10.1021/acs.inorgchem.8b00352.s001 Po-Jung Huang Po-Jung Huang Yoshiki Natori Yoshiki Natori Yasutaka Kitagawa Yasutaka Kitagawa Yoshihiro Sekine Yoshihiro Sekine Wataru Kosaka Wataru Kosaka Hitoshi Miyasaka Hitoshi Miyasaka One-Dimensional Chains of Paddlewheel-Type Dichromium(II,II) Tetraacetate Complexes: Study of Electronic Structure Influenced by σ- and π‑Donation of Axial Linkers American Chemical Society 2018 HOMO σ- donor strength Cr RCO p K b L ax II Electronic Structure Influenced 2018-04-11 16:48:37 Journal contribution https://acs.figshare.com/articles/journal_contribution/One-Dimensional_Chains_of_Paddlewheel-Type_Dichromium_II_II_Tetraacetate_Complexes_Study_of_Electronic_Structure_Influenced_by_-_and_Donation_of_Axial_Linkers/6128180 Paddlewheel-type carboxylate-bridged dichromium­(II,II) complexes possess intriguing properties such as high redox activity and thermally assisted paramagnetism. However, the relationship of their structures with electronic states and physical properties has not been extensively studied. In this work, we investigated a series of one-dimensional chain complexes based on the paddlewheel-type dichromium­(II,II) tetraacetate complex ([Cr<sub>2</sub><sup>II,II</sup>­(OAc)<sub>4</sub>] = [Cr<sub>2</sub><sup>II,II</sup>]) with pyridine/pyrazine-type organic linkers (μ<sub>2</sub>-L<sub>ax</sub>) having different σ- and π-donating abilities to clarify the electronic structure of [Cr<sub>2</sub><sup>II,II</sup>] assemblies. The chain compounds are stable in air, probably owing to their robust polymerized forms. X-ray crystallographic studies and magnetic measurements revealed that the basicity (p<i>K</i><sub>b</sub>) of L<sub>ax</sub>, which quantitatively correlates with the σ-donor strength of L<sub>ax</sub>, modulates the Cr–Cr and Cr–L<sub>ax</sub> distances and the energy separation (<i>E</i><sub>S–T</sub>) between the diamagnetic (singlet) and thermally populated paramagnetic (triplet) states. The Cr–Cr and Cr–L<sub>ax</sub> distances are strongly influenced by σ- and π-donation from L<sub>ax</sub>, while the frontier δ orbital makes only a small contribution to the structural features. Density functional theory calculations were conducted to clarify this issue. The calculations produced the following unanticipated results against the long-known model: (i) the σ bonding orbital is the HOMO and dominates bonding in the [Cr<sub>2</sub><sup>II,II</sup>] unit, (ii) the total Cr–Cr bond order is less than 1.0, and (iii) the δ orbital electron density is almost completely localized on the chromium sites. The computational results accurately predict the magnetic behavior and provide evidence for a new configuration of frontier orbitals in [Cr<sub>2</sub><sup>II,II</sup>­(RCO<sub>2</sub>)<sub>4</sub>­(L<sub>ax</sub>)<sub>2</sub>].