Question

The complexes shown below are catalysts for the water splitting process. It has been found that the carbonyl stretching frequency is a useful parameter to gauge the effect of ligand environment on the key step in the reaction. Describe the bonding characteristics of the N2P-ligand and the structures of the complexes, and predict how the electronic property (donating or withdrawing) of the substituent X is reflected by the carbonyl stretching frequency (Suresh et al. Inorg. Chem. 2015, 54, 11150). Et2NRuP(tBu)2 H CO

Answer 1

Covalent strength of the bond can be studied using IR spectroscopy. In the molecular orbital diagram of CO, we can find HOMO or highest occupied molecular orbital is a bonding molecular orbital and LUMO or lowest unoccupied molecular orbital is an antibonding molecular orbital. So the addition of electron density to the molecular orbital of CO makes the CO bond order less. Thus stretching frequency of CO decreases.

Usually, ligands of type PR₃ are π- acceptor type. So metal to CO back-donation will be less and thus CO stretching frequency increases because of more CO bond order. It is because, as PR₃ pulls electrons from metal and metal become electron deficient to donate to the molecular orbital of CO. Likewise NR₃ ligands also. So as X is electron donating, then CO stretching frequency decreases because ligand N₂P accepts less electron density from metal. but X is electron withdrawing then CO stretching frequency increases because N₂P ligand accepts more electron density from metal. Electron donating X group makes ligand environment more electron rich and thus less π acceptance fro metal orbital. This makes metal to CO back donation high and thus decrease in CO stretching frequency. Likewise, if X is electron withdrawing then the ligand environment become electron deficient and thus accept more electron density from metal. This makes the metal more electron deficient and less viable to back donate electron density to CO orbital. So CO stretching frequency increases.