A set of terms, definitions, and recommendations is provided for use in the classification of coordination polymers, networks, and metal-organic frameworks (MOFs). A hierarchical terminology is recommended in which the most general term is coordination polymer. Coordination networks are a subset of coordination polymers and MOFs a further subset of coordination networks. One of the criteria an MOF needs to fulfill is that it contains potential voids, but no physical measurements of porosity or other properties are demanded per se. The use of topology and topology descriptors to enhance the description of crystal structures of MOFs and 3D-coordination polymers is furthermore strongly recommended.

Coordination polymers (CPs) and metal-organic frameworks (MOFs) are among the most prolific research areas of inorganic chemistry and crystal engineering in the last 15 years, and yet it still seems that consensus is lacking about what they really are, or are not.

This paper is devoted to the determination of the spin distribution in the spin triplet ground state of [Cu2(t-Bupy)4(N3)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 N3 - bridges. The PND results have been discussed. The spin distribution in [Cu2(t-Bupy)4(N3)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 π 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.