The Weak Helps the Strong: Sigma-Holes and the Stability of MF₄·Base Complexes

Abstract

Bonding interactions between an electron-deficient region (a sigma-hole) on M and electron donors in MF4-Base complexes, where M = C, Si, Ge, Sn, and Pb, are examined and rationalized. These interactions are seen to transition from weak primarily noncovalent interactions for all bases when M = C to stronger primarily covalent bonds in adducts as the valence shell expands for the heavier M atoms. For M = Ge, Sn, and Pb, the complexes are particularly stable. The consistent axial preference in these systems is anticipated by previous studies and is readily explained from the vantage point of sigma-hole interactions. A series of bound complexes of common bases such as pyridine, tetrahydrofuran, and water are identified, some of which are even more stable than the SiF4·NH3 and SiF4·N(CH3)3 complexes that have already been identified experimentally. Sigma-hole bonding to di- and poly-substituted central atoms, perhaps on par with halogen bonding, is expected to become increasingly important as an ordering interaction in materials science and engineering. Group 14 compounds have distinct advantages in this respect.

Hexacoordinated Tetrel‐Bonded Complexes between TF4 (T=Si, Ge, Sn, Pb) and NCH: Competition between σ‐ and π‐Holes

“…[25,26,38,98,99] Previous studies have confirmed the trend that the tetrel bond grows stronger as T becomes larger, [30,32,33,47,100] but that the effect levels off between Sn and Pb. [45,101] An earlier work [45,102] supports the result noted here that the deformation energy induced within the Lewis acid molecule by formation of a tetrel bonded complex drops as the central tetrel atom grows in size. The ability of the heavier tetrel atoms to participate in two simultaneous noncovalent bonds is confirmed by earlier calculations [103] performed by Grabowski at the MP2 level, who also noted the possibility of both trans and cis structures, at least for Sn, for which the latter is more stable than the former by 3 kcal/mol, in nice agreement with our own findings.…”

Halogen Bonding: A Halogen-Centered Noncovalent Interaction Yet to Be Understood

“…Based solely on the arguments of the results of the MESP model, one might suggest [109,161,201,267] that the potentials of F in CF 4 are indicative of fluorine interacting only with electrophiles. From a later study reported elsewhere [158,268], it was found that that there are four positive σ-holes on the C atom directed along the outer extensions of the four F-C bonds, one would conclude that F• • • C(-F) in the crystal are genuine attractive contracts even though the r(F• • • C) are significantly longer-3.342 Å-than the sum of vdW radii of the C and F atoms (1.77 and 1.46 Å, respectively). While doing so, the importance of F• • • F interactions between the fluorine atoms of the interacting CF 4 molecules has probably to be neglected, as was done in the experimental studies [263][264][265][266].…”

Ability of IR and NMR Spectral Data to Distinguish between a Tetrel Bond and a Hydrogen Bond

“…And indeed it is to these heavier atoms that most previous work has been devoted [18][19][20][21][22][23][24][25][26] . In the case of the C atom, its ability to engage in a TB has typically been amplified by adding a number of electron-withdrawing substituents like F [27][28][29] rather than considering a CH3 group itself. Our own lab has contributed to the current understanding of tetrel bonding as well [30][31][32][33][34][35] including the issue of steric crowding 26,36 and the manner in which such interactions might be used to design an effective selective anion receptor [37][38][39][40] .…”