Stabilization of high oxidation states in transition metals.: 2.: WCl6 oxidizes [WF6]-, but would PtCl6 oxidize [PtF6]-?: An electrochemical and computational study of 5d transition metal halides:: [MF6]z versus [MCl6]z (M = Ta to Pt; z = 0, 1-, 2-)

The trends in redox potentials for isovalent series of 5d hexafluoro- and -chlorometalates, [MX6](0/-) and [MX6](-/2-) (M = Ta to Pt; X = F, Cl), are compared, including the previously unpublished electrochemistry of [IrF6](2-). For a given series, the trend in redox data can be understood in terms of the core charge of the metal and interelectronic terms. However, there is a marked convergence of the electrochemical redox potentials for isovalent series of [MF6](z/z-1) and [MCl6](z/z-1) (z = 0, 1-) complexes. Thus, while the oxidation potential of [TaF6](2-) is 1.6 V lower than that of [TaCl6](2-), the oxidation potential of [IrF6](2-) is only 0.5 V lower than that of [IrCl6](2-). The redox data correlate well with computed electron affinities of MX6 and [MX6](-) derived from density functional calculations. A fragmentation approach is adopted to analyze the electrochemical trends in terms of the properties of the metal center and trends in the metal-halide bonding. The observed convergence in redox data for isovalent [MX6](z/z-1) (X = F, Cl; Z = 0, 1-) series is rationalized in terms of the ability of the halide arrays to stabilize the two metal oxidation states involved. The ability of the chloride array to stabilize the higher metal oxidation state increases more rapidly along the third row transition metals than does that of the fluoride array. This counteracts the effects of metal core charge to produce the observed convergence. The computational results indicate that, for the later metals in their highest oxidation states, the redox-active orbital becomes increasingly halide based. In view of this, the stability of the neutral hexahalides with respect to the reductive elimination of X-2 was studied, and the results suggest that OsCl6 and IrCl6 are more likely to be stable as novel hexachlorides than PtCl6.