Metal–Metal Single Bonds with the Magnetic Anisotropy of Quadruple Bonds: A Systematic Series of Heterobimetallic Bismuth(II)–Rhodium(II) Formamidinate Complexes

The first set of five heterobimetallic MM′(form) 4 (form=formamidinate) complexes containing a BiRh core has been successfully synthesized. The Bi−Rh bond lengths lie between 2.5196(6) and 2.572(2) Å, consistent with Bi−Rh single bonds. All complexes have rich electrochemistry, with the [BiRh] 4+/5+ redox couples spanning approximately 700 mV and showing a strong correlation to remote ligand substitution. Visible spectroscopy showed two features for complexes 1 – 5 at approximately 459 and 551 nm, unique to BiRh paddlewheel complexes that are attributed to LMCT bands into the Bi−Rh σ* orbital. The large spin–orbit coupling (SOC) of Bi creates a massive Bi−Rh magnetic anisotropy, Δ χ , approximately −4800×10 −36  m 3 molecule −1 , which is the largest value reported for any single bond to date.