It is proposed that spin-crossing effects can dramatically affect reaction mechanisms, rate constants, branching ratios, and temperature behaviors of organometallic transformations. This phenomenon is termed two-state reactivity (TSR) and involves participation of spin inversion in the rate-determining step. While the present analysis is based on studies of transition metals under idealized conditions, several recent reports imply that TSR is by no means confined to the gas phase. In fact, participation of more than a single spin surface in the reaction pathways is proposed as a key feature in organometallic chemistry.

Two-State Reactivity as a New Concept in Organometallic Chemistry§

Transition-metal centers are the active sites for a broad variety of biological and inorganic chemical reactions. Notwithstanding this central importance, density-functional theory calculations based on generalized-gradient approximations often fail to describe energetics, multiplet structures, reaction barriers, and geometries around the active sites. We suggest here an alternative approach, derived from the Hubbard U correction to solid-state problems, that provides an excellent agreement with correlated-electron quantum chemistry calculations in test cases that range from the ground state of Fe-2 and Fe-2(-) to the addition elimination of molecular hydrogen on FeO+. The Hubbard U is determined with a novel self-consistent procedure based on a linear-response approach.

http://www.inquimae.fcen.uba.ar/groups/simulation/PDF/prl-ggau.pdf

Density functional theory in transition-metal chemistry: A self-consistent Hubbard U approach

The importance of the selective activation of alkanes for science and technology in the forthcoming decades does not need to be explicitly pointed out in this thematic issue of Chemical ReViews. Instead, we would like to summarize the development and the state-of-art of experimental and theoretical methods for the investigation of model reactions for alkane activation in the gas phase.1 However, before doing so let us address the question, how other scientists, both in academia as well as industry, can profit from such model studies in the gas phase, usually involving very small, charged species under conditions in a mass spectrometer which are very far from real catalysis. To this end, we refer to the synthesis of HCN as a reasonably simple example of how large-scale technical processes can be investigated in microscopic models. † In memoriam of Dr. Andreas Fiedler. * To whom correspondence should be addressed. E-mail: detlef.schroeder@ uochb.cas.cz, E-mail: roithova@natur.cuni.cz. ‡ Charles University in Prague. § Academy of Sciences of the Czech Republic. Jana Roithová (right), Ph.D., leads a group on the topic of reaction mechanisms at the Faculty of Science at the Charles University in Prague. Her research is based on the synergy of gas-phase experiments and theoretical calculations. She received her Ph.D. degree in the group of Prof. Zdenek Herman (J. Heyrovsky Institute of Physical Chemistry, Prague) on the topic of reactivity of small molecular dications in 2003. Since then she has made key contributions to superelectrophilic chemistry in the gas phase, with particular attention toward the activation of nonreactive substrates such as rare gases, nitrogen, and methane. Recently, she concentrates her research on the properties of redox-active molecules, their interactions with transition metals, and mechanisms of organometallic reactions. She is author of more than 80 papers and received, among others, a “L’Oréal for Women in Science” stipend and the Hlavka prize.

Selective activation of alkanes by gas-phase metal ions.

http://absimage.aps.org/image/MAR07/MWS_MAR07-2006-003168.pdf

Density Functional Theory in Transition Metal Chemistry: A Self-Consistent Hubbard U approach

This review focuses on noncovalent metal ion-ligand complexes and measurements of the bond energies of such species. The method utilized in this work is threshold collision-induced dissociation (CID), as achieved using a guided ion beam tandem mass spectrometer. Accurate determination of bond energies requires attention to many details of the experiments and data analysis. These details are discussed thoroughly and compared to other methods. A comprehensive listing of metal-ligand bond dissociation energies determined by threshold CID is provided. This list includes a variety of metals (alkalis, magnesium, aluminum, and first and second row transition metals), many different types of ligands, and variations in the number of ligands. The trends in these values are discussed, and we elucidate the importance of ion-dipole and ion-induced dipole interactions, chelation, different conformers and tautomers, steric interactions, solvation phenomena, and electronic effects such as hybridization and promotion.

