DFT/TD-DFT analysis of structural, electrochemical and optical data from mononuclear osmium and heterobinuclear osmium–ruthenium alkynyl complexes

Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of the molecular structures, optical absorption spectra, and spectroelectrochemical behavior of the complexes trans-[Os(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCH)Cl(dppe)2] (1), trans,trans-[(dppe)2ClOs(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashC)RuCl(dppe)2] (2), trans,trans-[(dppe)2ClOs(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashC)Ru(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCH)(dppe)2] (3), trans-[Os(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCH)Cl(dppe)2] (4), and trans,trans-[(dppe)2ClOs(Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashCC6H4-4-Ctriple bond; length of mdashC)RuCl(dppe)2] (5) were undertaken. The calculated structures for the mononuclear osmium complexes 1 and 4 are in good agreement with the X-ray data, a 0.3 Å lengthening of the Os–Cl bond in proceeding from the theoretical model for 4 to the experimental structure being rationalized on the basis of intermolecular H-bonding effects for the latter. For the non-oxidized forms of these five complexes, the lowest-energy bands are assigned to MLCT transitions along the main molecular axis from orbitals with mostly Os d, Cl p and alkynyl character to orbitals on the alkynyl backbone. The calculations indicate that the heterobimetallic complexes 2, 3 and 5 do not undergo metal-centred oxidation. Instead, the electron is lost from an orbital that is delocalized across the bridge and both metal centres. The mono-oxidized and di-oxidized species show new bands in the low-energy region arising from LMCT transitions, primarily corresponding to transitions from the dppe ligands to metal- and alkynyl-based orbitals. For structures involving two or more phenylethynyl units along the main axis of the molecule, the TD-DFT calculations are in significantly better agreement with the observed spectra when the phenyl groups adopt a non-coplanar conformation.