REGULAR ARTICLE The role of amide ligands in the stabilization of Pd(II) tricoordinated complexes: is the Pd–NR 2 bond order single or higher? Salvador Moncho Æ Gregori Ujaque Æ Pablo Espinet Æ Feliu Maseras Æ Agustı ´ Lledo ´s Received: 11 February 2009 / Accepted: 11 February 2009 / Published online: 6 March 2009 Ó Springer-Verlag 2009 Abstract The existence of tricoordinated Pd(II) com- plexes has been a matter of controversy for a long time. The recent X-ray characterization of a family of Pd com- plexes [PdArXL] allowed to certify the existence of true tricoordinated Pd(II) species. The unique role played by the amido ligand (X = NR 2 ), among a family of X ligands, was noticed in a previous computational work. Here, the influence of the R substituents at the amide and the nature of the Pd–N amido bond are theoretically analyzed. The relative stability of d 8 tricoordinated [PdLAr(NR 2 )] com- plexes versus d 8 tetracoordinated derivatives as a function of the R substituents is studied by analyzing the two most common ways to fill the vacant coordination site in a tricoordinated complex: solvent coordination (with tetra- hydrofuran as solvent), or dimerization giving [(l-NR 2 ) 2 Pd 2 L 2 Ar 2 ]) complexes. The nature of the Pd–N bonding interaction is analyzed using several theoretical schemes as molecular orbitals, QTAIM, ELF and NBO. Each of these schemes suggests that the order of the Pd–N bond in this family of complexes is higher than one. An asymmetric p interaction between the nitrogen lone pair and the LUMO over the tricoordinated Pd center is proposed as an important source of additional stabilization of tricoordi- nated species provided by amido ligands. Keywords Palladium chemistry Á Amide ligands Á Theoretical transition metal chemistry 1 Introduction Electronic population on the metallic center of transition metal complexes (both in intermediates and in transition states) is a crucial characteristic for the understanding of their properties and reactivity. Most organometallic com- plexes in their common oxidation state have a formal 18- electron counting, which is considered the standard rule, but late transition metals show a tendency to 16-electron counting. Thus, for isolated Pd(II) (d 8 ) complexes the square–planar 4-coordination with a formal 16-electron counting is absolutely dominant, although the less abun- dant five-coordination is not uncommon [1]. However, three-coordinated palladium(II) complexes are a real rarity hardly expected to be found as stable species, although they are often proposed as feasible intermediate structure in mechanistic proposals [2]. In this respect, theoretical cal- culations have proved to be highly useful in determining reaction mechanisms in organic [3–5] and organometallic chemistry [6, 7]. One kind of formally three-coordinated Pd(II) com- plexes frequently depicted as intermediates in very important catalytic cycles has a PdArXL stoichiometry with Ar = aromatic ring, X = anionic ligand, and L = phosphine. In practice, with unexceptional ligands the fourth coordination site is admittedly satisfied either by Dedicated to Prof. Santiago Olivella on the occasion of his 65th birthday. S. Moncho Á G. Ujaque Á F. Maseras Á A. Lledo ´s (&) Departament de Quı ´mica, Facultat de Cie `ncies, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Barcelona, Spain e-mail: agusti@klingon.uab.es P. Espinet IU CINQUIMA/Quı ´mica Inorga ´nica, Facultad de Ciencias, Universidad de Valladolid, 47071 Valladolid, Spain F. Maseras Institute of Chemical Research of Catalonia (ICIQ), 43007 Tarragona, Spain 123 Theor Chem Acc (2009) 123:75–84 DOI 10.1007/s00214-009-0539-7