I. Introduction 601 A. Hydride Is the Smallest Ligand and the Only One To Make a Pure Single Bond to a Metal 601 B. H Is Easy To Locate through Quantum Calculations but Hard To Locate through Experimental Techniques 602 C. Hydride Complexes Are Full of Surprises 603 D. H 2 Is the Ideal Model for a σ Bond Coordinating to a Metal 603 E. H 2 Is the Ideal Model for Activation of a Single Bond 603 F. Scope and Limitations of the Review 603 II. Small Gas-Phase Systems 603 III. H as an Ideal Ligand 605 A. General Approach for Bonding in Transition Metal Complexes 605 B. The d 0 ML 6 Case 606 C. The d 6 ML 5 Case 606 D. Other Systems 607 IV. "Computational Crystallography" of Hydride Complexes, a Cost-Effective Method for High-Quality Structural Determination 607 A. Good Success with Simplified Models 607 B. Improving the Model 610 V. The Dihydrogen Saga 611 A. Dihydrogen as a Ligand 611 B. Dihydrogen versus Dihydride 613 C. Theoretical Tools for Analysis of the H‚‚‚H Interaction 614 VI. Interactions with M−H and with M−H 2 . Hydrogen Bonds Again! 615 VII. Hydrogen Exchange Processes 617 A. Pairwise Exchange 617 B. Polytopal Rearrangements 619 C. Site Exchange with H Atoms outside the Coordination Sphere 619 D. Dihydrogen Rotation 620 E. Exchange Processes in M(H)(H 2 ) Complexes 622 VIII. Quantum Exchange Couplings in Polyhydrides 623 A. Physical Origin of Quantum Exchange Couplings 623 B. Simulation of Quantum Exchange Couplings 624 IX. Stretched Dihydrogen Complexes 625 A. The Electronic Point of View 626 B. The Dynamic Point of View 627 X. Breaking the H−H Bond by Transition Metal Complexes 627 A. Oxidative Addition 628 B. σ Bond Metathesis 629 XI. Methodological Peculiarities in the Study of Polyhydride Systems 629 XII. Conclusions and Perspectives 631 XIII. Acknowledgment 631 XIV. References 631 10.

Palladium-catalyzed C-C cross-coupling reactions (Suzuki-Miyaura, Negishi, Stille, Sonogashira, etc.) are among the most useful reactions in modern organic synthesis because of their wide scope and selectivity under mild conditions. The many steps involved and the availability of competing pathways with similar energy barriers cause the mechanism to be quite complicated. In addition, the short-lived intermediates are difficult to detect, making it challenging to fully characterize the mechanism of these reactions using purely experimental techniques. Therefore, computational chemistry has proven crucial for elucidating the mechanism and shaping our current understanding of these processes. This mechanistic elucidation provides an opportunity to further expand these reactions to new substrates and to refine the selectivity of these reactions. During the past decade, we have applied computational chemistry, mostly using density functional theory (DFT), to the study of the mechanism of C-C cross-coupling reactions. This Account summarizes the results of our work, as well as significant contributions from others. Apart from a few studies on the general features of the catalytic cycles that have highlighted the existence of manifold competing pathways, most studies have focused on a specific reaction step, leading to the analysis of the oxidative addition, transmetalation, and reductive elimination steps of these processes. In oxidative addition, computational studies have clarified the connection between coordination number and selectivity. For transmetalation, computation has increased the understanding of different issues for the various named reactions: the role of the base in the Suzuki-Miyaura cross-coupling, the factors distinguishing the cyclic and open mechanisms in the Stille reaction, the identity of the active intermediates in the Negishi cross-coupling, and the different mechanistic alternatives in the Sonogashira reaction. We have also studied the closely related direct arylation process and highlighted the role of an external base as proton abstractor. Finally, we have also rationalized the effect of ligand substitution on the reductive elimination process. Computational chemistry has improved our understanding of palladium-catalyzed cross-coupling processes, allowing us to identify the mechanistic complexity of these reactions and, in a few selected cases, to fully clarify their mechanisms. Modern computational tools can deal with systems of the size and complexity involved in cross-coupling and have a continuing role in solving specific problems in this field.

Of the several hundred examples of transition metal dihydrogen complexes that have been reported to date, the vast majority have H-H distances of less than 1.0 Angstrom. A small number of complexes have been reported with distances in the range of 1.1 to 1.5 Angstrom. These complexes have been termed elongated dihydrogen complexes. In this review, experimental methods for structure determination of such complexes are summarized, along with computational approaches which have proven useful in understanding the structures of these molecules.

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.

