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A question of identity: A palladium complex featuring a carbenic S,C,S‐based pincer ligand is reported (see scheme). DFT calculations show that, unlike classical carbene complexes, coordination occurs through donation from a d orbital at the metal into the empty np orbital of the carbene fragment, with the nσ orbital essentially acting as a nonbonding orbital. The complex is shown to have nucleophilic character.

The complexes Os(η 5-C5H5)Cl{η 2-HC⋮CC(OH)R2}(PiPr3) (R = Ph (1a), Me (1b)) react with TlPF6 to give [Os(η 5-C5H5){η 2-HC⋮CC(OH)R2}(PiPr3)]PF6 (R = Ph (2a), Me (2b)). The structures of 1a and 2a have been determined by X-ray diffraction. The comparative study of the data reveals a shortening of the Os−C(alkyne) distances on going from 1a to 2a, whereas the acetylenic bond lengths remain almost identical. Comparison of their 1H and 13C{1H} NMR spectra shows that the HC⋮ proton resonances and the chemical shifts of the acetylenic carbon atoms of 2a and 2b are substantially shifted toward lower field than are those of 1a and 1b. DFT calculations were carried out on Os(η 5-C5H5)Cl(η 2-HC⋮CR)(PH3) (R = H (A), R = CH3 (A CH3 )) and [Os(η 5-C5H5)(η 2-HC⋮CR)(PH3)]+ (R = H (B), R = CH3 (B CH3 )) model systems in order to study the differences in bonding nature of the two parent alkyne complexes, 1 and 2. Calculations give geometries very close to the X-ray-determined ones, and by using the GIAO method we succeed in qualitatively reproducing the experimental 1H and 13C chemical shifts. Both structural and spectroscopic changes can be explained by the participation of the acetylenic second π orbital (π⊥) in the metal−alkyne bonding. As we go from 1 to 2 or from A to B, the extraction of the chloride ligand transforms the 2-electron-donor alkyne ligand to a 4-electron-donor ligand, with both the π|| and the π⊥ orbitals donating to the metal and stabilizing the otherwise 16-electron unsaturated complex 2. Calculations also predict an increase of dissociation energies of the alkyne, and an enhancement in the energy of rotation of the alkyne, for complex B. Finally, Bader's atoms in molecules (AIM) analysis shows that differences in coordination nature are also reflected in the topological properties of electron density.

The problem of intramolecular hydrogen atom exchange in the OsH3(BH4)(PR3)2 system is examined from both theoretical and experimental points of view, through ab initio MO calculations on the OsH3(BH4)(PH3)2 system at the MP2, MP4, and CCSD(T) computational levels and variable-temperature 1H NMR studies on the OsH3(BH4)(P i Pr3)2 complex. Three different exchange processes are fully characterized from a theoretical point of view through location of intermediates and transition states. Experimental results supporting the existence of these three different exchange processes and providing definitely its intramolecular nature are also presented.

The interaction between H2 and M(CO) n (PH3)5 - n (M = Cr, Mo, W; n = 0, 3, 5) metal fragments has been studied by means of CCSD(T)//B3LYP calculations. Three steps in the dihydrogen addition path that starting from the ML5 and H2 separated fragments leads to a stable dihydride have been considered:  (i) dihydrogen coordination; (ii) cleavage of the H−H bond in a dihydrogen-like structure, leading to a PB1 cis-dihydride; (iii) reorganization of the pentagonal bipyramidal cis-dihydride formed to a more stable PB2 dihydride structure. From the thermodynamic results and the energy profiles for the oxidative addition the nine complexes under study can be classified in three groups:  (i) only dihydrogen observable; M(CO)5H2 (M = Cr, Mo, W); and M(CO)3(PH3)2H2 (M = Cr, Mo); (ii) equilibrium between dihydrogen and PB2 dihydride:  W(CO)3(PH3)2H2 and M(PH3)5H2 (M = Cr, Mo); (iii) only dihydride observable; W(PH3)5H2. The different behavior for dihydrogen addition is related to the energetics of the M−H2 and M−H bonds and to the singlet−triplet separation in the ML5 fragment.

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The palladium-catalyzed allylation of primary amines has been investigated by DFT calculations (B3PW91, PCM method), and two potential mechanisms were studied. The first mechanism relies on the formation of cationic hydridopalladium complexes. Their formation involves a metal-assisted formal (1,3) shift of a proton from the nitrogen atom of an ammonium to the Cbeta carbon atom. The second part of the cycle relies on a ligand exchange through a pentacoordinated 18VE hydridopalladium complex. The last step likely proceeds through a bimolecular pathway and formally consists of a proton transfer from the allylammonium to the alcohol group of the complex. The second mechanism, which is closer to that currently admitted for nucleophilic allylic substitutions, relies on the decomplexation of the coordinated allylammonium and appears to be favored. This catalytic cycle was recomputed on model complexes varying the ligands, and a charge decomposition analysis was carried out to assess the influence of the electronic properties of the ligands. To compare our results with competitive experiments, CDA calculations were also performed on real ligands. In agreement with experimental observations, this process was found to be strongly ligand dependent, decomplexation being favored by strong pi-acceptor ligands. These calculations led us to show experimentally that complex [Pd(P(OPh)(3))(2)(eta(3)-C(3)H(5))][OTf] is an efficient catalyst for this allylation. Finally, this catalytic process proved to be sensitive to the nature of the amine, with poorly basic amines favoring the re-formation of the catalytic precursor.

The bis-2,5-diphenylphosphole xantphos ligand (XDPP) 1 reacts with the [AuCl(tht)] complex to afford the monocoordinated [Au(XDPP)Cl] 2 and the dicoordinated chelate species [Au(XDPP)Cl] 3. Addition of AgOTf on this mixture, at room temperature, affords the cationic [Au(XDPP)][OTf] complex 4 which was fully characterized. An X-ray crystal structure analysis confirms the bent structure of this 14 VE [ML(2)](+) complex. Reaction of 4 with HSiMe(2)Ph in tetrahydrofuran at -78 degrees C yields the dinuclear [(XDPP)Au-H-Au(XDPP)](+) cationic complex 5, in which the hydride bridges the two [Au(XDPP)](+) metal fragments. In 5, the Au-P bond lengths are different and the phosphorus atoms which are located nearly trans to the hydride ligand exhibit significantly shorter P-Au bond lengths. Reaction of 4 with DSiMe(2)Ph to form the [(XDPP)Au-D-Au(XDPP)](+) complex 6 allowed to unambiguously ascribe the chemical shift of the deuteride in (2)H NMR (delta = 7.0 ppm with a (2)J(DP) = 8.4 Hz. The electronic structure of the [(XDPP)Au-H-Au(XDPP)](+) complex was studied through density functional theory calculations. An orbital analysis is developed in which complex 5 is viewed as the combination of two 12 electrons fragments [Au(XDPP)](+) with H(-). This analysis reveals that the hydride interacts in a bonding way with the sigma MO between the two gold atoms and in an antibonding way with a combination of d orbitals at the metal centers. This simple description allows to rationalize the inequivalence of the two types of P-Au bonds in 5.