Database Analysis of Transition Metal Carbonyl Bond Lengths:  Insight into the Periodicity of π Back-Bonding, σ Donation, and the Factors Affecting the Electronic Structure of the TM−C⋮O Moiety

Analysis of the relationships between TM−C and C⋮O distances of the more than 20 000 structures reported to the CSD has revealed new experimentally derived insights into the bonding in these systems. The databases of structures were investigated by a combination of DFT and statistical methods. The different abilities of transition metals to both accept and donate electrons are reflected in differences in their data sets. There are significant changes in gradient of the curves representing TM−C versus C⋮O scatter plots. One such change in gradient occurs at the bond length corresponding to C⋮O in its unbound state. There are also significant differences between the second and third transition series. The analysis provides a structural means of probing the distribution of electrons in a metal−carbonyl fragment and provides important insights into the periodicity of back-bonding, σ donation, and the ability of the carbonyl to stabilize different metal oxidation states.