Hydrogen bond
Hydrogen bonds can vary in strength from weak (1–2 kJ mol) to strong (161.5 kJ mol in the ion ). Typical enthalpies in vapor include:
N-heterocyclic silylene
Atoms in Molecules bond energy at critical point calculations show that the N to Si bonds in an NHSi are significantly ionic, unlike the highly covalent N-C bond in an NHC. Furthermore, the charge density and the Laplacian (∇) at the bond critical points between the silylene and carbene are dissimilar. Silylenes have much lower charge density between Si and N, with a positive Laplacian from Si to N indicating a more ionic interaction, while the Laplacian is negative going from carbon to nitrogen in an NHC indicating a more covalent bond. Upon coordination to a Group 11 metal the covalent character in the Si or C to metal bond is small, but not insignificant, again with a higher covalent character in the carbenic system. Similarly, silicon has a higher Laplacian between it and the group 11 metals than does carbon, indicating a less covalent metal interaction in silyenes as compared to carbenes.