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.