The X-C…Y Carbon Bond

Abstract

The CH3X (X more electronegative group/atom than C) molecules have their C atom partially positive which enables it to bond with another atom/group of atoms having an electron rich region, Y. This leads to a penta-coordinate carbon having X-C…Y ‘carbon bond’ in addition to the four covalent bonds around the central carbon. The X-C…Y carbon bond is very similar to the X-H…Y hydrogen bond. The X-C bond lengthens and there is a red-shift in X-C stretching frequency. The ∠X-C…Y is linear. There is an overlap between X-C σ* orbital and the donor orbital in Y, which leads to the weakening X-C bond and red-shift. Analysis of the electron density topology shows a bond critical point between C and Y. It also shows that there is mutual penetration of electron clouds of the C and Y atoms, leading to bond formation. The X-C…Y carbon bond is an intermediate in SN2 reactions, very much like the X-H…Y hydrogen bond being an intermediate in proton transfer reactions. Moreover, the carbon bonded H2O…CH3OH geometry is a text-book example for the ill-defined hydrophobic interactions, as the H2O approaches methanol from the hydrophobic CH3 side. Similar interactions involving Si and Ge containing molecules exist and these have all been named ‘tetrel bonds’, extending halogen bonding (Group 17), chalcogen bonding (Group 16) and pnicogen bonding (Group 15) to Group 14. While such interactions involving elements from all groups are likely, we expect that the carbon bonding would be next only to hydrogen bonding in its importance, as C and H are the elements chosen by nature and life as we know is largely based on these elements. This chapter summarizes the results on carbon bonding since it was proposed about a year ago.