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 ( and ) present in the Pt-NHC bond. Supported by computational analyses, the percentage of 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 back-acceptors than their unsaturated NHC congeners (IPr and IMes). The synergistic effect between back-donation and 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.