Homoleptic Organoderivatives of High-Valent Nickel(III)

Homoleptic perhalophenyl derivatives of divalent nickel complexes with the general formula [NBu4]2[NiII(C6X5)4] [X=F (1), Cl (2)] have been prepared by low‐temperature treatment of the halo‐complex precursor [NBu4]2[NiBr4] with the corresponding organolithium reagent LiC6X5. Compounds 1 and 2 are electrochemically related by reversible one‐electron exchange processes with the corresponding organometallate(III) compounds [NBu4][NiIII(C6X5)4] [X=F (3), Cl (4)]. The potentials of the [NiIII(C6X5)4]−/[NiII(C6X5)4]2− couples are +0.07 and −0.11 V for X=F or Cl, respectively. Compounds 3 and 4 have also been prepared and isolated in good yield by chemical oxidation of 1 or 2 with bromine or the amminium salt [N(C6H4Br‐4)3][SbCl6]. The [NiIII(C6X5)4]− species have SP‐4 structures in the salts 3 and 4, as established by single‐crystal X‐ray diffraction methods. The [NiII(C6F5)4]2− ion in the parent compound 1 has also been found to exhibit a rather similar SP‐4 structure. According to their SP‐4 geometry, the NiIII compounds (d7) behave as S=1/2 systems both at microscopic (EPR) and macroscopic levels (ac and dc magnetization measurements). The spin Hamiltonian parameters obtained from the analysis of the magnetic behavior of 3 and 4 within the framework of ligand field theory show that the unpaired electron is centered mainly on the metal atom, with >97 % estimated d z 2 contribution. Thermal decomposition of 3 and 4 proceeds with formation of the corresponding C6X5C6X5 coupling compounds. Oxidation of the organonickel(II) compounds [NBu4]2[NiIIR4] (R=C6F5, C6Cl5) takes place at the metal, as derived from the analysis of the magnetic properties of the oxidized species. The latter appear as the first homoleptic σ‐organonickel(III) derivatives. They have been found to exhibit SP‐4 geometry and to undergo thermally‐induced CC reductive coupling processes.