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 backdonation 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.

Mixed NHC-phosphane palladium(0) complexes [(NHC)Pd(PR(3))] (NHC: N-heterocyclic carbene) are synthesized directly from commercially available reagents, with the possibility to tune the nature of both the NHC and the phosphane. Reaction of [(NHC)Pd(allyl)Cl] (palladium source) and PR(3), in the presence of a base afforded, in isopropanol, [(NHC)Pd(PR(3))] in good yields. We found that the nature of the solvent played a key role in the efficient reduction of the Pd(II) precursor to Pd(0). Supported by experimental evidence we propose that the reduction step is driven by the isopropoxide anion formed in situ from isopropanol and a base. Detection of acetone in the reaction mixture confirms that the isopropoxide anion acts as the reducing agent. Moreover, different bases proved efficient for the reaction. The structures of the complexes were unambiguously confirmed by X-ray analysis. Exposure of these complexes to air does not lead to decomposition, but to the oxo-complex [(NHC)Pd(PR(3))(O(2))], which is stable both in the solid state and in solution.

Enthalpies of chalcogen atom transfer to Mo(N[t-Bu]Ar)3, where Ar = 3,5-C6H3Me2, and to IPr (defined as bis-(2,6-isopropylphenyl)imidazol-2-ylidene) have been measured by solution calorimetry leading to bond energy estimates (kcal/mol) for EMo(N[t-Bu]Ar)3 (E = S, 115; Se, 87; Te, 64) and EIPr (E = S, 102; Se, 77; Te, 53). The enthalpy of S-atom transfer to PMo(N[ t-Bu]Ar) 3 generating SPMo(N[t-Bu]Ar)3 has been measured, yielding a value of only 78 kcal/mol. The kinetics of combination of Mo(N[t-Bu]Ar)3 with SMo(N[t-Bu]Ar)3 yielding (mu-S)[Mo(N[t-Bu]Ar)3]2 have been studied, and yield activation parameters Delta H (double dagger) = 4.7 +/- 1 kcal/mol and Delta S (double dagger) = -33 +/- 5 eu. Equilibrium studies for the same reaction yielded thermochemical parameters Delta H degrees = -18.6 +/- 3.2 kcal/mol and Delta S degrees = -56.2 +/- 10.5 eu. The large negative entropy of formation of (mu-S)[Mo(N[t-Bu]Ar)3]2 is interpreted in terms of the crowded molecular structure of this complex as revealed by X-ray crystallography. The crystal structure of Te-atom transfer agent TePCy3 is also reported. Quantum chemical calculations were used to make bond energy predictions as well as to probe terminal chalcogen bonding in terms of an energy partitioning analysis.