Exploiting coordination geometry to selectively predict the r-donor and p-acceptor abilities of ligands: A back-and-forth journey between electronic properties and spectroscopy

Through an analysis of eighty tetrahedral and square-planar metal carbonyls of general formula [M(CO)(L0)(L)2] including newly synthesized chlorocarbonyl rhodium complexes with chelating atropoisomeric diphosphanes, we show how coordination geometry can switch the carbonyl stretching frequency into a selective probe of the r-donor and p-acceptor abilities of the ligands. We thus provide a framework whereby the r-donation and p-backdonation constituents of the Dewar–Chatt–Duncanson model can be quantitatively predicted through spectroscopic data on coordinated CO moieties and vice versa.