The cobalt(II) in [Co(NCS)2(4‐methoxypyridine)2]n are linked by pairs of thiocyanate anions into linear chains. In contrast to a previous structure determination, two crystallographically independent cobalt(II) centers have been found to be present. In the antiferromagnetic state, below the critical temperature (Tc=3.94 K) and critical field (Hc=290 Oe), slow relaxations of the ferromagnetic chains are observed. They originate mainly from defects in the magnetic structure, which has been elucidated by micromagnetic Monte Carlo simulations and ac measurements using pristine and defect samples. The energy barriers of the relaxations are Δτ1=44.9(5) K and Δτ2=26.0(7) K for long and short spin chains, respectively. The spin excitation energy, measured by using frequency‐domain EPR spectroscopy, is 19.1 cm−1 and shifts 0.1 cm−1 due to the magnetic ordering. Ab initio calculations revealed easy‐axis anisotropy for both CoII centers, and also an exchange anisotropy Jxx/Jzz of 0.21. The XXZ anisotropic Heisenberg model (solved by using the density renormalization matrix group technique) was used to reconcile the specific heat, susceptibility, and EPR data.

Reaction of Ni(NCS)2 with different co‐ligands based on pyridine derivatives substituted in 4‐position leads to the formation of 1D coordination polymers with the composition [Ni(NCS)2(co‐ligand)2]n, in which the metal cations are coordinated octahedrally by two S‐ and two N‐bonding thiocyanate anions as well as two neutral co‐ligands and are linked into chains by µ‐1,3‐bridging thiocyanate anions. Dependent on the nature of the co‐ligand, novel isomers with linear and corrugated chains are observed, in which the Ni cations are trans‐ or cis‐cis‐trans coordinated. This also includes a dimer and two isomeric chain compounds with 4‐chloropyridine as ligand, of which one is metastable at room‐temperature. For all these compounds, together with several additional Ni(NCS)2 chain compounds, the values of the intrachain exchange interaction J are extracted based on magnetic measurements. The values of J for 13 different Ni(NCS)2 compounds are compared with the structural results to investigate if there are some simple relations between the magnetic exchange, the metal coordination and the chain geometry. The low temperature magnetic properties of the Ni(NCS)2 dimer and the chain compounds are also discussed based on magnetic and specific heat measurements.

Reaction of Co­(NCS)2 with different coligands leads to the formation of three compounds with the general composition [Co­(NCS)2(L)2] n (L = aniline (1), morpholine (2), and ethylenethiourea (3)). In all of these compounds the cobalt­(II) cations are octahedrally coordinated by two trans thiocyanate N and S atoms and the apical donor atoms of the coligands and are linked into linear chains by pairs of anionic ligands. The magnetic behavior was investigated by a combination of static and dynamic susceptibility as well as specific-heat measurements, computational studies, and THz-EPR spectroscopy. All compounds show antiferromagnetic ordering as observed for similar compounds with pyridine derivatives as coligands. In contrast to the latter, for 1–3 significantly higher critical temperatures and no magnetic single-chain relaxations are observed, which can be traced back to stronger interchain interactions and a drastic change in the magnetic anisotropy of the metal centers. These results are discussed and compared with those of the pyridine-based compounds, which provides important insights into the parameters that govern the magnetic behavior of such one-dimensional coordination polymers.

Two different isomers of [Co­(NCS)2(4-chloropyridine)2] n (3C and 3L) were synthesized from solution and by thermal decomposition of Co­(NCS)2(4-chloropyridine)2(H2O)2 (2), which show a different metal coordination leading to corrugated chains in 3C and to linear chains in 3L. Solvent mediated conversion experiments prove that 3C is thermodynamically stable at room temperature where 3L is metastable. Magnetic measurements reveal that the magnetic exchange in 3L is comparable to that observed for previously reported related chain compounds, whereas in 3C with corrugated chains, the ferromagnetic interaction within the chains is strongly suppressed. The magnetic ordering takes place at 2.85 and 0.89 K, for 3L and 3C, respectively, based on specific heat measurements. For 3L the field dependence of magnetic relaxations in antiferromagnetically ordered ferromagnetic chains is presented. In addition, 3L is investigated by FD-FT THz-EPR spectroscopy, revealing a ground to first excited state energy gap of 14.0 cm–1. Broken-symmetry DFT calculations for 3C and 3L indicate a ferromagnetic intrachain interaction. Ab initio CASSCF/CASPT2/RASSI-SO computational studies reveal significantly different single-ion anisotropies for the crystallographically independent cobalt­(II) centers in 3C and 3L. Together with the geometry of the chains this explains the magnetic properties of 3C and 3L. The ab initio results also explain the weaker exchange interaction in 3C and 3L as compared to previously reported [Co­(NCS)2(L)2] n compounds with linear chains.

Invited for the cover of this issue is the group of Michał Rams at Jagiellonian University (Kraków, Poland) and colleagues at Christian‐Albrechts‐Universität zu Kiel, Friedrich‐Schiller‐Universität Jena, and Helmholtz‐Zentrum Berlin. The image represents a 1D coordination polymer with Co(II) spins that are flipped by photons during an EPR experiment. Read the full text of the article at 10.1002/chem.201903924.