SUMMARY OF KEY CONCEPTS

•	Searching the CSD for molybdenum carbonyl complexes and monitoring the MoC and CO bonds illustrates there is an effect whereby as the MC bond distance lengthens, the corresponding CO bond distance shortens (and vice versa).

•	The complementarity of this effect can be seen when the MoC distance is plotted against the CO distance in a statistical analysis package such as Vista: a linear relationship between the two distances is observed.

•	This phenomenon is called pi back-bonding and can be explained by electron donation between bonding and anti-bonding orbitals.

•	Pi back-bonding is found not only in MoCO complexes, but also other transition metal CO complexes.

•	The strength of pi back bonding in a metal carbonyl complex can be monitored using IR spectroscopy.

•	For optimal pi back bonding to occur, the metal must be in a low oxidation state.