Design of a structural database for homoleptic transition metal complexes

The interest and possible contents of a structural database for homoleptic metal complexes (HCD) is analyzed with the help of a set of some 1400 crystal and gas-phase structural data sets. Some examples of stereochemical and bonding trends that can be obtained from such a data collection are given. Attention is paid also to pseudohomoleptic complexes. These are defined as compounds which are formally homoleptic, in the sense that all the ligands around a metal are chemically equivalent, but in one case two very different bonding situations are found, notably due to Jahn–Teller distortions. Another case of pseudohomoleptic complexes correspond to those molecules that are described as homoleptic but that show short intra- or intermolecular contacts to extra donor groups, making it effectively heteroleptic.