Quantum chemical calculations at the MP2/TZVPP//BP86/SVP level are reported for the first and second proton affinities (PAs) of divalent carbon-donor molecules. The molecules investigated are imidazol-2-ylidenes ("normal" NHCs) and the tautomeric imidazol-4/5-ylidenes ("abnormal" NHCs). PAs are also calculated for acyclic and cyclic carbodiphosphoranes, carbophosphoranesulfide, unsaturated and saturated carbodicarbenes, tetraaminoallenes and carbon suboxide. The results are discussed in terms of divalent carbon(II) compounds (carbenes) CR(2), which have one lone electron pair at carbon, and carbon(0) compounds CL(2), which have two lone pairs at carbon and two C<--L donor-acceptor bonds. Divalent C(0) compounds (carbones) not only have very high first PAs, but the second PA is also large and strong enough to isolate doubly protonated C(0) species as salts in a condensed phase. The first PA of divalent carbon(II) compounds (carbenes) are also large. However, they have much smaller second PAs than the divalent carbon(0) compounds. The divalent C(0) character of a compound is not always obvious when the bonding situation in the equilibrium geometry is considered. This is the case, for example, for tetraaminoallenes (TAAs). Protonation of TAAs changes the bonding situation of the central moiety from doubly bonded (R(2)N)(2)C=C=C(NR(2))(2) to a donor-acceptor description (R(2)N)(2)C-->C(H(+))(n)<--C(NR(2)) [n = 1, 2]. The atomic partial charge at the carbon donor atom does not correlate with the PA and the trend of the second PA may be quite different from the trend of the first. The trends of the first and second PA correlates quite well with the eigenvalues of the highest-lying carbon lone-pair orbitals.

DFT calculations at the BP86/TZ2P level were carried out to analyze quantitatively the metal-ligand bonding in transition-metal complexes that contain imidazole (IMID), imidazol-2-ylidene (nNHC), or imidazol-4-ylidene (aNHC). The calculated complexes are [Cl4TM(L)] (TM = Ti, Zr, Hf), [(CO)5TM(L)] (TM = Cr, Mo, W), [(CO)4TM(L)] (TM = Fe, Ru, Os), and [ClTM(L)] (TM = Cu, Ag, Au). The relative energies of the free ligands increase in the order IMID < nNHC < aNHC. The energy levels of the carbon sigma lone-pair orbitals suggest the trend aNHC > nNHC > IMID for the donor strength, which is in agreement with the progression of the metal-ligand bond-dissociation energy (BDE) for the three ligands for all metals of Groups 4, 6, 8, and 10. The electrostatic attraction can also be decisive in determining trends in ligand-metal bond strength. The comparison of the results of energy decomposition analysis for the Group 6 complexes [(CO)5TM(L)] (L = nNHC, aNHC, IMID) with phosphine complexes (L = PMe3 and PCl3) shows that the phosphine ligands are weaker sigma donors and better pi acceptors than the NHC tautomers nNHC, aNHC, and IMID.

Quantum chemical calculations using DFT at the BP86/TZ2P level of theory are reported for the complexes (PH 3 ) 2 ClM-L where L is an N-heterocyclic ligand and M a group-10 metal Ni, Pd and Pt. The ligands comprise pyridyl groups or carbenes derived from the pyridine, quinolidine or isoquinolidine systems wherein the nitrogen atom is either adjacent to the carbene carbon atom or it is in a remote (meta or para, or in the adjacent ring) position. Comparative calculations include the isomeric ligands of the well-known five-membered Nheterocyclic carbene. The nature of the metal-ligand interactions are investigated by energy decomposition analysis (EDA). The EDA results suggest that the nature of the metalcarbene bonds in the complexes shows little variation when the position of the nitrogen atom in pyridylidenes is adjacent (ortho) or remote (meta or para). It changes even very little

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Reductive complexation of RuCl 3 and OsCl 3 in the presence of phosphinine C 5 H 5 P and arsenine C 5 H 5 As yields the species trans-Cl 2 (η 1 -C 5 H 5 E) 4 M II (M = Ru, E = P, As; M = Os, E = P). trans-Cl 2 (η 1 -C 5 H 5 As) 4 Ru constitutes the first arsenine complex of a late transition metal. Further reduction and isolation of the binary complexes (C 5 H 5 E) n M 0 (n = 2, 4, 5) failed, presumably because of metal-ligand bond cleavage. According to X-ray diffraction analysis, trans-Cl 2 (η 1 -C 5 H 5 P) 4 Ru (3) features two pairs of coplanar trans-phosphinine ligands,