We report the synthesis, X-ray structure, and solution behavior of a "nanosquare" formed by four repeating ( i Pr 2 BiI) units. The title complex [ i Pr 2 BiI] 4 (1) formed spontaneously as the major product from the reaction of i Pr 3 Bi with MI 2 (M = Mn, Co, Fe). Complex 1 was then more directly synthesized via bromination of i Pr 3 Bi to form ( i Pr 2 BiBr), followed by iodide substitution. The X-ray structure of 1 exhibited acute Bi−I−Bi bonds (∼85−95°) that enabled formation of the hollow square motif. Synthetic control experiments with R 2 BiX (R = i Pr, Bn; X = I, Br) revealed that the identities of both the halide and bismuth R group are determinants for square formation: the bromo-substituted [ i Pr 2 BiBr] n (2) and benzyl-substituted [Bn 2 BiI] n (3) variants formed only polymers as determined by X-ray crystallography. Density functional theory calculations revealed nearly pure porbital-based bonding molecular orbitals on both iodide and bismuth, facilitating the ∼90°/180°bonding motifs.

Herein, we test the hypothesis that neutral, heavy-atom stibine donors can increase the extent of spin-orbit coupling on light, 3d transition metal. To this end, we developed a novel synthetic route toward coordinating a paramagnetic 3d metal ioncobalt­(II)with neutral stibine ligands. Such complexes have not been reported in the literature due to the weak σ donor strength of stibines and the hard–soft mismatch between a 3d metal and a 5p ligandwhich herein has been overcome using alkylated Sb donors. Magnetometry of [(Sb i Pr2Ph)2Co­(I)2] (1) reveals that 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 an 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 always higher in the presence of a heavy pnictogen as compared with a heavy halide. Ab initio ligand field theory 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 the D value beyond geometric perturbation is operative, but its precise mechanism(s) of action remains obscure.