Triplet bismuthinidenes featuring unprecedented giant and positive zero field splittings

Isolation of triplet pnictinidenes, which bear two unpaired electrons at the pnictogen centers, has long been a great challenge due to their intrinsic high reactivity. Herein, we report the syntheses and characterizations of two bismuthinidenes M^sFluind^Bu-Bi (3) and M^sFluind*-Bi (4) stabilized by sterically encumbered hydrindacene ligands. They were facilely prepared through reductions of the corresponding dichloride precursors with 2 molar equivalents of potassium graphite. The structural analyses revealed that 3 and 4 contain a one-coordinate bismuth atom supported by a Bi-C single σ bond. As a consequence, the remaining two Bi 6p orbitals are nearly degenerate, and 3 and 4 possess triplet ground states. Experimental characterizations with multinuclear magnetic resonance, magnetometry and near infrared spectroscopy coupled to wavefunction based ab initio calculations concurred to evidence that there exist giant and positive zero field splittings (>4300 cm^-1) in their S = 1 ground states. Hence even at room temperature the systems almost exclusively populate the lowest-energy nonmagnetic Ms = 0 level, which renders them seemingly diamagnetic.