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.

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 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.