The elementary stages of organic reactions catalyzed by acids, bases, nucleophilic catalysts, metal complexes, solid metals and their compounds in gas-phase or liquid-phase heterogeneous and homogeneous processes are the formation and transformation of various organic and organometallic intermediates, as well as metal complexes. Organic intermediate compounds include carbenium ions R +, carbonium RH 2 +, carbo-anions R-, anion- and cation radicals, radicals and biradicals R, R:, as well as molecular complexes of organic donor and acceptor molecules (DA), which are called also complexes with renos of the charge. In homogeneous and heterogeneous catalysis by metal complexes (metal complex catalysis) of organic reactions, intermediates are complex (coordination) compounds with organic and inorganic ligands, organometallic compounds with an M-C bond, which in most cases are coordination compounds. A similar situation takes place in the case of “two-dimensional” chemistry on the surface of solid metal catalysts. Let us consider the main types of reactions of metal complexes and organometallic compounds.
Elementary stages involving metal complexesThe reactions of metal complexes can be divided into three groups:
a) reactions of electron transfer;
b) ligand substitution reactions;
c) reactions of coordinated ligands.
Electron transfer reactions
Two mechanisms are realized in reactions of electron transfer - the outer-sphere mechanism (without changes in the coordination spheres of the donor and acceptor) and the bridging (inner-sphere) mechanism, leading to changes in the coordination sphere of the metal.
Let us consider the outer-sphere mechanism using the example of octahedral complexes of transition metals. In the case of symmetric reactions ( G 0 = 0)
the rate constants vary in a very wide range of values - from 10-12 to 10 5 L · mol-1 · sec-1, depending on the electronic configuration of the ion and the extent of its resetting in the course of the process. In these reactions, the principle of least movement is very clearly manifested - the least change in the valence shell of the participants in the reaction.
In the electron transfer reaction (1) (Co * is an isotope of the Co atom)
(symmetric reaction), Co 2+ (d 7) ᴨȇ transforms into Co 3+ (d 6). The electronic configuration (valence shell) does not change during this transfer
6 electrons at the threefold degenerate binding level remain unchanged (), and from the antibonding e g level, one electron is removed.
Second order rate constant for reaction (1) k 1 = 1.1 lmol - 1 sec - 1. Since Phen (phenanthroline) is a strong ligand, the maximum number of 7 d-electrons are paired (spin-paired state). In the case of a weak ligand NH 3, the situation changes dramatically. Co (NH 3) n 2+ (n = 4, 5, 6) is in a spin-unpaired (high-spin) state.
The stronger complex Co (NH 3) 6 3+ (stronger than Co (NH 3) 6 2+ ~ 10 30 times) is in a spin-coupled state, like the complex with Phen. In this regard, in the process of electron transfer, the valence shell should be strongly restructured and, as a result, k= 10 - 9 lmol - 1 sec - 1. The degree of conversion of Co 2+ to Co 3+, equal to 50%, is achieved in the case of the Phen ligand in 1 second, and in the case of NH 3 in ~ 30 years. Obviously, a stage with such a rate (formally elementary) can be excluded from the set of elementary stages when analyzing the reaction mechanisms.
The magnitude G for the electron transfer reaction during the formation of a collision complex, according to the Marcus theory, it includes two components and
The first term is the energy of reorganization of M-L bonds within the complex (the length and strength of the bond with a change in the valence state). The quantity includes the energy of the rearrangement of the outer solvation shell in the process of changing the coordinates M-L and the charge of the complex. The smaller the change in the electronic environment and the smaller the change in the M-L length, the lower, the larger the ligands, the lower and, as a result, the higher the rate of electron transfer. The value for the general case can be calculated using the Marcus equation
where. For = 0.
In the case of the inner-sphere mechanism, the electron transfer process is facilitated, since one of the ligands of the first complex forms a bridging complex with the second complex, displacing one of the ligands from it
The rate constants of such a process are 8 orders of magnitude higher than the constants for the reduction of Cr (NH 3) 6 3+. In such reactions, the reducing agent must be a labile complex, and the ligand in the oxidizing agent must be capable of forming bridges (Cl-, Br-, I-, N 3 -, NCS-, bipy).
Ligand Substitution ReactionsOne of the most important stages in metal complex catalysis - the interaction of the substrate Y with the complex - occurs according to three mechanisms:
a) Replacement of the ligand with a solvent. This stage is usually depicted as the dissociation of the complex
The essence of the process in most cases is the replacement of the ligand L with a solvent S, which is then easily replaced by a substrate molecule Y
b) Attachment of a new ligand along the free coordinate with the formation of an associate followed by dissociation of the replaced ligand
c) Synchronous substitution (type S N 2) without the formation of an intermediate
In the case of Pt (II) complexes, the reaction rate is very often described by the two-route equation
where k S and k Y- rate constants of processes proceeding by reactions (5) (with solvent) and (6) with ligand Y. For example,
The last stage of the second route is the sum of three fast elementary stages - cleavage of Cl-, addition of Y, and cleavage of the H2O molecule.
