Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts

One-electron oxidation of known (tBu3P)2M (1, M = Pd; 2, M = Pt) with [Ph3C][HCB11Cl11] leads to two-coordinate, monovalent cations of the formula [(tBu3P)2M][HCB11Cl11] (3, M = Pd; 4, M = Pt), which also possess linear geometry but with elongated M–P bonds. Spectroscopic and computational studies consistently show that the unpaired electron of the d9 configuration of 3 and 4 belongs to largely non-bonding orbitals: the s/dz2 hybrid for Pd and the degenerate dx2-y2/dxy pair for Pt. We show that molecular-orbital-based arguments alone are incapable of predicting or rationalizing the observed M–P bond lengthening on oxidation; correct prediction and rationalization are achieved only by inclusion of electrostatic and Pauli effects. This emphasizes the dangers of interpreting any perturbative changes in bond metrics solely on the basis of energies and occupancies of molecular orbitals; the inclusion of electrostatic and Pauli components is essential to providing a more complete picture. [Display omitted] •Two-coordinate, cationic complexes of monovalent Pd and Pt are synthesized•The monovalent cations possess longer, stronger M–L bonds than their zero-valent analogs•Theoretical consideration of electrostatic and Pauli effects offers an explanation Homogeneous catalysis is an indispensable part of chemistry’s role in solving sustainability issues and innovation on many fronts. Most homogeneous catalysts are molecular transition-metal complexes, whose behavior is typically viewed via the prism of metal–ligand interactions. Proper understanding of chemical bonding in these situations is crucial to progress in catalysis. Organophosphines are among the most widely utilized ligands, especially for precious metals such as palladium (Pd) and platinum (Pt). This work presents characterization of unusual pairs of Pd and Pt complexes that differ only by a single unit of charge and have the metal bound to only two phosphorus atoms. Seemingly contradictory to common logic, removal of an electron from a complex results in significant elongation and in strengthening of metal–phosphorus bonds. Experimental and computational analysis provides a rationalization for this phenomenon, which could influence consideration of more complex systems. Ozerov and colleagues describe the synthesis and characterization of linear, two-coordinate, cationic phosphine complexes of monovalent Pd and Pt. Comparison of the structures of these complexes to the neutral, zero-valent analogs revealed