ABSTRACT:The use of the hypervalent iodine reagents in oxidative processes has become a staple in modern organic synthesis. Frequently, the reactivity of λ 3 iodanes is further enhanced by acids (Lewis or Brønsted). The origin of such activation, however, has remained elusive. Here, we use the common combination of PhI(OAc)2 with BF3·Et2O as model to fully explore this activation phenomenon. In addition to the spectroscopic assessment of the dynamic acid-base interaction, for the first time the putative PIDA·BF3 complex has been isolated and its structure determined by X-Ray diffraction. Consequences of such activation are discussed from a structural and electronic (DFT) points of views, including the origins of the enhanced reactivity.

The binuclear complexes of d8 transition metal ions of type [L2M(μ-XR n )2ML2] (n = 0 to 2) show a wide variety of bending angles between the coordination planes of the two metal atoms. A theoretical and structural analysis of those compounds with unsubstituted bridges, [L2M(μ-X)2ML2], tries to unravel the factors that determine the degree of bending of such compounds. A rationale is given for the structures of 139 crystallographically independent molecules. A driving force for bending of the molecules is the attractive metal···metal interaction that results from donor−acceptor interactions between the d z 2 and p z orbitals of the two metal atoms and is modulated by the nature of (a) the metal atom, (b) the terminal ligands, and (c) the bridging atoms. In all cases the energy difference between the planar and the bent form is within the 10 kcal/mol range, and steric effects seem to be important in preventing bending only for the bulkiest terminal ligands. For the studied bridging atoms, the stability of the bent form increases in the order O < S > Se > Te and F < Cl < Br < I, and for different metal atoms it increases in the order Ni(II) < Pd(II) < Pt(II) < Rh(I) < Ir(I).

Gold(I) complexes featuring electron acceptor ligands such as phosphites and phosphoramidites catalyze the [4C+2C] intramolecular cycloaddition of allenedienes. The reaction is chemo- and stereoselective, and provides trans-fused bicyclic cycloadducts in good yields. Moreover, using novel chiral phosphoramidite-based gold catalysts it is possible to perform the reaction with excellent enantioselectivity. Experimental and theoretical data dismiss a cationic mechanism involving intermediate II and suggest that the formation of the [4C+2C] cycloadducts might arise from a 1,2-alkyl migration (ring contraction) in a cycloheptenyl Au-carbene intermediate (IV), itself arising from a [4C+3C] concerted cycloaddition of the allenediene. Therefore, these [4C+2C] allenediene cycloadditions and the previously reported [4C+3C] counterparts most likely share such cycloaddition step, differing in the final 1,2-migration step.

With the goal of understanding how the nature of the tridentate macrocyclic supporting ligand influences the relative stability of isomeric mu-eta 2:eta 2-peroxo- and bis(mu-oxo)dicopper complexes, a comparative study was undertaken of the O2 reactivity of Cu(I) compounds supported by the 10- and 12-membered macrocycles, 1,4,7-R3-1,4,7-triazacyclodecane (R3TACD; R = Me, Bn, iPr) and 1,5,9-triisopropyl-1,5,9-triazacyclododecane (iPr3TACDD). While the 3-coordinate complex [(iPr3TACDD)Cu]SbF6 was unreactive with O2, oxygenation of [(R3TACD)Cu(CH3CN)]X (R = Me or Bn; X = ClO4- or SbF6-) at -80 degrees C yielded bis(mu-oxo) species [(R3TACD)2Cu2(mu O)2]X2 as revealed by UV-vis and resonance Raman spectroscopy. Interestingly, unlike the previously reported system supported by 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN), which yielded interconverting mixtures of peroxo and bis(mu-oxo) compounds (Cahoy, J.; Holland, P. L.; Tolman, W. B. Inorg. Chem. 1999, 38, 2161), low-temperature oxygenation of [(iPr3TACD)Cu(CH3CN)]SbF6 in a variety of solvents cleanly yielded a mu-eta 2:eta 2-peroxo product, with no trace of the bis(mu-oxo) isomer. The peroxo complex was characterized by UV-vis and resonance Raman spectroscopy, as well as an X-ray crystal structure (albeit of marginal quality due to disorder problems). Intramolecular attack at the alpha C-H bonds of the substituents was indicated as the primary decomposition pathway of the oxygenated compounds through examination of the decay kinetics and the reaction products, which included bis(mu-hydroxo)- and mu-carbonato-dicopper complexes that were characterized by X-ray diffraction. A rationale for the varying results of the oxygenation reactions was provided by analysis of (a) the X-ray crystal structures and electrochemical behavior of the Cu(I) precursors and (b) the results of theoretical calculations of the complete oxygenated complexes, including all ligand atoms, using combined quantum chemical/molecular mechanics (integrated molecular orbital molecular mechanics, IMOMM) methods. The size of the ligand substituents was shown to be a key factor in controlling the relative stabilities of the peroxo and bis(mu-oxo) forms, and the nature of this influence was shown by both theory and experiment to depend on the ligand macrocycle ring size.