Synthesis, Structural, and Magnetic Characterization of Linear and Bent Geometry Cobalt(II) and Nickel(II) Amido Complexes: Evidence of Very Large Spin–Orbit Coupling Effects in Rigorously Linear Coordinated Co²⁺

The complexes M­(II)­{N­(H)­Ar^Prⁱ₆}₂ (M = Co, <b>1</b> or Ni, <b>2</b>; Ar^Prⁱ₆ = C₆H₃-2,6­(C₆H₂-2,4,6-Prⁱ₃)₂), which have rigorously linear, N–M–N = 180°, metal coordination, and M­(II)­{N­(H)Ar^Me₆}₂ (M = Co, <b>3</b> or Ni, <b>4</b>; Ar^Me₆ = C₆H₃-2,6­(C₆H₂-2,4,6-Me₃)₂), which have bent, N–Co–N = 144.1(4)°, and N–Ni–N = 154.60(14)°, metal coordination, were synthesized and characterized to study the effects of the metal coordination geometries on their magnetic properties. The magnetometry studies show that the linear cobalt­(II) species <b>1</b> has a very high ambient temperature moment of about 6.2 μ_B (cf. spin only value = 3.87 μ_B) whereas the bent cobalt species <b>3</b> had a lower μ_B value of about 4.7 μ_B. In contrast, both the linear and the bent nickel complexes <b>2</b> and <b>4</b> have magnetic moments near 3.0 μ_B at ambient temperatures, which is close to the spin only value of 2.83 μ_B. The studies suggest that in the linear cobalt species <b>1</b> there is a very strong enhanced spin orbital coupling which leads to magnetic moments that broach the free ion value of 6.63 μ_B probably as a result of the relatively weak ligand field and its rigorously linear coordination. For the linear nickel species <b>2</b>, however, the expected strong first order orbital angular momentum contribution does not occur (cf. free ion value 5.6 μ_B) possibly because of π bonding effects involving the nitrogen p orbitals and the d_<i>xz</i> and d_<i>yz</i> orbitals (whose degeneracy is lifted in the <i>C</i>_2<i>h</i> local symmetry of the Ni­{N­(H)­C­(<i>ipso</i>)}₂ array) which quench the orbital angular momentum

Synthesis, Structural, and Magnetic Characterization of Linear and Bent Geometry Cobalt(II) and Nickel(II) Amido Complexes: Evidence of Very Large Spin–Orbit Coupling Effects in Rigorously Linear Coordinated Co²⁺

The complexes M­(II)­{N­(H)­Ar^Prⁱ₆}₂ (M = Co, <b>1</b> or Ni, <b>2</b>; Ar^Prⁱ₆ = C₆H₃-2,6­(C₆H₂-2,4,6-Prⁱ₃)₂), which have rigorously linear, N–M–N = 180°, metal coordination, and M­(II)­{N­(H)Ar^Me₆}₂ (M = Co, <b>3</b> or Ni, <b>4</b>; Ar^Me₆ = C₆H₃-2,6­(C₆H₂-2,4,6-Me₃)₂), which have bent, N–Co–N = 144.1(4)°, and N–Ni–N = 154.60(14)°, metal coordination, were synthesized and characterized to study the effects of the metal coordination geometries on their magnetic properties. The magnetometry studies show that the linear cobalt­(II) species <b>1</b> has a very high ambient temperature moment of about 6.2 μ_B (cf. spin only value = 3.87 μ_B) whereas the bent cobalt species <b>3</b> had a lower μ_B value of about 4.7 μ_B. In contrast, both the linear and the bent nickel complexes <b>2</b> and <b>4</b> have magnetic moments near 3.0 μ_B at ambient temperatures, which is close to the spin only value of 2.83 μ_B. The studies suggest that in the linear cobalt species <b>1</b> there is a very strong enhanced spin orbital coupling which leads to magnetic moments that broach the free ion value of 6.63 μ_B probably as a result of the relatively weak ligand field and its rigorously linear coordination. For the linear nickel species <b>2</b>, however, the expected strong first order orbital angular momentum contribution does not occur (cf. free ion value 5.6 μ_B) possibly because of π bonding effects involving the nitrogen p orbitals and the d_<i>xz</i> and d_<i>yz</i> orbitals (whose degeneracy is lifted in the <i>C</i>_2<i>h</i> local symmetry of the Ni­{N­(H)­C­(<i>ipso</i>)}₂ array) which quench the orbital angular momentum

Two-Coordinate, Quasi-Two-Coordinate, and Distorted Three Coordinate, T-Shaped Chromium(II) Amido Complexes: Unusual Effects of Coordination Geometry on the Lowering of Ground State Magnetic Moments

