Taming Neutral Silylidenebismuthanes (Si═Bi) with a Low-Coordinate Bismuth Atom

Unsaturated main-group compounds containing heteroleptic double bonds of bismuth remain exceedingly rare due to inherent p_π-p_π bond weakness. Here we report the synthesis and characterization of the first isolable silylidenebismuthane complexes, L(Me₃Si)Si═Bi(SiMe₃) and L(Me₃Si)Si═Bi(SiⁱPr₃) ((E)-1a,b) (L = PhC(N^tBu)₂), containing a neutral but strongly polarized silicon-bismuth double bond. The complexes were obtained as the E isomers in 55% and 60% yield through the salt metathesis reaction of LSi^IICl with the corresponding potassium bis(silyl)bismuthanide complexes. Remarkably, LSi^II-Bi(SiMe₃)₂ and LSi^II-Bi(SiMe₃)(SiⁱPr₃) could not be observed but undergo a trimethylsilyl group migration from the Bi^III atom to the Si^II atom to form a Si═Bi moiety containing σ³λ⁴-coordinate Si^II and σ²λ²-coordinate Bi^I atoms. The reactions of (E)-1a,b with [W(CO)₅(thf)] cause the (E) → (Z) isomerization with respect to the silyl groups attached to furnish the corresponding terminal Bi→W(CO)₅ complexes (Z)-2a,b, in which the Si═Bi π bonding interaction is significantly weakened. This and the nature of the Si═Bi bonds are corroborated by DFT calculations.