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:
Agostic interaction
Agostic interactions are best demonstrated by crystallography. Neutron diffraction data has shown that C−H and M┄H bond distances are 5-20% longer than expected for isolated metal hydride and hydrocarbons. The distance between the metal and the hydrogen is typically 1.8–2.3 Å, and the M┄H−C angle falls in the range 90°–140°. The presence of a H NMR signal that is shifted upfield from that of a normal aryl or alkane, often to the region normally assigned to hydride ligands. The coupling constant "J" is typically lowered to 70–100 Hz versus the 125 Hz expected for a normal sp carbon–hydrogen bond.
On the basis of experimental and computational studies, the stabilization arising from an agostic interaction is estimated to be 10–15 kcal/mol. Recent calculations using compliance constants point to a weaker stabilisation (<10 kcal/mol). Thus, agostic interactions are stronger than most hydrogen bonds. Agostic bonds sometimes play a role in catalysis by increasing 'rigidity' in transition states. For instance, in Ziegler–Natta catalysis the highly electrophilic metal center has agostic interactions with the growing polymer chain. This increased rigidity influences the stereoselectivity of the polymerization process.