A spectroscopic and computationally minimal approach to the analysis of charge-transfer processes in conformationally fluxional mixed-valence and heterobimetallic complexes

Class II mixed-valence bimetallic complexes {[Cp′(PP)M]C≡C−C≡N[M′(PP)′Cp′]}²⁺ (M, M′=Ru, Fe; PP=dppe, (PPh₃)₂; Cp′=Cp*, Cp) exist as conformational ensembles in fluid solution, with a population of structures ranging from cis- to trans-like geometries. Each conformer gives rise to its own series of low-energy intervalence charge-transfer (IVCT) and local d–d transitions, which overlap in the NIR region, giving complex band envelopes in the NIR absorption spectrum, which prevent any meaningful attempt at analysis of the band shape. However, DFT and time-dependent (TD)DFT calculations with dispersion-corrected global-hybrid (BLYP35-D3) or local hybrid (lh-SsirPW92-D3) functionals on a small number of optimised structures chosen to sample the ground state potential energy hypersurfaces of each of these complexes has proven sufficient to explain the major features of the electronic spectra. Although modest in terms of computational expense, this approach provides a more accurate description of the underlying molecular electronic structure than would be possible through analysis of the IVCT band by using the static point-charge model of Marcus–Hush theory and derivatives, or TDDFT calculations from a single (global) minimum energy geometry.