Relative Strengths of a Pnicogen and a Tetrel Bond and Their Mutual Effects upon One Another

The ability of the T and Z atoms of TR₃ZR₂ to engage in a noncovalent interaction with NH₃ is assessed by DFT calculations, where the T atom refers to C, Si, and Ge; Z = As, Sb, and P; and substituents R = H and F. In most instances, the tetrel bond (TB) is both stronger and shorter than the pnicogen bond (ZB). These two bond strengths can be equalized, or preference shifted to the ZB, if F substituents are placed on the Z and H on the T atoms. Employing C as the T atom results in a very weak TB, with the ZB clearly favored energetically. The simultaneous formation of both TB and ZB weakens both, particularly the latter, but both bonds survive intact. Geometric and spectroscopic perturbations of the subunits reflect the two types of noncovalent bonds.

Origins and properties of the tetrel bond

The tetrel bond (TB) recruits an element drawn from the C, Si, Ge, Sn, Pb family as electron acceptor in an interaction with a partner Lewis base. The underlying principles that explain this attractive interaction are described in terms of occupied and vacant orbitals, total electron density, and electrostatic potential. These principles facilitate a delineation of the factors that feed into a strong TB. The geometric deformation that occurs within the tetrel-bearing Lewis acid monomer is a particularly important issue, with both primary and secondary effects. As a first-row atom of low polarizability, C is a reluctant participant in TBs, but its preponderance in organic and biochemistry make it extremely important that its potential in this regard be thoroughly understood. The IR and NMR manifestations of tetrel bonding are explored as spectroscopy offers a bridge to experimental examination of this phenomenon. In addition to the most common σ-hole type TBs, discussion is provided of π-hole interactions which are a result of a common alternate covalent bonding pattern of tetrel atoms.

Nature and Strength of M-H···S and M-H···Se (M = Mn, Fe, & Co) Hydrogen Bond

The significance of dispersion contribution in the formation of strong hydrogen bonds (H-bonds) can no more be ignored. It was illustrated that less electronegative and electropositive H-bond acceptors such as S, Se, and Te are also capable of forming strong N-H···Y H-bonds, mostly due to the high polarizabilities of H-bond acceptor atoms. Herein, for the first time, we report the evidence of formation of nonconventional M-H···Y H-bonds between metal hydrides (M-H, M = Mn, Fe, Co) and chalcogen H-bond acceptors (Y = O, S, or Se). The nature and the strength of unusual M-H···Y H-bonds were revealed by several quantum chemical calculations and H-bond descriptors. The structural parameters, electron density topology, donor-acceptor natural bond orbital (NBO) interaction energies, and spectroscopic observables such as M-H stretching frequencies and ¹H chemical shifts are well-correlated to manifest the existence and strength of M-H···Y H-bonding. The M-H···Y H-bonds are dispersive in nature, and the computed H-bond energies are found to be in the range from ∼5 to 30 kJ/mol, which can be compared to those of the conventional H-bonds such as O-H···O, N-H···O, and N-H···O═C H-bonds, etc.