The nature of Pd-carbene and Pd-halogen bonds in (bisNHC)PdX2 type catalysts: insights from density functional theoryElectronic supplementary information (ESI) available: Selected structural parameters, results of second order perturbation theory analysis, back-bonding interaction, natural hybrid orbital analysis, HOMO, LUMO orbitals, charge transfer values, EDA results, bond dissociation energy values and Cartesian coordinates of all the complexes are given in the supplementary information. See

Methylene bridged palladium biscarbene halide complexes have proven to be excellent catalysts in cross coupling reactions and in activation of alkanes. These types of complexes are widely used in decomposition and reductive elimination reactions because of their more stable carbene-Pd bond. The electronic structure and bonding of such methylene bridged palladium biscarbene complexes (LnPdX 2 ) (Ln = NHC) have been investigated using density functional theory. The calculated results reveal that the nature of the substituents as well as the coordinating halide ion determine the strength of the Pd-C carbene and Pd- X bonds. These two bonds can be fine-tuned to achieve better catalytic activity through proper substituents. In effect, this can be done by making the Pd-C carbene bond much stronger through σ-donation and π-back donation and also by making the Pd-X bond weaker by choosing a suitable halide (X). Pd-carbene and Pd-halogen play a key role in the catalytic activity of (bisNHC)PdX 2 complexes and they can be fine-tuned with proper substitution in the carbene moiety and choosing a weakly coordinating halide ion.