Zero-field splitting in metal complexes

When two paramagnetic transition metal ions are present in the same molecular entity, the magnetic properties can be totally different from the sum of the magnetic properties of each ion surrounded by its nearest neighbors. These new properties depend on the nature and the magnitude of the interaction between the metal ions through the bridging ligands. If both ions have an unpaired electron (e.g. Cu2+ ions), then the molecular state of lowest energy is either a spin singlet or a spin triplet. In the former case, the interaction is said to be antiferromagnetic, in the latter case ferromagnetic. The nature and the order of magnitude of the interaction can be engineered by judiciously choosing the interacting metal ions and the bridging and terminal ligands, and, thus, by the symmetry and the delocalization of the orbitals centered on the metal ions and occupied by the unpaired electrons (magnetic orbitals). The first success in this “molecular engineering” of bimetallic compounds was in the synthesis of a Cu2+VO2+ heterobimetallic complex in which the interaction is purely ferro-magnetic. The same strategy could be utilized for designing molecular ferromagnets, one of the major challenges in the area of molecular materials. Another striking result is the possibility of tuning the magnitude of the interaction through a given bridging network by modifying the nature of the terminal ligands, which, in some way, play the role of “adjusting screws”. By careful selection of the bridging and terminal ligands, a very large antiferro-magnetic interaction can be achieved, even if the metal ions are far away from each other. Some sulfur-containing bridges are especially suitable in this respect.

Dinuclear Complexes with Predictable Magnetic Properties

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

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Spin state switching in iron coordination compounds

Constructing magnetic molecular solids by employing three-atom ligands as bridges

The field of heteropolymetallic systems with magnetic metal centers occupies a crossing point between biology and physics. For instance the Cu(II)-Fe(III) interaction in cytochrome oxidase is of the same nature as the Cu(II)-Mn(II) interaction in a novel system which could be the first molecular ferromagnet. The mechanism of the interaction is discussed, both from a phenomenological view point using a spin Hamiltonian, and from an orbital view point. An orbital model for the isotropic interaction is presented. It is based on the concept of natural magnetic orbitals. The mechanism of the anisotropic and antisymmetric interactions is more briefly treated. The role of the Zeeman perturbation is then considered in relation with the magnetic and EPR properties of the heterobimetallic complexes. Several examples are presented to emphasize that the nature, ferro- or antiferromagnetic of the isotropic interaction is controlled by the symmetry of the magnetic orbitals. The concept of overlap density is introduced. It permits an estimation of the magnitude of the ferromagnetic stabilization in the case of orthogonality of the magnetic orbitals. The Cu(II)-Fe(III) interaction, in relation to the situation encountered in cytochrome oxidase, the Cu(II)-Ni(II) interaction and a few additional selected examples are discussed. A section deals with the case where one of the interacting ions has an orbital degeneracy. Afterwards, the heterotrinuclear complexes are studied. The important concept of regular and irregular spin state structure is developped and the Mn(II)Cu(II)Mn(II) triad is presented as a spectacular example of irregular spin state structure. A section is devoted to the ordered bimetallic chains. The theory is presented, both at a qualitative and quantitative levels and the already reported compounds of this kind are discussed. One of them may be considered as one of the first molecular ferromagnets. The last but one section concerns the systems with even more subtle spin orders. In conclusion, the vast perspectives of this area are outlined.

