Factors affecting d-block metal-ligand bond lengths: toward an automated library of molecular geometry for metal complexes

Metal-ligand (M-L) bond lengths for a range of ligands (carboxylates, chlorides, pyridines, water, tertiary phosphines, and alkenes) and a variety of metals have been retrieved from the Cambridge Structural Database, CSD. Analysis of the factors which affect M-L bond lengths (for example, ligand coordination mode, oxidation state, metal coordination number and geometry, spin and Jahn-Teller effects, and ligand trans to M-L bond) shows that it is generally possible to subdivide the M-L data sets systematically to obtain better defined, unimodal, bond length distributions with means and sample standard deviations (SSDs) which reflect the nature of the bond in question. Typically, the SSDs for the M-L data sets can be reduced to 0.04-0.05 A by these methods. This work is an extension to tables of bond lengths in organometallic compounds and coordination complexes published in 1989. The importance of the factors which affect M-L bond lengths for particular metal-ligand groups are discussed. From the case studies reported, an algorithm is proposed by which compilation of a library of molecular geometry for metal complexes may be automated. The points that need to be considered to produce such a molecular library from the data stored in the CSD are discussed. The development of such a library would allow users to retrieve chemically well-defined geometric data rapidly and accurately. This should be of use, for example, to crystallographers and molecular modelers.