The recent excitement in the study of transition metals is in part due to improved methods of studying, both experimentally and theoretically, the plethora of low-lying electronic states and their effects on reaction properties.

The different gas-phase reactivity that transition metals show depending on the spin, electron configuration, and even spin-orbit level has been discussed extensively by Armentrout and co-workers. As they have pointed out in those works, studies of excited electronic states of transition metal ions can help validate the molecular orbital ideas that are used routinely to understand the activation of covalent bonds by transition metals. Thus, such studies are a perfect field for interaction between experiment and theory.

The reactions of transition metal cations and water have received much attention recently in large part due to two curious effects. The first is that the early transition metal cations (Sc⁺, Ti⁺ and V⁺) are more reactive than their oxides, while the contrary occurs with the late metals (Cr⁺, Mn⁺ and Fe⁺). Even more interesting, however, is that the primary product observed in the reverse reaction (MO⁺ + H₂ --> M⁺ + H₂O) is a low-spin excited state of the cation, meaning that spin, rather than reaction energetics, is the overriding constraint of the reaction.

With regards to the importance of spin in these reactions evidence comes from both the forward reaction (M⁺ + H₂ -->  MO⁺ + H₂ ) and its reverse. In the forward transition metal cation plus water reaction, two factors point to the importance of the spin-state. The first is that if ground-state high-spin cations are used for the reaction, the observed MO⁺ product is, nevertheless in its low-spin ground state. Of course, if an excited-state low-spin cation is used the MO⁺ product will also be in its ground state.

However, the grand difference between using high-spin or low-spin cations as reactants is not the final product but rather the efficiency of the reaction. While the high-spin cations do react to give low-spin MO⁺ products, the efficiency is actually quite low. On the other hand, the reaction starting with a low-spin cation proceeds rapidly and efficiently.

There is also a point of difference between Sc⁺, Ti⁺, and V⁺ that is rather interesting. The ground-state high-spin cations of Sc+ and Ti+ do produce (though inefficiently) low-spin MO⁺. However, the ground state V⁺ (5D(d4)) cation does not make the spin crossing to produce triplet VO+. The first quintet excited state of V⁺(5F(sd3)), on the other hand, follows the trend established by the high-spin Sc⁺ and Ti⁺ cations, i.e. production of low-spin MO⁺, though be it inefficiently. Of question is whether this is due to the occupation scheme (both high-spin ground state Sc⁺ and Ti⁺ have a singly occupied s orbital) or due simply to the extra kick of energy available in the 5F state of V+. We have addressed this question in our discussion of the potential energy surfaces.

From the experimental data we gather that the reaction pathway is a low-spin pathway and that somewhere along the line the high-spin complex must undergo a spin-forbidden crossing. The reaction pathway demonstrated in our earlier work is: Ti⁺ + H₂O --> Ti⁺-OH₂ --> HTi + OH --> (H2)-TiO⁺ --> TiO⁺ + H_2. In the Ti⁺-OH₂ ion-molecule complex the high-spin state still lies below the low-spin state, but in the HTi + OH intermediate that situation is reversed implying that the high- and low-spin surfaces cross between these two moieties.

Another topic of interest concerning these reactions is the process of H₂ elimination (or addition in the reverse reaction). Two possibilities were proposed: elimination from an H₂MO⁺ intermediate, or elimination from a four-centered transition state. Our earlier work on the high- and low-spin Ti⁺ + H₂O reaction does predict a final intermediate before H₂ elimination in which the H atoms are most closely associated with the Ti atom. However, no H-Ti covalent sigma bonds existed. Instead the intermediate immediately preceding H₂ elimination could be viewed as an ion-molecule complex with some donation of electron density from the H-H sigma bond to the Ti atom and some back donation from a Ti d orbital to the H-H sigma* orbital.

Please refer to the Publications section or drop us a note to learn more about our research in this field.