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