When are tricoordinated PdII species accessible? Stability trends and mechanistic consequences

The ease of access to Pd(II) tricoordinated species (whether intermediates or transition states) in organometallic and catalytic reactions has been assessed with DFT methods to analyze the relative stability of tricoordinated [PdArXL] complexes versus their tetracoordinated derivatives formed by two most common processes of filling the fourth coordination site: solvent coordination (with tetrahydrofuran), or dimerization to give [Pd2Ar2(micro-X)2L2]. The effect of each ligand (L=PH3, PMe3, PPh3, PtBu3, 1-AdPtBu2; Ar=C6F5, C6H5, C6H4OH, C6H4OCH3, C6H4NH2, C6H2(NH2)3; X=F, Cl, Br, I, OH, SH, NH2, PH2, CH3) on these two processes has been systematically considered, and the results have been compared with the experimental information available. The trends observed, match the experimental results and suggest that: 1) the formation of bridged dimeric complexes is strongly preferred; 2) electronic effects are in general less important compared to steric effects; 3) when steric effects prevent formation of bridges and coordination of a fourth external ligand, intramolecular agostic interactions are established with C--H groups of one ligand; 4) as an exception, for X=NR2 true tricoordinated complexes, not showing agostic interactions, become stable. In the later case NR2 seems to act as pi-donor with its lone pair to the empty orbital at the fourth coordination site of palladium, thus avoiding a true 14e configuration for the tricoordinated PdII complex.