Electronic properties of n-heterocyclic carbene (NHC) ligands: Synthetic, structural, and spectroscopic studies of (NHC)Platinum(II) complexes

N-Heterocyclic carbene complexes of platinum(II) have been synthesized, notably monocarbene complexes cis-[(IPr)Pt(dmso)(Cl)(2)], 6, cis-[(IMes)Pt(dmso)(Cl)(2)], 7, cis- [(SIPr)Pt(dmso)(Cl)(2)], 8, cis[(SIMes)Pt(dmso)(Cl)(2)], 9, and cis- [(TTP)Pt(dmso)(Cl)(2)], 10. All complexes have been fully characterized by multinuclear NMR spectroscopy. Complex 7, 9, and 10 have been characterized by X-ray crystallography. The data obtained have allowed for the differentiation between electronic contributions (a and T) present in the Pt-NHC bond. Supported by computational analyses, the percentage of pi back-donation from the metal to the NHC is found to be on the order of 10%. More interestingly, we find that saturated NHC (SIPr and SIMes) are more efficient pi back-acceptors than their unsaturated NHC congeners (IPr and lMes). The synergistic effect between pi back-donation and sigma donation present in the saturated NHC systems results in increased electron density at the platinum center compared to the bonding situation in the unsaturated NHC examples.