In planar square complexes of transition metals, the trans effect, formulated by II Chernyaev, is observed - the effect of LT on the rate of substitution of the ligand in the trans position to the LT ligand. For Pt (II) complexes, the trans effect increases in the series of ligands:
H 2 O ~ NH 3< Cl- ~ Br- < I- ~ NO 2 - ~ C 6 H 5 - < CH 3 - <
< PR 3 ~ AsR 3 ~ H- < олефин ~ CO ~ CN-.
The presence of the kinetic trans effect and thermodynamic trans effect explains the possibility of the synthesis of inert isomeric complexes of Pt (NH 3) 2 Cl 2:
Coordinated ligand reactions§ Reactions of electrophilic substitution (S E) of hydrogen by a metal in the coordination sphere of the metal and processes inverse to them
SH - H 2 O, ROH, RNH 2, RSH, ArH, RCCH.
Even H 2 and CH 4 molecules are involved in reactions of this type
§ Insertion reactions L on the M-X link
In the case of X = R (organometallic complex), metal-coordinated molecules are also incorporated into the M-R bond (L - CO, RNC, C 2 H 2, C 2 H 4, N 2, CO 2, O 2, etc.). Insertion reactions are the result of an intramolecular attack of a nucleophile X on a molecule coordinated by - or - type. Reverse reactions - reactions - and - elimination
§ Reactions of oxidative addition and reductive elimination
M 2 (C 2 H 2) M 2 4+ (C 2 H 2) 4-
Apparently, in these reactions there is always a preliminary coordination of the attached molecule, but this is not always possible to fix. In this regard, the presence of a free site in the coordination sphere or a site associated with a solvent, which is easily replaced by a substrate, is an important factor affecting the reactivity of metal complexes. For example, bis-allyl complexes of Ni are good precursors of catalytically active particles, since a complex with a solvent appears due to the easy reductive elimination of bis-allyl, the so-called. “Bare” nickel. The role of empty seats is illustrated by the following example:
§ Reactions of nucleophilic and electrophilic addition to - and - metal complexes
Reactions of organometallic compoundsAs intermediates in catalytic reactions, there are both classical organometallic compounds with MC, M = C, and MC bonds, and nonclassical compounds in which the organic ligand is coordinated to the 2, 3, 4, 5, and 6 -types, or is an element of electron-deficient structures - bridging СН 3 and С 6 Н 6 -groups, non-classical carbides (Rh 6 C (CO) 16, C (AuL) 5 +, C (AuL) 6 2+, etc.).
Among the digital mechanisms for classical -metallic compounds, we note several mechanisms. Thus, 5 mechanisms of electrophilic substitution of a metal atom at the M-C bond have been established.
electrophilic substitution with nucleophilic assistance
AdE Attachment-Elimination
AdE (C) Attachment to the C atom in sp 2 -hybridization
AdE (M) Addition oxidative to metal
Nucleophilic substitution at a carbon atom in demetallation reactions of organometallic compounds occurs as a redox process:
The participation of an oxidizing agent in such a stage is possible.
CuCl 2, p-benzoquinone, NO 3 - and other compounds can serve as such an oxidizing agent. Here are two more elementary stages typical for RMX:
hydrogenolysis of the M-C bond
and homolysis of the M-C bond
An important rule relating to all reactions of complex and organometallic compounds and associated with the principle of least motion is Tolman's 16-18 electron shell rule (Section 2).
Coordination and organometallic compoundson the surfaceAccording to modern concepts, complexes and organometallic compounds are formed on the surface of metals, similar to compounds in solutions. For surface chemistry, the participation of several surface atoms in the formation of such compounds and, of course, the absence of charged particles are essential.
Surface groups can be any atoms (H, O, N, C), groups of atoms (OH, OR, NH, NH 2, CH, CH 2, CH 3, R), coordinated molecules CO, N 2, CO 2, C 2 H 4, C 6 H 6. For example, upon adsorption of CO on a metal surface, the following structures were found:
The C 2 H 4 molecule on the metal surface forms β-complexes with one center and di-bonded ethylene bridges M-CH 2 CH 2 -M, i.e. essentially metal cycles
On the surface of Rh, for example, during the adsorption of ethylene, the following processes of conversion of ethylene occur as the temperature rises:
Reactions of surface intermediates include the stages of oxidative addition, reductive elimination, insertion, - and - elimination, hydrogenolysis of M-C and C-C bonds, and other reactions of the organometallic type, but without the appearance of free ions. The tables show the mechanisms and intermediates of surface transformations of hydrocarbons on metals.
Table 3.1. Catalytic reactions involving the cleavage of the C-C bond.
Legend:
Alkyl, metal cycle;
Carben, allyl;
Carbyne, vinyl.
Table 3.2. Catalytic reactions involving the formation of a C-C bond.
Designations: see table. 3.1.
The formation of all the above organometallic compounds on the surface of metals was confirmed by physical methods.
