The carbene triel bond is predicted and characterized by theoretical calculations. The C lone pair of N‐heterocyclic carbenes (NHCs) is allowed to interact with the central triel atom of TrR3 (Tr = B and Al; R = H, F, Cl, and Br). The ensuing bond is very strong, with an interaction energy of nearly 90 kcal/mol. Replacement of the C lone pair by that of either N or Si weakens the binding. The bond is strengthened by electron‐withdrawing substituents on the triel atom, and the reverse occurs with substitution on the NHC. However, these effects do not strictly follow the typical pattern of F > Cl > Br. The TrR3 molecule suffers a good deal of geometric deformation, requiring on the order of 30 kcal/mol, in forming the complex. The R(C···Tr) bond is quite short, for example, 1.6 Å for Tr = B, and shows other indications of at least a partially covalent bond, such as a high electron density at the bond critical point and a good deal of intermolecular charge transfer.

The ability of the F atom of HC≡CF, H2C=CHF and H3CCH2F to serve as an electron donor to the triel (Tr) atom of TrR3 in the context of a triel bond is assessed by ab initio calculations. The triel bond formed by Csp3—F is strongest, as high as 30 kcal/mol, followed by Csp2—F, and then by Csp—F whose triel bonds can be as small as 1 kcal/mol. The noncovalent bond strength diminishes in the order Tr = Al > Ga > B, consistent with the intensity of the π-hole above the Tr atom in the monomer. The triel bond strength of the Al and Ga complexes increases along with the electronegativity of the R substituent but is largest for R=H when Tr=B. Electrostatics play the largest role in the stronger triel bonds, but dispersion makes an outsized contribution for the weakest such bonds.

The complexes between R 3 Tr (Tr = B, Al, and Ga; R = H, F, Cl, and Br) and H 2 X (X = O, S, and Se) were theoretically studied. The interaction energies of R 3 AlÁ Á ÁH 2 X and R 3 GaÁ Á ÁH 2 X are consistent with the electronegativity of the halogen atom R (R 6 ¼ H), but an opposite dependence is found for R 3 BÁ Á ÁH 2 X. The triel bond of R 3 TrÁ Á ÁH 2 X is weaker for the heavier chalcogen donor. The dependence of triel bonding strength on the triel atom is complicated, depending on the nature of R and X. The methyl substitution of H 2 X causes a substantial increase in the interaction energy from −5.74 kcal/mol to −22.88 kcal/mol, and its effect is relevant to the nature of Tr, X, and R groups. For the S and Se donors, the increased percentage of interaction energy is almost the same due to the methyl substitution, which is larger than that of the O analogue. In most triel-bonded complexes, electrostatic dominates and polarization has comparable contribution. However, polarization plays a dominant role in R 3 BÁ Á ÁR 0 2 S and R 3 BÁÁÁR 0 2 Se (R = Cl and Br; R 0 = H and Me).Triel bond is a directionally attractive interaction between a Group IIIA atom (triel) and a Lewis base. [1] Although the name of triel bond was proposed until 2014, the electron-deficient property of the triel atom has been recognized for a long time. [2][3][4] Generally, the triel-containing molecules are trivalent triel compounds such as trihydrides and trihalides. A π-hole is defined as a region of the positive molecular electrostatic potential (MEP) which is perpendicular to the center of a planar molecule or the planar portion of a molecular framework. Triel bonding belongs to a π-hole interaction since the electron-deficient region in the triel bond is related to the outer vacant p orbital, which is perpendicular to the plane of the molecule. The π-hole on the triel atom was firstly found in the analysis of its MEP map by Murray et al. [5] The presence of π-hole is responsible for the directionality of the triel bonding. Many experimental and theoretical studies have focused on triel bond due to its significant roles in numerous chemical reactions. [6][7][8][9][10] In general, triel bond is very strong, even being close to a partially covalent bond. Leopold et al. [11] performed a systematical investigation of the triel-bonded complexes and named them as "partially bonded complexes" because their equilibrium distances are intermediate between vander Waals interactions and chemical bonds. Interestingly, remarkable changes occur when the triel-bonded complexes are in different phases. For example, when F 3 BÁ Á ÁCH 3 CN is in the gaseous state, the length of B N bond is 2.01 Å, but it shortens to be 1.63 Å in the condensed state. [12] This dramatic shortening was ascribed to the cooperativity. Lots of attention was then paid to the cooperativity between triel bonding and other types of interactions including hydrogen bonding, [13] halogen bonding, [14] chalcogen bonding, [15] pnicogen bonding, [16] tetrel bonding, [17] and regium b...

Complexes were formed by pairing ZCl 3 (Z = P, As, or Sb) with C 2 R 4 (R = H, F, or CN). The first interaction present is a pnicogen bond between the Z atom and the CC π-bond. This bond weakens as the H atoms of ethylene are replaced by electron-withdrawing F and CN, and the potential above the alkene switches from negative to positive. In the latter two cases, another set of noncovalent bonds is formed between the Cl lone pairs of ZCl 3 and the π*(CC) antibonding orbital as well as with the F or CN substituents. The growing strength of these interactions, coupled with a large dispersion energy, more than compensates for the weak pnicogen bond in C 2 (CN) 4 , with its repulsion being between areas of positive charge on each subunit, making its complexes with ZCl 3 very strong, as high as 25 kJ/mol. The pnicogen bond in C 2 F 4 is weaker than in C 2 H 4 , and its subsidiary lone pair−π bonds are weaker than in C 2 (CN) 4 , thus the complexes of this alkene with ZCl 3 are the weakest of the set.