Magnetism of the heteropolymetallic systems

I. Paramagnetism: The Curie Law.- A. Introduction.- B. Diamagnetism and Paramagnetism.- C. Magnetic Moment of a Magnetic Ion Subsystem.- D. Some Curie Law Magnets.- E. Susceptibilities of the Lanthanides.- F. Temperature Independent Paramagnetism.- References.- II. Thermodynamics and Relaxation.- A. Introduction.- B. Thermodynamic Relations.- C. Thermal Effects.- D. Adiabatic Demagnetization.- E. Relaxation Time and Transition Probability.- F. Spin-lattice Relaxation Processes.- G. Susceptibility in Alternating Fields.- H. Adiabatic Susceptibilities.- References.- III. Paramagnetism: Zero-Field Splittings.- A. Introduction.- B. Schottky Anomalies.- C. Adiabatic Demagnetization.- D. Van Vleck's Equation.- E. Paramagnetic Anisotropy.- F. Effective Spin.- G. Direct Measurement of D.- H. Electron Paramagnetic Resonance (EPR).- References.- IV. Dimers and Clusters.- A. Introduction.- B. Energy Levels and Specific Heats.- C. Magnetic Susceptibilities.- D. Copper Acetate and Related Compounds.- E. Some Other Dimers.- F. EPR Measurements.- G. Clusters.- H. The Ising Model.- References.- V. Long-Range Order.- A. Introduction.- B. Molecular Field Theory of Ferromagnetism.- C. Thermal Effects.- D. Molecular Field Theory of Antiferromagnetism.- E. Ising, XY, and Heisenberg Models.- F. Critical Point Exponents.- G. Cu(NO3)2*21/2H2O.- H. Dipole-Dipole Interactions.- I. Exchange Effects on Paramagnetic Susceptibilities.- J. Superexchange.- References.- VI. Short-Range Order.- A. Introduction.- B. One-Dimensional or Linear Chain Systems.- C. Two-Dimensional or Planar Systems.- D. Long-Range Order.- References.- VII. Special Topics: Spin-Flop, Metamagnetism, Ferrimagnetism and Canting.- A. Introduction.- B. Phase Diagrams and Spin-Flop.- C. Metamagnetism.- D. Ferrimagnetism.- E. Canting and Weak Ferromagnetism.- References.- VIII. Selected Examples.- A. Introduction.- B. Some Single Ion Properties.- 1. Ti3+.- 2. V3+.- 3. VO2+.- 4. Cr3+.- 5. Mn2+.- 6. Fe3+.- 7. Fe2+ and Cr2+.- 8. Co2+.- 9. Ni2+.- 10. Cu2+.- 11. Lanthanides.- C. Some Examples.- 1. Iron(III) Methylammonium Sulfate.- 2. CaCu(OAc)4*6H2O.- 3. Hydrated Nickel Halides.- 4. Hydrated Nickel Nitrates.- 5. Tris Dithiocarbamates of Iron(III).- 6. Spin-3/2 Iron(III).- 7. Manganous Acetate Tetrahydrate.- 8. [M(C5H5NO)6]ClO4)2.- 9. NiX2L2.- 10. [(CH3)3NH]MX3*2H2O.- References.- Append.- A. Physical Constants.- B. Hyperbolic Functions.- Formula Index.

Magnetic Properties of Transition Metal Compounds

The angular variation of the EPR linewidths in the crystallographic ac and ab planes of CaCu(OAc) 4 ·6H 2 O is studied In discussing the results it is demonstrated that the magnetic properties of this compound need not be described with a linear-chain model as suggested earlier

The angular dependence of the EPR linewidths in calcium copper acetate hexahydrate

Pulsed-field magnetization curves for Ni(C5H5NO)6(ClO4)2 and [C(NH2)3]V(SO4)2⊙6H2O show a separation of the ground-state singlet and the upper doublet of D/gαB = 41.8 and 40.6 kOe, respectively. Additional measurements of X(ω, H, T) evidence pronounced spin-lattice relaxation effects for the V3+ salt, as well as cross-relaxations within the S = 1 multiplet.

Susceptibility and magnetization of the singlet-ground-state systems Ni(C5H5NO)6(ClO4)2 and [C(NH2)3]V(SO4)2⊙6H2O as a function of field, frequency and temperature

Magnetic ordering in [Ni en3] (NO3)2

Spin-lattice relaxation times have been measured for CaCu(CH 3 COO) 4 .6H 2 O at 4.2 K and 2.0 K in external fields up to 50 kOe. An anomalous shortening of the observed times is seen at approximately 18.5 kOe for a powdered specimen, while for a single crystal a number of such anomalies are detected. It is suggested that the anomalies occur because of the interaction between the copper spin system and a rotator system, arising due to the hindered internal rotation of the methyl group of the acetate ligand. Results on the deuterated compound CaCu(CD 3 COO) 4 .6H 2 O are in agreement with this suggestion.

R.L. Carlin

Papers

Magnetic Properties of Transition Metal Compounds

The angular dependence of the EPR linewidths in calcium copper acetate hexahydrate

Susceptibility and magnetization of the singlet-ground-state systems Ni(C5H5NO)6(ClO4)2 and [C(NH2)3]V(SO4)2⊙6H2O as a function of field, frequency and temperature

Magnetic ordering in [Ni en3] (NO3)2

Tunnelling resonance effects in non-resonant spin-lattice relaxation experiments