**Preface to "Tetrel Bonds"**

After many decades of study of the H-bond which probed every facet of its origin and behavior, researchers realized there are a group of very similar interactions that had largely defied recognition. It was realized that the replacement of the bridging proton of the H-bond by a halogen atom led to a stabilizing interaction of strength comparable to the H-bond itself. The stability of this halogen bond was based in part on the anisotropic distribution of electron density around the halogen atom that allowed a positive region on an otherwise negatively charged atom. It was not long after that point that research turned to close parallels where the halogen atom was itself replaced by members of the chalcogen and pnicogen families, whose bonding properties were likewise parallel to those of halogen bonds. A recent addition to this pantheon is the tetrel bond, a family that encompasses Si, Ge, Sn, and the ubiquitous and immensely important C atom.

It is the tetrel bond that serves as the focus of this volume. Papers collected from a recent Special Issue of Molecules cover this topic from a range of perspectives. The first two papers concentrate on some of the theoretical aspects that contribute to the bond, providing a lens by which to understand it and how it might be used to predict certain chemical properties. Coverage next turns toward some of the structural and spectroscopic aspects of these bonds, from an understanding based on vibrational modes, to effects of geometrical distortions, and to specific lessons learned from analysis of crystal structures.

The C atom is of special importance in all of chemistry and biochemistry, and the next three papers focus attention on the tetrel bonds specifically centered on C. One common element of these papers is to alert the community to not only the widespread presence of tetrel bonds but also the importance of these bonds to methyl groups. Along with the acceptance that the presence of a negative charge on the electron donor will strengthen a H-bond, the next two contributions illustrate that the same phenomenon is true of tetrel bonds and describe how this bond enhancement can be applied to advantage. The last five papers offer a broad perspective, describing how tetrel bonds fit into the general picture of noncovalent bonds in general. This section also includes a visualization as to the roles played by π-systems, not only in the tetrel bond donor but also in the partner nucleophile.

> **Steve Scheiner** *Editor*
