Ancillary Ligand Effects upon Dithiolene Redox Noninnocence in Tungsten Bis(dithiolene) Complexes

An
expanded set of compounds of the type [W­(S2C2Me2)2L1L2]n (n = 0: L1 = L2 = CO, 1; L1 = L2 = CNtBu, 2; L1 = CO, L2 = carbene, 3; L1 = CO, L2 = phosphine, 4; L1 = L2 = phosphine, 5. n = 2–: L1 = L2 =
CN–, [6]2–) has been
synthesized and characterized. Despite isoelectronic formulations,
the compound set reveals gradations in the dithiolene ligand redox
level as revealed by intraligand bond lengths, υCCchelate, and rising edge energies in the sulfur K-edge X-ray absorption
spectra (XAS). Differences among the terminal series members, 1 and [6]2–, are comparable
to differences seen in homoleptic dithiolene complexes related by
full electron transfer to/from a dithiolene-based MO. The key feature
governing these differences is the favorable energy of the CO π*
orbitals, which are suitably positioned to overlap with tungsten d
orbitals and exert an oxidizing effect on both metal and dithiolene
ligand via π-backbonding. The CN– π*
orbitals are too high in energy to mix effectively with tungsten and
thus leave the filled dithiolene π* orbitals unperturbed. This
work shows how, and the degree to which, the redox level of a noninnocent
ligand can be modulated by the choice of ancillary ligands(s).