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⁺

Abstract

The complexes M(II){N(H)Ar(Pr(i)(6))}(2) (M = Co, 1 or Ni, 2; Ar(Pr(i)(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Pr(i)(3))(2)), which have rigorously linear, N-M-N = 180°, metal coordination, and M(II){N(H)Ar(Me(6))}(2) (M = Co, 3 or Ni, 4; Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(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 1 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 3 had a lower μ(B) value of about 4.7 μ(B). In contrast, both the linear and the bent nickel complexes 2 and 4 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 1 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 2, 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(xz) and d(yz) orbitals (whose degeneracy is lifted in the C(2h) local symmetry of the Ni{N(H)C(ipso)}(2) array) which quench the orbital angular momentum.

Synthesis, Structural, Spectroscopic, and Magnetic Characterization of Two-Coordinate Cobalt(II) Aryloxides with Bent or Linear Coordination

“…45 The crude red product was then extracted with hot toluene, concentrated, and left to stand undisturbed overnight at 25°C to yield dark red crystals that were suitable for X-ray analysis. X-ray crystallography showed that the product 2b had a cobalt(II) occupancy of 84.6(2)%; however, a second synthesis of the OAr iPr 4 derivative under similar conditions but where the resulting crude product was heated to ca.165°C at 5 × 10 −2 Torr for 2 h resulted in a dark red residue and colorless crystals which had sublimed on the upper walls of the flask. The red solid was then recrystallized from hot toluene, which afforded compound 2a with a cobalt(II) occupancy of 93.5(3)% as indicated by X-ray diffraction.…”

“…The bent structure of 1, in contrast to the linear structures of 2a and 2b, is primarily a result of the smaller size of the OAr Me 6 ligand in comparison to OAr iPr 4 , which permits secondary Co--C interaction between the relatively electron-poor cobalt(II) and electron-rich flanking suggest that other factors are probably involved. 52,53 It is possible that dispersion forces involving interactions between C−H moieties of the isopropyl groups from the two terphenyl substituents are strong enough to stabilize linear geometry at the metal, as observed in 2a, 2b, and Fe(OAr iPr 4 ) 2 36 and in M{N(H)Ar iPr6 } 2 , where M = V, 22 Cr, 24 Mn, 54 Fe, 8 Co, 4 and Ni. 4 The effects of dispersion forces produced by the less ramified Ar Me 6 substructure may be insufficient to induce linear geometry, and the lower steric requirement of Ar Me 6 in comparison to Ar iPr 4 or Ar iPr6 permits significant bending of the divalent metal geometries to form a pseudotetrahedral arrangement with the ipso-carbons as demonstrated in the compounds 1, Co{N(H)Ar Me 6 } 2 , 4 and Fe{N(H)Ar Me 6 } 2 .…”

“…Alternatively, the ν(M−O) bands for 1 at 390 cm −1 and 2a at 380 cm −1 are comparable to the amide analogues at 385 cm −1 for Co{N(H)Ar Me 6 } 2 4 and 380 cm −1 for Co{N(H)Ar iPr6 } 2 . 4 The characteristic ν(H−O) band at 3540 cm −1 for HOAr Me 6 43 and 3530 cm −1 for HOAr iPr4 44 does not appear in the IR spectra of 1 and 2a respectively, demonstrating that free ligand either is…”

The Instability of Ni{N(SiMe3)2}2: A Fifty Year Old Transition Metal Silylamide Mystery

“…Its instability is especially striking because several other stable, homoleptic Ni(II) amides are known. These include [{Ni(NPh2)2}2], [14] the borylamides Ni{N(R)BMes2}2 (R = Ph or Mes), [15,16] the primary terphenyl amides Ni{N(H)Ar Me6 }2, [17] Ni{N(H)Ar iPr4 }2, [18] and Ni{N(H)Ar iPr6 }2 [17] (Ar . [19,20] Computations indicated that the latter and its Fe and Co analogues are probably stabilized by attractive interligand dispersion forces.…”

“…1.821 Å). [17,18] Originally, Bürger and Wannagat reported that blood red 'Ni{N(SiMe3)2}2' (probably Ni{N(SiMe3)2}2(THF)) turned black after a short time at room temperature. [2] We also found that 1 and 2 became black within 30 minutes at ca.…”