Probing the Donor Properties of Pincer Ligands Using Rhodium Carbonyl Fragments: An Experimental and Computational Case Study

Metal carbonyls are commonly employed probes for quantifying the donor properties of monodentate ligands. With a view to extending this methodology to mer‐tridentate “pincer” ligands, the spectroscopic properties [ν(CO), δ (13C), (1) J (RhC)] of rhodium(I) and rhodium(III) carbonyl complexes of the form [Rh(pincer)(CO)][BAr(F) (4)] and [Rh(pincer)Cl(2)(CO)][BAr(F) (4)] have been critically analysed for four pyridyl‐based pincer ligands, with two flanking oxazoline (NNN), phosphine (PNP), or N‐heterocyclic carbene (CNC) donors. Our investigations indicate that the carbonyl bands of the rhodium(I) complexes are the most diagnostic, with frequencies discernibly decreasing in the order NNN > PNP > CNC. To gain deeper insight, a DFT‐based energy decomposition analysis was performed and identified important bonding differences associated with the conformation of the pincer backbone, which clouds straightforward interpretation of the experimental IR data. A correlation between the difference in carbonyl stretching frequencies Δν(CO) and calculated thermodynamics of the Rh(I)/Rh(III) redox pairs was identified and could prove to be a useful mechanistic tool.