The Pnicogen Bond: Its Relation to Hydrogen, Halogen, and Other Noncovalent Bonds

Abstract

Among a wide range of noncovalent interactions, hydrogen (H) bonds are well known for their specific roles in various chemical and biological phenomena. When describing conventional hydrogen bonding, researchers use the notation AH···D (where A refers to the electron acceptor and D to the donor). However, the AH molecule engaged in a AH···D H-bond can also be pivoted around by roughly 180°, resulting in a HA···D arrangement. Even without the H atom in a bridging position, this arrangement can be attractive, as explained in this Account. The electron density donated by D transfers into a AH σ* antibonding orbital in either case: the lobe of the σ* orbital near the H atom in the H-bonding AH···D geometry, or the lobe proximate to the A atom in the HA···D case. A favorable electrostatic interaction energy between the two molecules supplements this charge transfer. When A belongs to the pnictide family of elements, which include phosphorus, arsenic, antimony, and bismuth, this type of interaction is called a pnicogen bond. This bonding interaction is somewhat analogous to the chalcogen and halogen bonds that arise when A is an element in group 16 or 17, respectively, of the periodic table. Electronegative substitutions, such as a F for a H atom opposite the electron donor atom, strengthen the pnicogen bond. For example, the binding energy in FH(2)P···NH(3) greatly exceeds that of the paradigmatic H-bonding water dimer. Surprisingly, di- or tri-halogenation does not produce any additional stabilization, in marked contrast to H-bonds. Chalcogen and halogen bonds show similar strength to the pnicogen bond for a given electron-withdrawing substituent. This insensitivity to the electron-acceptor atom distinguishes these interactions from H-bonds, in which energy depends strongly upon the identity of the proton-donor atom. As with H-bonds, pnicogen bonds can extract electron density from the lone pairs of atoms on the partner molecule, such as N, O, and S. The π systems of carbon chains can donate electron density in pnicogen bonds. Indeed, the strength of A···π pnicogen bonds exceeds that of H-bonds even when using strong proton donors such as water with the same π system. H-bonds typically have a high propensity for a linear AH···D arrangement, but pnicogen bonds show an even greater degree of anisotropy. Distortions of pnicogen bonds away from their preferred geometry cause a more rapid loss of stability than in H-bonds. Although often observed in dimers in the gas phase, pnicogen bonds also serve as the glue in larger aggregates, and researchers have found them in a number of diffraction studies of crystals.

Close contacts involving germanium and tin in crystal structures: experimental evidence of tetrel bonds

“…Chalcogen bonds (CBs) have been studied in silico [25,26] and in the solid [27], liquid [28], and gas [29] phases. Pnictogen bonds (PBs) have received much attention in studies performed in silico [30] and in the solid [31]; and the aerogen bond (AB) is the most recently discovered subset of σ-hole interactions [32].…”

Intra-/Intermolecular Bifurcated Chalcogen Bonding in Crystal Structure of Thiazole/Thiadiazole Derived Binuclear (Diaminocarbene)PdII Complexes

“…The hydrogen [1,2], halogen [3][4][5][6][7][8][9][10], chalcogen [11][12][13][14][15][16][17][18][19][20][21][22][23][24], pnictogen [25][26][27][28][29][30][31], tetrel [32] bonding, stacking [33][34][35][36][37], cation/anion-π [38][39][40][41][42], and metallophilic interactions [43][44][45] play key roles in many chemical, physical, and biochemical processes, due to their ability to control structures and properties of associates and supramolecular systems.…”

Ab initio and DFT studies of the interaction between carbonyl and thiocarbonyl groups: the role of S···O chalcogen bonds

“…Depending on the nature of the bridging atoms [1][2][3], these interactions are commonly designed as halogen [4][5][6][7][8][9], chalcogen [10][11][12][13][14][15][16] or pnicogen [17][18][19][20] bonds. The attractive force has been attributed to an anisotropic distribution of electron density around the bridging X atom, characterized by a crown of positive electrostatic potential along the extension of the Y-X bond (σ-hole) or in areas perpendicular to it (π-hole) [6-8, 21, 22].…”