The synthesis and characterization of the mononuclear chromium­(II) terphenyl substituted primary amido-complexes Cr­{N­(H)­Ar^Prⁱ₆}₂ (Ar^Prⁱ₆ = C₆H₃-2,6-(C₆H₂-2,4,6-^<i>i</i>Pr₃)₂ (<b>1</b>), Cr­{N­(H)­Ar^Prⁱ₄}₂ (Ar^Prⁱ₄ = C₆H₃-2,6-(C₆H₃-2,6-^<i>i</i>Pr₂)₂ (<b>2</b>), Cr­{N­(H)­Ar^Me₆}₂ (Ar^Me₆ = C₆H₃-2,6-(C₆H₂-2,4,6-Me₃)₂ (<b>4</b>), and the Lewis base adduct Cr­{N­(H)­Ar^Me₆}₂(THF) (<b>3</b>) are described. Reaction of the terphenyl primary amido lithium derivatives Li­{N­(H)­Ar^Prⁱ₆} and Li­{N­(H)­Ar^Prⁱ₄} with CrCl₂(THF)₂ in a 2:1 ratio afforded complexes <b>1</b> and <b>2</b>, which are extremely rare examples of two coordinate chromium and the first stable chromium amides to have linear coordinated high-spin Cr²⁺. The reaction of the less crowded terphenyl primary amido lithium salt Li­{N­(H)­Ar^Me₆} with CrCl₂(THF)₂ gave the tetrahydrofuran (THF) complex <b>3</b>, which has a distorted T-shaped metal coordination. Desolvation of <b>3</b> at about 70 °C gave <b>4</b> which has a formally two-coordinate chromous ion with a very strongly bent core geometry (N–Cr–N= 121.49(13)°) with secondary Cr--C­(aryl ring) interactions of 2.338(4) Å to the ligand. Magnetometry studies showed that the two linear chromium species <b>1</b> and <b>2</b> have ambient temperature magnetic moments of about 4.20 μ_B and 4.33 μ_B which are lower than the spin-only value of 4.90 μ_B typically observed for six coordinate Cr²⁺. The bent complex <b>4</b> has a similar room temperature magnetic moment of about 4.36 μ_B. These studies suggest that the two-coordinate chromium complexes have significant spin–orbit coupling effects which lead to moments lower than the spin only value of 4.90 μ_B because λ (the spin orbit coupling parameter) is positive. The three-coordinated complex <b>3</b> had a magnetic moment of 3.79 μ_B

“Chemical Metamagnetism”: From Antiferromagnetic PrCo₂P₂ to Ferromagnetic Pr_0.8Eu_0.2Co₂P₂ via Chemical Compression

“Chemical Metamagnetism”: From Antiferromagnetic PrCo₂P₂ to Ferromagnetic Pr_0.8Eu_0.2Co₂P₂ via Chemical Compressio

Measurement of Extreme Hyperfine Fields in Two-Coordinate High-Spin Fe²⁺ Complexes by Mössbauer Spectroscopy: Essentially Free-Ion Magnetism in the Solid State

Mössbauer studies of three two-coordinate linear high-spin Fe²⁺ compounds, namely, Fe­{N­(SiMe₃)­(Dipp)}₂ (<b>1</b>) (Dipp = C₆H₃-2,6-^<i>i</i>Pr₂), Fe­(OAr′)₂ (<b>2</b>) [Ar′ = C₆H₃-2,6-(C₆H₃-2,6-^<i>i</i>Pr₂)₂], and Fe­{C­(SiMe₃)₃}₂ (<b>3</b>), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (<b>1</b>–<b>3</b>), as well as zero- and applied-field heat-capacity measurements (<b>3</b>). As <b>1</b>–<b>3</b> are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn–Teller theorem need not necessarily apply. We find that the resulting very large unquenched orbital angular momentum leads to what we believe to be the largest observed internal magnetic field to date in a high-spin iron­(II) compound, specifically +162 T in <b>1</b>. The latter field is strongly polarized along the directions of the external field for both longitudinal and transverse field applications. For the longitudinal case, the applied field increases the overall hyperfine splitting consistent with a dominant orbital contribution to the effective internal field. By contrast, <b>2</b> has an internal field that is not as strongly polarized along a longitudinally applied field and is smaller in magnitude at ca. 116 T. Complex <b>3</b> behaves similarly to complex <b>1</b>. They are sufficiently self-dilute (e.g., Fe···Fe distances of ca. 9–10 Å) to exhibit varying degrees of slow paramagnetic relaxation in zero field for the neat solid form. In the absence of EPR signals for <b>1</b>–<b>3</b>, we show that heat-capacity measurements for one of the complexes (<b>3</b>) establish a <i>g</i>_eff value near 12, in agreement with the principal component of the ligand electric field gradient being coincident with the <i>z</i> axis

Oligonuclear Fe Complexes (Fe, Fe₄, Fe₆, Fe₉) Derived from Tritopic Pyridine Bis-Hydrazone LigandsStructural, Magnetic, and Mössbauer Studies

Tri-topic pyridine bis-hydrazone ligands produce polynuclear complexes with Fe­(II) and Fe­(III) salts with varying nuclearity and metal ion oxidation states. Mononuclear, tetranuclear, hexanuclear, and nonanuclear examples are discussed using structural, magnetic and Mössbauer data. In one case, although X-ray data suggest a [3 × 3] Fe₉ grid (space group <i>P</i>4₂/<i>n</i>), careful examination of the structure, in conjunction with magnetic and Mössbauer data, indicates an unusual situation where the corner and center sites are present at unit occupancy, whereas side site occupancy is ∼0.6