Bismuthinidenes are a class of organobismuth compounds, analogous to carbenes. These compounds have the general form R-Bi, with two lone pairs of electrons on the central bismuth(I) atom. Due to the unusually low valency and oxidation state of +1, most bismuthinidenes are reactive and unstable, though in recent decades, both transition metals and polydentate chelating Lewis base ligands have been employed to stabilize the low-valent bismuth(I) center through steric protection and π donation either in solution or in crystal structures. Lewis base-stabilized bismuthinidenes adopt a singlet ground state with an inert lone pair of electrons in the 6s orbital. A second lone pair in a 6p orbital and a single empty 6p orbital make Lewis base-stabilized bismuthinidenes ambiphilic. Recently, a triplet bismuthinidene is reported by Cornella et al.

The comparatively large covalent radius of bismuth results in weaker bonds between bismuth and other elements.

The earliest examples of bismuthinidene complexes used transition metal chemistry to stabilize the Bi(I) center. These methods generally use simple bismuth(I) halides or methylbismuth to ligate to tungsten, manganese, and chromium carbonyl complexes. These complexes were occasionally found to oligomerize, forming Bi-Bi single or double bonds to form bismuthane or bismuthene moieties. One of the first examples of a monomeric bismuthinidene was discovered by Balasz et al., who used R = 2-(dimethylaminomethyl)phenyl to chelate a Bi(I) center through a combination of strong C-Bi and weak N-Bi interactions. Although the molecule quickly formed a cyclic oligomer, upon reaction with two equivalents of tungsten pentacarbonyl, monomeric crystalline RBi[W(CO)₅]₂ was isolated.

Reduction of ArBi^IIICl₂ (R = 2,6-bis[N-(2’,6’-dimethylphenyl)ketimino]phenyl) gives the Bi(I) derivative. This complex is referred to as "Dostál's bismuthinidene". Many analogs have been prepared by varying the substituents on the aryl or the two imine arms.

Monomeric bismuthinidene can also be prepared with a bidentate N,C-ligand. Reduction of the bismuth dichloride [C₆H₂-2-(CH=N-2’,6’-iPr₂C₆H₃)-4,6-(tBu)₂]BiCl₂ by two equivalents of K[B(iBu)₃H] gives the dark violet bismuthinidene. In contrast to the earlier transition metal-stabilized [2-(Me₂NCH2)C₆H₄]Bi[W(CO)₅]₂, the tert-butyl group ortho to the bismuth atom in this N,C-coordinated bismuthinidene sterically block the partially empty p-type orbital on the bismuth atom, kinetically stabilizing it without the use of transition metals. In addition, calculated nucleus-independent chemical shift indices (NICS) and anisotropy of current-induced density (ACID) analysis show that the BiC₃N ring of the molecule was stabilized by a certain degree of aromatic character due and may be classified as a benzazabismole to the delocalization of six π electrons, despite the nominally dative Bi-N bond. Unlike N,C,N-coordinated bismuthinidenes, this N,C-coordinated species requires the pendant nitrogen atom to be in an imine group, as replacement of the Dipp-substituted imine arm with a diethyl-substituted amine arm resulted in rapid dimerization to a dibismuthene species.

Cyclic alkyl amino carbenes have also been used to generate bismuthinidenes.

Bismuthinidene is a carbene analogs. The structural and electronic properties of bismuthinidenes are in large part driven by the inert-pair effect, i.e. the large energy gap between the bismuth atom's 6s and 6p orbitals disfavors the formation of sp hybrid orbitals. In contrast, for the lighter congeners phosphinidenes, their smaller phosphorus 3s-3p energy gap favors a triplet ground state,

Dostál's N,C,N-stabilized bismuthinidene is the most commonly bismuthinidene in the literature. Optimization of the tert-butyl imino version of this compound at the M06/cc-pVTZ level of theory reveals that, as in other nontrigonal pnictogen compounds, the central bismuth atom is coplanar with the N,C,N-coordinating ligand, adopting a T-shaped C_2v coordination mode. The Wiberg bond index (WBI) between the bismuth and carbon atoms is 1.09, while the Bi-C bond distance is 2.2156 Å, slightly shorter than the sum of these atoms’ single-bonded covalent radii (Σ_cov(Bi,C) = 2.26 Å). On the other hand, the WBI of the Bi-N bonds is only 0.34, and the Bi-N bond distance is 2.500 Å, significantly longer than the sum of these atoms’ covalent radii (Σ_cov(Bi,N) = 2.22 Å). This agrees with calculations based on the quantum theory of atoms in molecules (QTAIM), which show that the electron density at the bond critical point between Bi and N is only 0.049, significantly lower than the electron density of 0.114 at the Bi-C bond critical point. Natural bond orbital (NBO) calculations show that these weaker dative bonds arise from weak σ donation of the nitrogen atoms’ lone pairs into an empty 6p orbital on the central bismuth atom. These two n_N → p*_Bi interactions stabilize the bismuthinidene by as much as 382 kJ/mol. Additionally, the amount of sigma donation from the pendant nitrogen atoms may be increased or decreased by replacing the tert-butyl groups on the pendant nitrogen atoms with aryl groups containing electron-donating groups or electron-withdrawing groups, respectively. One lone pair resides in the bismuth atom's 6s orbital and generally remains inert, while the other resides in the 6p orbital oriented perpendicular to the plane of the central ring, which also comprises the highest occupied molecular orbital (HOMO).