Question

The relative donor effect of simple ligands can be determined by measuring the carbonyl stretching frequencies of their metal carbonyl coordination complexes. Imagine you have prepared a series of L2-Ir(CO)Cl complexes and measured the carbonyl stretches. Assign the stretches (2102, 2054, 2032, 2014, 1995 cm -1 ) to the complexes containing the following ligands (assume that all ligands are purely σ-donors): NPh3, PPh3, pyridine, 4-fluoropyridine, 4-bromopyridine. Explain your reasoning!

Answer 1

Ligands that are trans to a carbonyl can have a large effect on the ability of the CO ligand to effectively π-backbond to the metal. The better the carbonyl π-backbond to the metal the lower will be the carbonyl stretching frequency as the system moves from having a M-CO to a M=C=O. So the higher the strength of the trans ligand binding to the metal the lower will be the strength of CO binding to the metal and higher will be the CO stretching frequency.

Since in this case we are only considering the ligands to be purely σ-donors. Then the better the σ-donor capability of a ligand to the metal the stronger will be its bonding to the metal and by trans effect weaker will be its bonding to the trans CO. The higher will be the CO stretching frequency. Now amoung the ligands given the NPh₃ is the most basic, PPh₃ is next basuc followed by pyridine, 4-bromopyridine and finally 4-fluoropyridine. The more basic the ligand the more electron density it can donate to the central metal. The stronger its σ-donation capacity.

So 2102 will be assigned to the complex with NPh3.

2054 will be assigned to PPh3

2032 will be asigned to pyridine

2014 will be assigned to 4-bromopyridine

1995 will be assigned to 4-fluoropyridine