Interactions between C-H and N-H bonds and d(8) square planar metal complexes: Hydrogen bonded or agostic?

The structural characteristics of C-H...M and N-H...M groups in square planar d(8)-ML(4) metal complexes are examined and compared with those for C-H...M groups in unambiguously agostic d(6)-ML(5) systems. The N-H...M(d(8)) systems have the linear geometry typical of hydrogen bonds, not the bent geometry typical of agostic species. As proposed by Brammer, the N-H...M(d(8)) systems are best regarded as hydrogen bonds, but the C-H...M(d(8)) systems are intermediate in geometry between the hydrogen bonded N-H...M systems and the true agostic C-H...M of the d(6) cases and are neither typical of hydrogen bonds nor typical agostic species.