Both misconceptions are common and make this field unusually devoid of clarity. When it comes to chemical reactions, however, most people are either biased by the forbidden character of such changes and overemphasise their difficulty, or instead dismiss spin as being unimportant. The well-known phenomenon of spin equilibria in crossover compounds 1 highlights that spin changes are in fact common in transition metal chemistry. Thus, spin changes tend to be more strictly forbidden when the affected electrons are localised on light atoms such as carbon, less so when they are on transition metals. Such a change is often referred to as ‘forbidden’, but in fact, it is more appropriate to say that it is associated with a certain degree of spin-forbiddenness, which is determined by the magnitude of the spin-orbit coupling term (SOC) of the system’s Hamiltonian. When some of the electrons in the reactants or the products are unpaired, this may lead to a change of the total electronic spin during the reaction. In 1999, he took up a temporary lectureship in Bristol his research interests are in computational chemistry and reaction mechanisms, particularly in transition metal chemistry.ġ Introduction In chemical reactions, bonding electrons get redistributed so that chemical bonds are broken and formed. He spent two years as a Humboldt fellow with Helmut Schwarz at the Technische Universität Berlin, carrying out experimental and computational studies of gas-phase ion-molecule reactions, then one year with Benny Gerber at the Hebrew University, Jerusalem. He studied Chemistry and Philosophy at the Université Catholique de Louvain, completing his PhD on organic reaction mechanisms with Heinz Viehe in 1995. Jeremy Harvey was born in Britain in 1969 but moved to Belgium aged five. He has held Visiting Professor positions at the Technische Universität München (1993–94), Tokyo Metropolitan University (1995), the Inorganic Chemistry Laboratory at Oxford (1998), and Los Alamos National Laboratory (20). He started his career at the University of Maryland, College Park, where he rose in the ranks to Full Professor, and moved to his current Professorial position in 1996. He spent one year with Geoffrey Wilkinson (exchange student, 1983–84) and two years with Al Cotton (research associate, 1985–87). Rinaldo Poli obtained his PhD in 1985 from the Scuola Normale Superiore in Pisa under the supervision of Fausto Calderazzo. Density functional theory, especially using gradient-corrected and hybrid functionals, performs reasonably well for the difficult problem of predicting the energy splitting between different spin states of transition metal complexes, although careful calibration is needed. This computational approach is of great help in understanding the mechanisms of spin-forbidden reactions, provided that accurate calculations can be used to predict the relevant PESs. To explain these observations, we discuss the shape of the different potential energy surfaces (PESs) involved, and the energy of the minimum energy crossing points (MECPs) between them. Spin-forbidden oxidative addition of ethylene to occurs competitively with ligand association. For example, ligand substitution in is faster than expected, whereas addition of dinitrogen to is slow. These reactions may proceed faster, slower-or at the same rate as-otherwise equivalent processes in which spin is conserved. Many reactions of transition metal compounds involve a change in spin.
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