We present results that show that the formation of the so called 18-electron complexes of palladium(0) is completely driven by back donation from palladium to the phosphine ligands. Although Pd(PPh3)4 has been one of the most commonly used catalysts for constructing carbon-carbon bonds via cross coupling reactions since the 1970's, there are still things we do not fully understand. In solid phase the preferred coordination number is 4 while in solution palladium loses one ligand and is mainly present as the tri-coordinate complex. Moreover, it is believed that yet another ligand must dissociate before any reaction occur. We have analyzed the bonding between palladium and phosphines, and have found two striking effects. 1) When the first and second phosphine coordinates the energy from donation to palladium increases, as does the back-donation. However, when the third and fourth phosphine coordinate the energy from donation decreases while the back donation continues to increase, and actually becomes the dominant term for the tetra-coordinate complex. This result suggests that it is not further donation of electrons from the phosphines that drives the formation of the tri- and tetracoordinate complexes but actually back-donation. 2) For describing the full system it is crucial to use a method that is capable of describing dispersion. We find that both M06 and B3LYP-D perform excellently, while B3LYP fails completely to reproduce the experimental results.

Dispersion and back-donation gives tetracoordinate Pd(PPh<sub>3</sub>)<sub>4</sub>

243rd National Meeting (2012)

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Dispersion and back-donation gives tetracoordinate Pd(PPh3)4

Dispersion and back-donation gives tetracoordinate Pd(PPh₃)₄