, short Pt-Tl bond distances in the [(NC)_5Pt-Tl(CN)_n]~(n-) complexes: 2.60(1), 2.62(1), and 2.64(1) A for n = 1-3, respectively. Thus, the Pt-Tl bond distance increases with increasing number of cyano ligands on the thallium atom. In all three complexes the thallium atom and five cyano ligands, with a mean Pt-C distance of 2.00-2.01 A, octahedrally coordinate the platinum atom. In the hydrated [(NC)_5Pt-Tl(CN)(H_2O)_4]~- species the thallium atom coordinates one cyano ligand, probably as a linear Pt-Tl-CN entity with a Tl-C bond distance of 2.13(1) A, and possibly four loosely bound water molecules with a mean Tl-O bond distance of about 2.51 A. In the [(NC)_5Pt-Tl(CN)_2]~(2-) species, the thallium atom probably coordinates the cyano ligands trigonally with two Tl-C bond distances at 2.20(2) A, and in [(NC)_5Pt-Tl(CN)_3]~(3-) Tl coordinates tetrahedrally with three Tl-C distances at 2.22(2) A. EXAFS data were reevaluated for previously studied mononuclear thallium(III)-cyano complexes in aqueous solution, [Tl(CN)_2(H_2O)_4]~+, [Tl(CN)_3(H_2O)], and [Tl(CN)_4]~-, and also for the solid K[Tl(CN)_4] compound. A comparison shows that the Tl-C bond distances are longer in the dinuclear complexes [(NC)_5Pt-Tl(CN)_n]~(n-) (n = 1-3) for the coordination number. Relative oxidation states of the metal atoms were estimated from their ~(195)Pt and ~(205)Tl chemical shifts, confirming that the [(NC)_5Pt-Tl(CN)_n]~(n-) complexes can be considered as metastable intermediates in a two-electron-transfer redox reaction from platinum(II) to thallium(III). Vibrational spectra were recorded and force constants from normal-coordinate analyses are used for discussing the delocalized bonding in these species.