Paramagnetic 18-Valence-Electron Alkylcyclopentadienylnickel(II) Bromide Dimers

The paramagnetic behavior of 18-electron cyclopentadienylnickel­(II) complexes of the [CpNi­(μ-Br)]₂ type with two unpaired electrons per metal ion has been investigated by ¹H and ¹³C NMR spectroscopy of [Cp‴Ni­(μ-Br)]₂ (<b>1</b>; Cp‴ = 1,3,4-<i>t</i>Bu₃C₅H₂), [³CpNi­(μ-Br)]₂ (<b>2</b>; ³Cp = 1,3,4-<i>i</i>Pr₃C₅H₂), and [⁴CpNi­(μ-Br)]₂ (<b>3</b>; ⁴Cp = 2,3,4,5-<i>i</i>Pr₄C₅H). The tri-<i>tert</i>-butylcyclopentadienyl derivative <b>1</b> crystallizes in the triclinic space group <i>P</i>1̅ and has been investigated by X-ray crystallography. Solid-state magnetic susceptibility measurements of <b>3</b> revealed an effective magnetic moment at room temperature of 4.04 μ_B, confirming the presence of two d⁸ nickel­(II) ions. While antiferromagnetic coupling via the bromo bridges is weak (<i>J</i> = −2.4 cm^–1), zero-field splitting is substantial (<i>D</i> = +48.2 cm^–1). NMR spectra of complexes <b>1</b>–<b>3</b> show signals with half-widths up to 3600 Hz within a spectral window exceeding 500 ppm (¹H) or 2200 ppm (¹³C). An analysis of the spectra gave insight into the spin delocalization, the equilibrium orientation of the <i>i</i>Pr substituents, and the presence of different conformers of compound <b>3.</b> Paramagnetic behavior has also been observed for the Cp* derivative [Cp*Ni­(μ-Br)]₂ (Cp* = C₅Me₅) by ¹H NMR spectroscopy. The presence of two unpaired electrons is discussed in terms of the weak ligand field originating from the combined interaction of poorly π accepting alkylcyclopentadienyl and π donating bromo ligands with the nickel­(II) center