A family of mononuclear tetrahedral cobalt(II) thiourea complexes, [Co(L)](NO) (1) and [Co(L)](ClO) where x = 2 (2), 3 (3), 4 (4) (where L = thiourea, L = 1,3-dibutylthiourea, L = 1,3-phenylethylthiourea, and L = 1,1,3,3-tetramethylthiourea), has been synthesized using a rationally designed synthetic approach, with the aim of stabilizing an Ising-type magnetic anisotropy (-D). On the basis of direct-current, alternating-current, and hysteresis magnetic measurements and theoretical calculations, we have identified the factors that govern the sign and magnitude of D and ultimately the ability to design a single-ion magnet for a tetrahedral cobalt(II) ion. To better understand the magnetization relaxation dynamics, particularly for complexes 1 and 2, dilution experiments were performed using their diamagnetic analogues, which are characterized by single-crystal X-ray diffraction with the general molecular formulas of [Zn(L)](NO) (5) and [Zn(L)](ClO) (6). Interestingly, intermolecular interactions are shown to play a role in quenching the quantum tunneling of magnetization in zero field, as evidenced in the hysteresis loop of 1. Complex 2 exhibits the largest U value of 62 cm and reveals open hysteresis loops below 4 K. Furthermore, the influence of the hyperfine interaction on the magnetization relaxation dynamics is witnessed in the hysteresis loops, allowing us to determine the electron/nuclear spin S(Co) = //I(Co) = / hyperfine coupling constant of 550 MHz, a method ideally suited to determine the hyperfine coupling constant of highly anisotropic metal ions stabilized with large D value, which are otherwise hard to determine by conventional methods such as electron paramagnetic resonance.

Four novel mononuclear tetrahedral cobalt(II) complexes containing exocyclic mesoionic ligands of molecular formulae [Co(II)(L1)(X)2(MeCN)] X = Cl (1) or Br (2) and [Co(II)(L2)(X)2(MeCN)], X = Cl (3) or Br (4) have been reported. It is found that simple substitution of L1 (O donor in 1 and 2) by L2 (S donor in 3 and 4) results in switching of the single ion magnetic anisotropy parameter (D) from positive to negative, with a significant change in magnitude.

The reaction of [Co(II)(NO3)2]·6H2O with the nitroxide radical, 4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition-metal complex [Co(II)(L(•))2](NO3)2 (1), which has been investigated using temperature-dependent magnetic susceptibility, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, density functional theory (DFT) calculations, and variable-temperature X-ray structure analysis. Magnetic susceptibility measurements and X-ray diffraction (XRD) analysis reveal a central low-spin octahedral Co(2+) ion with both ligands in the neutral radical form (L(•)) forming a linear L(•)···Co(II)···L(•) arrangement. This shows a host of interesting magnetic properties including strong cobalt-radical and radical-radical intramolecular ferromagnetic interactions stabilizing a S = (3)/2 ground state, a thermally induced spin crossover transition above 200 K and field-induced slow magnetic relaxation. This is supported by variable-temperature EPR spectra, which suggest that 1 has a positive D value and nonzero E values, suggesting the possibility of a field-induced transverse anisotropy barrier. DFT calculations support the parallel alignment of the two radical π*NO orbitals with a small orbital overlap leading to radical-radical ferromagnetic interactions while the cobalt-radical interaction is computed to be strong and ferromagnetic. In the high-spin (HS) case, the DFT calculations predict a weak antiferromagnetic cobalt-radical interaction, whereas the radical-radical interaction is computed to be large and ferromagnetic. The monocationic complex [Co(III)(L(-))2](BPh4) (2) is formed by a rare, reductively induced oxidation of the Co center and has been fully characterized by X-ray structure analysis and magnetic measurements revealing a diamagnetic ground state. Electrochemical studies on 1 and 2 revealed common Co-redox intermediates and the proposed mechanism is compared and contrasted with that of the Fe analogues.

