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