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Spin Density Maps in the Triplet Ground State of [Cu2(t-Bupy)4(N3)2](ClO4)2 (t-Bupy = p-tert-butylpyridine):  A Polarized Neutron Diffraction Study

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This paper is devoted to the determination of the spin distribution in the spin triplet ground state of [Cu-2(t-Bupy)(4)(N-3)(2)](ClO4)(2), With t-Bupy = p-tert-butylpyridine. The crystal structure, previously solved at room temperature from X-ray diffraction, has been redetermined at 18 K from unpolarized neutron diffraction. The structure consists of binuclear cations in which Cu2+ ions are doubly bridged by azido groups in the 1,1-fashion, and noncoordinated perchlorate anions. The experimental spin distribution has been determined from polarized neutron diffraction (PND) at 1.6 K under 50 kOe. The spin populations have been found to be strongly positive on the Cu2+ ions, weakly positive on the terminal and bridging nitrogen atoms of the azido groups as well as on the nitrogen atoms of the t-Bupy ligands, and weakly negative on the central nitrogen atoms of the N-3(-) bridges. The PND results have been discussed. The spin distribution in [Cu-2(t-Bupy)(4)(N-3)2](ClO4)(2) has been analyzed as resulting from a spin delocalization from the Cu2+ ions toward the azido bridges, to which a spin polarization effect within the azido pi orbitals is superimposed. The experimental data have been compared to the results of DFT calculations. The spin density map is qualitatively reproduced; however, the DFT calculations overestimate the spin delocalization from the Cu2+ ions toward the peripheral and bridging ligands.