We test the hypothesis that neutral, heavy-atom stibine donors can impart spin-orbit coupling to a light, 3d transition metal. To this end, we developed a novel synthetic route towards coordinating a paramagnetic 3d metal ion — cobalt(II) — with neutral stibine ligands. Such complexes do not occur in literature due to the weak σ donor strength of stibines, and the hard-soft mismatch between a 3d metal and a 5p ligand — which has been overcome using alkylated Sb donors. Magnetometry of [(SbiPr2Ph)2Co(I)2] (1) reveals the stibine complex 1 exhibits a higher magnitude D value (D = |24.96| cm–1) than the spectroscopically derived value for the corresponding phosphine complex 3 (D = –13.13 cm–1), indicative of large zero-field splitting. CASSCF/NEVPT2 calculations corroborate the experimental D values for 1 and 3, predicting D = –31.9 and –8.9 cm–1, respectively. A re-examination of magnetic parameters across the entire series [(ER3)2Co(X)2] (E = P→Sb; X = Cl→I) reveals that (i) increasingly heavy pnictogens lead to increased X–Co–X bond angle, which is correlated with larger magnitude D values; and (ii) for a given X–Co–X bond angle, the D value is higher in presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory (AILFT) calculations for 1 (stibine complex) and 3 (phosphine complex) reveal no substantial differences in spin-orbit coupling (ζ = 479.2, 480.2 cm–1) or Racah parameter (B = 947.5, 943.9 cm–1), an indicator of covalency. Thus, some ‘heavy atom effect’ on D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.

Synthesis and magnetism of antimony- vs phospine-supported cobalt(II) halides

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