Iron versus Ruthenium: Clarifying the Electronic Differences between Prototypical Mixed-Valence Organometallic Butadiyndiyl Bridged Molecular Wires

The
electronic structures of the prototypical bimetallic buta-1,3-diyn-1,4-diyl-bridged
radical cation complexes [{M­(dppe)­Cp′}2(μ-CCCC)]+ (M= Fe, Cp′ = Cp* (1a), Cp (1b); M = Ru, Cp′ = Cp* (2a), Cp (2b)) have been (re)­investigated using a combination of UV–vis–NIR
and IR spectroelectrochemistry, and quantum chemical calculations
based on both dispersion-corrected global (BLYP35-D3) and local (Lh-SsirPW92-D3)
hybrid functionals. Following analysis of new and existing data, including
the IR-active ν­(CC) bands, the iron compounds [1]+ are reclassified as valence-trapped (Robin
and Day Class II) mixed-valence complexes, in contrast to the ruthenium
complexes [2]+, which are delocalized (Robin
and Day Class III) systems. All members of the series exist as a thermally
populated distribution of conformers in solution, and the overlapping
spectroscopic profiles make the accurate extraction of the parameters
necessary for the analysis of [1]+ and [2]+ within the framework of the Marcus–Hush
model extremely challenging. Analysis of the spin-density distributions
from a range of conformational minima provides an alternative representation
of the degree of charge localization, and a comparison between members
of the series is presented.