The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. As in organic chemistry, the mechanisms of transition metal reactions are typically inferred from experiments that examine the concentration dependence of the incoming and outgoing ligands on the reaction rate, the detection of intermediates, and the stereochemistry of the reactants and products. The resulting rate law is: \[Rate= \frac{k_{1}k_{2}[Y][ML_{n}]}{k_{-1}[L] + k_{2}[Y]}\]. If the rate determining step is the dissociation of L from the complex, then the concentration of Y does not affect the rate of reaction, leading to the first-order rate law: Illustration of the dissociative ligand substitution mechanism for an ML6 complex. Watch the recordings here on Youtube! In contrast, the slightly more compact ion [Ni(H2O)6]2+ ion exchanges water via the Id mechanism.[23]. L n − 1M − L − L, k1 ⇌ + L, k − 1 L n − 1M − + Y, k2 → L n − 1M − Y. Highly charged cationic complexes tend to form ion pairs with anionic ligands, and these ion pairs often undergo reactions via the Ia pathway. Let's consider a very commmon and simple ligand exchange reaction, which is the substitution of one water molecule for another in an octahedral [M(H2O)6]n+ complex. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0. The Trans Effect, which is connected with the associative mechanism, controls the stereochemistry of certain ligand substitution reactions. Henry Taube, who studied the mechanisms of ligand exchange reactions in simple test tube experiments, classified transition metal complexes as labile if their reaction half-life was one minute or less, and inert if they took longer to react. In the case of Cr3+ and V2+, the energy penalty for distorting the complex away from octahedral symmetry - to make, for example, a 5- or 7-coordinate intermediate - is particularly high. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Examples of associative mechanisms are commonly found in the chemistry of d8 square planar metal complexes, e.g. This mechanism is illustrated below for ligand substitution on an octahedral ML 6 complex. Complexes that undergo associative substitution are typically either coordinatively unsaturated or contain a ligand that can change its bonding to the metal, e.g. On the timescale of most laboratory experiments, the Taube definition of lability is a useful one for classifying reactions into those that have low and high activation energies. As we will see, the crystal field stabilization energy (CFSE) plays a key role in determining the activation energy and therefore the rate of ligand substitution. Kinetically, however, the complex is labile, meaning that it can exchange its ligands rapidly. These compounds (ML4) bind the incoming (substituting) ligand Y to form pentacoordinate intermediates ML4Y, which in a subsequent step dissociate one of their ligands. When we think about the reactions of transition metal complexes, it is important to recall the distinction between their thermodynamics and kinetics. [ "article:topic", "showtoc:no", "Berry pseudorotation", "license:ccbysa" ], For p-block elements, faster exchange occurs with larger ions (e.g., Ba. The labilization of trans ligands is attributed to electronic effects and is most notable in square planar complexes, but it can also be observed with octahedral complexes. The trans effect refers to the labilization (making more reactive) of ligands that are trans to certain other ligands, the latter being referred to as trans-directing ligands. The third and final equatorial site is occupied by the departing trans ligand, so the net result is that the kinetically favored product is the one in which the ligand trans to the one with the largest trans effect is eliminated. Thus, the entropy of activation is negative, which indicates an increase in order in the transition state. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. For example, when CH3Cl is reacted with the hydroxyl ion (OH-), it will lead to the formation of the original molecule called methanol with that hydroxyl ion. As it does so, it replaces a weaker nucleophile which then becomes a leaving group; The remaining positive or partially positive atom becomes an electrophile. [18] The cis effect is most often observed in octahedral complexes. In this case, we can simplify the problem by assuming a low steady-state concentration of the MLn intermediate. The electrostatically held nucleophilic incoming ligand can exchange positions with a ligand in the first coordination sphere, resulting in net substitution. In the case of an octahedral complex, this reaction would be first order in ML6 and zero order in Y, but only if the highest energy transition state is the one that precedes the formation of the ML5 intermediate. The first step is typically rate determining. In homogeneous catalysis, the associative pathway is desirable because the binding event, and hence the selectivity of the reaction, depends not only on the nature of the metal catalyst but also on the molecule that is involved in the catalytic cycle. The dynamic range of ligand substitution rates is enormous, spanning at least 15 orders of magnitude. In organic (and inorganic) chemistry, nucleophilic substitution is a fundamental class of reactions in which a nucleophile selectively bonds with or attacks the positive or partially positive charge on an atom or a group of atoms. Dissociation of Y results in no reaction, but dissociation of L results in net substitution, yielding the d8 complex ML3Y. Since the products (except for the label) are the same as the reactants, we know that ΔG° = 0 and Keq = 1 for this reaction. The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate. If, on the other hand, one starts from Pt(NH3)42+, the trans product is obtained instead: The trans effect in square complexes can be explained in terms of the associative mechanism, described above, which goes through a trigonal bipyramidal intermediate. Associative reactions follow second order kinetics: the rate of the appearance of product depends on the concentration of both ML4 and Y. For example the exchange between a 13C labeled CN- ion and a bound CN- ligand occurs on the timescale of tens of milliseconds: \[\ce{[Ni(CN)4]^{2-}_{(aq)} + *CN^{-}_{(aq)} -> [Ni(CN)3(*CN)]^{2-} + CN^{-}_{(aq)}} \: \: k_{exchange} \approx 10^{2}M^{-1}s^{-1}\] What we find is that octahedral complexes that have high CFSE (Cr3+, V2+) tend to be inert. Missed the LibreFest? Ligands with a high kinetic trans effect are in general those with high π acidity (as in the case of phosphines) or low-ligand lone-pair–dπ repulsions (as in the case of hydride), which prefer the more π-basic equatorial sites in the intermediate. Based on the rules we developed for calculating the CFSE of transition metal complexes, we can now predict the trends in ligand substitution rates: Ligand Substitution Mechanisms. Some authors prefer the term trans influence to distinguish this from the kinetic effect,[19] while others use more specific terms such as structural trans effect or thermodynamic trans effect.