Noncovalent metal–ligand bond energies as studied by threshold collision‐induced dissociation

The study of the reaction of water with the first-row transition-metal ions is continued in this work. Here we report the study of the reaction of water with the middle (Cr+, Mn+, and Fe+) first-row transition-metal cations in both high- and low-spin states. In agreement with experimental observations, the oxides are predicted to be more reactive than the metal ions, and no exothermic products are observed. Formation of endothermic products is examined. An in-depth analysis of the reaction paths leading to the observed products is given, including various minima, and several important transition states. All results have been compared with existing experimental and theoretical data, and our earlier works covering the (Sc+, Ti+, V+) + H2O reactions to observe existent trends for the early first-row transition-metal ions. The MO+ + H2 energy relative to M+ + H2O increases through the series from left to right. Additionally, the Fe+ case is seen to be significantly different from the entire Sc+−Mn+ series bec...

Reactivity of cr+(6s, 4d), mn+(7s, 5s), and fe+(6d, 4f) : reaction of cr+,mn+, and fe+ with water

The potential energy surfaces corresponding to the dehydrogenation reaction of H 2 O, NH 3 , and CH 4 molecules by Fe + ( 6 D, 4 F) cation have been investigated in the framework of the density functional theory in its B3LYP formulation and employing a new optimized basis set for iron. In all cases, the low-spin ion-dipole complex, which is the most stable species on the respective potential energy hypersurfaces, is initially formed. In the second step, a hydrogen shift process leads to the formation of the insertion products, which are more stable in a low-spin state. From these intermediates, three dissociation channels have been considered. All of the results have been compared with existing experimental and theoretical data. Results show that the three insertion pathways are significantly different, although spin crossings between high- and low-spin surfaces are observed in all cases. The topological analysis of the electron localization function has been used to characterize the nature of the bonds for all of the minima and transition states along the paths.

Theoretical Study of Two-State Reactivity of Transition Metal Cations: The ``Difficult'' Case of Iron Ion Interacting with Water, Ammonia, and Methane

In this paper we conclude the study of the reaction of water with the first row transition metal ions. We report the study of the reaction of water with the late (Co+, Ni+, and Cu+) first row transition metal cations in both high- and low-spin states. In agreement with experimental observations, no exothermic products are found and the oxides are predicted to be more reactive than the metal ions. Formation of endothermic products is examined. An in-depth analysis of the reaction paths possible for these reactions is given, including various minima and several important transition states. All results have been compared with existing experimental and theoretical data, and our earlier works covering the (Sc+-Fe+) + H2O reactions to observe existent trends for the first row transition metal ions.

Reactivity of Co+(3F,5F), Ni+(2D,4F), and Cu+(1S,3D): Reaction of Co+, Ni+, and Cu+ with Water

The study of the reaction of water with the early first-row transition metal ions has been completed in this work, in both high- and low-spin states. In agreement with experimental observations, the only exothermic products are the low-lying states MO+ + H2; formation of other endothermic products is also examine. An in-depth analysis of the reaction paths leading to each of the observed products is given, including various minima and several important transition states. All results have been compared with existing experimental data and our earlier work covering the Ti+ + H2O reaction in order to observe existent trends for the early first-row transition metal ions.

Reactivity of Sc+(3D,1D) and V+(5D,3F): Reaction of Sc+ and V+ with Water

The reaction of Ti+(4F,2F) + OH2 has been studied in detail for both doublet and quartet spin states. The only exothermic products are TiO+(2Δ) and H2; formation of several endothermic products is also examined. An in-depth analysis of the reaction paths leading to each of the observed products is given, including various singlet, triplet, and quartet minima, several important transition states, and a discussion of the two H2 elimination mechanisms proposed in the literature. The experimentally observed spin-forbidden crossing is given a possible explanation. Throughout this work comparison to experimental results in energetics, reaction products, and suggested mechanisms has been central.

Arantxa Irigoras

Papers

Reactivity of cr+(6s, 4d), mn+(7s, 5s), and fe+(6d, 4f) : reaction of cr+,mn+, and fe+ with water

Theoretical Study of Two-State Reactivity of Transition Metal Cations: The ``Difficult'' Case of Iron Ion Interacting with Water, Ammonia, and Methane

Reactivity of Co+(3F,5F), Ni+(2D,4F), and Cu+(1S,3D): Reaction of Co+, Ni+, and Cu+ with Water

Reactivity of Sc+(3D,1D) and V+(5D,3F): Reaction of Sc+ and V+ with Water

On the Reactivity of Ti+(4F,2F). Reaction of Ti+ with OH2