Interactions between CH and NH bonds and d8 square planar metal complexes: hydrogen bonded or agostic?

The structural characteristics of CH…M and NH…M groups in square planar d8-ML4 metal complexes are examined and compared w those for CH…M groups in unambiguously agostic d6-ML5 systems. The NH…M(d8) systems have the linear geometry typical o bonds, not the bent geometry typical of agostic species. As proposed by Brammer, the NH…M(d8) systems are best regarded as hydrogen bonds, but the CH…M(d8) systems are intermediate in geometry between the hydrogen bonded NH…M systems and the true agostic CH…M of the d6 cases and are neither typical of hydrogen bonds nor typical agostic species.