Two oxazolidine nitroxide complexes of cobalt(II), [Co(II)(L(•))2](B(C6F5)4)2·CH2Cl2 (1) and [Co(II)(L(•))2](B(C6F5)4)2·2Et2O (2), where, L(•) is the tridentate chelator 4,4-dimethyl-2,2-bis(2-pyridyl)oxazolidine N-oxide, have been investigated by crystallographic, magnetic, reflectivity, and theoretical (DFT) methods. This work follows on from a related study on [Co(II)(L(•))2](NO3)2 (3), a multifunctional complex that simultaneously displays magnetic exchange, spin crossover, and single molecule magnetic features. Changing the anion and the nature of solvation in the present crystalline species leads to significant differences, not only between 1 and 2 but also in comparison to 3. Structural data at 123 and 273 K, in combination with magnetic data, show that at lower temperatures 1 displays low-spin Co(II)-to-radical exchange with differences in fitted J values in comparison to DFT (broken symmetry) calculated J values ascribed to the sensitive influence of a tilt angle (θ) formed between the Co(dz(2)) and the trans-oriented O atoms of the NO radical moieties in L(•). Spin crossover in 1 is evident at higher temperatures, probably influenced by the solvate molecules and crystal packing arrangement. Complex 2 remains in the high-spin Co(II) state between 2 and 350 K and undergoes antiferromagnetic exchange between Co-radical and radical-radical centers, but it is difficult to quantify. Calculations of the magnetic orbitals, eigenvalue plots, and the spin densities at the Co and radical sites in 1 and 2 have yielded satisfying details on the mechanism of metal-radical and radical-radical exchange, the radical spins being in π*NO orbitals.

A new simple and conceptual theoretical scheme is proposed for estimating oneelectron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

Mononuclearf our coordinate Co II complexes have drawn ag reat deal of attention as they often exhibit excellent single-ion magnet (SIM) properties. Among the reported complexes, the axial zero-field splitting parameter (D)w as found to vary drastically both in terms of the sign as well as strength. There are variousp roposals in this respects uch as structural distortions,h eaviera tom substitution, metalligand covalency,t uning secondary coordination sphere, etc. that are expected to control the D values. To assess the importance of structurald istortions vs. heaviera tom substitution effect, herew eh ave undertaken detailed theoretical studies based on the ab initio CASSCF/NEVPT2 methodt o estimate zero-field splitting parameters for twelve complexes reported in the literature. Our test set includes the {Co II X 4 } (where X = O, S, Se) core structure where the D value was found to vary from + 19 to À118cm À1 .B ased on the structural variation, we have classified the complexes into three types (I-III)w here type I complexes were found to exhibit the largestn egative D value as desired for SIMs. The other two types (II and III)o fc omplexes have been found to be inferior with respect to type I. The secondary coordination spherew as also found to influence D,a ss ubstitution on the secondary coordination sphere atom was found to significantly alter the magnitude of D values. Particularly,t wo structuralp arameters, namely,t he dihedrala ngle between the two ligand planes and the ffX-Co-X polar angle were found to heavily influence the sign and strength of D values. Our analysis clearly reveals that these structural factors are much more important than the heavier atom substitution,o r metal-ligand covalency.Alarge variation in the D and E/D values among these complexesd espite possessing av ery close structuralsimilarity offers an exquisite playground for a chemistt odesign and develop new-generation Co II -based SIMs.

One‐electron redox processes in ligands that exhibit non‐innocent behaviour invariably leads to the generation of radical species, which, when coupled with redox‐active metal centres, can lead to strong magnetic exchange, ambiguous electronic structures and even spin crossover (SCO). We have chosen two iron complexes [FeII(L·)2](BF4)2 (1) and [FeIII(L–)2](BPh4) (2) [with L· = 4‐dimethyl‐2,2‐di(2‐pyridyl)oxazolidine N‐oxide] as the basis for an extensive theoretical study to determine the influence and interplay of the strong magnetic exchange, the redox non‐innocent behaviour and any possible spin transitions. A series of calculations established the low‐spin character of the Fe in both the complexes and gave us valuable insight into the electronic structure of 1 and 2 along with their one‐ and two‐electron reduced species. An attempt has been made to relate the bonding features to the energy difference between different spin states and to the influence of the exchange coupling on any possible SCO properties.

Overall these experiments give coherent results with T1/2 located between 320 and 335 K, a narrow range in comparison to the solid-state. DFT calculations allowed rationalizing the differences in the magnetic differences. The molecular orbital and spin density calculations reveal that the presence of the C=O group between the imidazolyl units in the ligands of 1-3 leads to an extended aromatic system, an effective -acceptor effect, stabilizing a LS state and reducing the LS-HS gap in comparison to 4.