Metal–ligand bonding and metal atom dynamics in Fe–Fe and Ru–Fe triple-decker sandwich complexes

Three structurally related bimetallic triple-decker complexes of iron and ruthenium have been studied by temperature-dependent 57Fe Mössbauer Effect (ME) spectroscopy and their redox-behavior has been elucidated. The triple-decker compounds are [Cp′FeCp′FeCp′]BF4, [CpRuCp′FeCp′]PF6 and [Cp*RuCp′FeCp′]PF6 where Cp = C5H5, Cp′ = C5Me4H and Cp* = C5Me5. The metal atom (Fe) root-mean-square-vibrational-amplitude has been determined over the temperature range ∼90 < T < 300 K, and is found to be essentially isotropic in all three cases. The ME spectrum for [Cp′FeCp′FeCp′]BF4 is similar to the spectra for the two Ru–Fe compounds indicating that the substitution of Ru for Fe has only a minor effect on the hyperfine and associated parameters. Similarly, analysis of the metal atom dynamics for [CpRuCp′FeCp′]PF6 and [Cp*RuCp′FeCp′]PF6 shows that ring substitution has a negligible effect on the bonding parameters. A comparison of the three triple-decker compounds with the mono-iron diamagnetic compound Cp*FeCp* and the paramagnetic [Cp′FeCp′]BF4 compound is also detailed.