Questions for self-control
1) How is the rule of the least change in the valence shell of a metal manifested in the course of ES in reactions of electron transfer?
2) Why do coordination vacancies promote effective interaction with the substrate?
3) List the main types of coordinated ligand reactions.
4) Give the mechanisms of electrophilic substitution in the reactions of organometallic compounds with HX.
5) Give examples of surface organometallic compounds.
6) Give examples of the participation of metal-carbene surface complexes in the transformations of hydrocarbons.
Advanced Study Literature
1. Temkin ON, Kinetics of catalytic reactions in solutions of metal complexes, M., MITHT, 1980, Part III.
2. Callman J., Higedas L., Norton J., Finke R., Organometallic Chemistry of Transient Metals, M., Mir, 1989, vol. I, vol. II.
3. Moiseev II, -Complexes in the oxidation of olefins, M., Nauka, 1970.
4. Temkin ON, Shestakov GK, Treger YA, Acetylene: Chemistry. Reaction mechanisms. Technology. M., Chemistry, 1991, 416 p., Section 1.
5. Henrici-Olive G., Olive S., Coordination and catalysis, M., Mir, 1980, 421 p.
6. Krylov OV, Matyshak VA, Intermediate compounds in heterogeneous catalysis, M., Nauka, 1996.
7. Zaera F., An Organometallic Guide to the Chemistry of Hydrocarbon Moities on Transition Metal Surfaces., Chem. Rev., 1995, 95, 2651-2693.
8. Bent B.E., Mimicking Aspects of Heterogeneous Catalysis: Generating, Isolating, and Reacting Proposed Surface Intermediates on Single Crystals in Vacuum, Chem. Rev. 1996, 96, 1361-1390.
One of the most important stages in metal complex catalysis - the interaction of the substrate Y with the complex - occurs according to three mechanisms:
a) Replacement of the ligand with a solvent. This stage is usually depicted as the dissociation of the complex
The essence of the process in most cases is the replacement of the ligand L with a solvent S, which is then easily replaced by a substrate molecule Y
b) Attachment of a new ligand along the free coordinate with the formation of an associate followed by dissociation of the replaced ligand
c) Synchronous substitution (type S N 2) without the formation of an intermediate
In the case of Pt (II) complexes, the reaction rate is very often described by the two-route equation
where k S and k Y- rate constants of processes proceeding by reactions (5) (with solvent) and (6) with ligand Y. For instance,
The last stage of the second route is the sum of three fast elementary stages - the cleavage of Cl -, the addition of Y, and the elimination of the H2O molecule.
In planar square complexes of transition metals, the trans effect, formulated by II Chernyaev, is observed - the effect of LT on the rate of substitution of the ligand in the trans position to the LT ligand. For Pt (II) complexes, the trans effect increases in the series of ligands:
H 2 O ~ NH 3 The presence of the kinetic trans effect and thermodynamic trans effect explains the possibility of the synthesis of inert isomeric complexes of Pt (NH 3) 2 Cl 2: Reactions of electrophilic substitution (S E) of hydrogen by a metal in the coordination sphere of a metal and processes inverse to them SH - H 2 O, ROH, RNH 2, RSH, ArH, RCCH. Even H 2 and CH 4 molecules are involved in reactions of this type Insertion reactions L by connection M-X In the case of X = R (organometallic complex), metal-coordinated molecules are also incorporated into the M-R bond (L – CO, RNC, C 2 H 2, C 2 H 4, N 2, CO 2, O 2, etc.). The insertion reactions are the result of an intramolecular attack of a nucleophile X on a molecule coordinated by the-or-type. Reverse reactions - reactions of-and-elimination Oxidative addition and reductive elimination reactions M 2 (C 2 H 2) M 2 4+ (C 2 H 2) 4– Apparently, in these reactions there is always a preliminary coordination of the attached molecule, but this is not always possible to fix. Therefore, the presence of a free site in the coordination sphere or a site associated with a solvent, which is easily replaced by a substrate, is an important factor affecting the reactivity of metal complexes. For example, bis--allyl complexes of Ni are good precursors of catalytically active particles, since a complex with a solvent appears due to the easy reductive elimination of bis-allyl, the so-called. “Bare” nickel. The role of empty seats is illustrated by the following example: Reactions of nucleophilic and electrophilic addition to - and-metal complexes As intermediates in catalytic reactions, there are both classical organometallic compounds with MC, M = C, and MC bonds, and nonclassical compounds in which the organic ligand is coordinated to the 2, 3, 4, 5, and 6 -types, or is an element of electron-deficient structures - bridging СН 3 and С 6 Н 6 -groups, non-classical carbides (Rh 6 C (CO) 16, C (AuL) 5 +, C (AuL) 6 2+, etc.). Among the specific mechanisms for classical -organometallic compounds, we note several mechanisms. Thus, 5 mechanisms of electrophilic substitution of a metal atom at the M-C bond have been established. electrophilic substitution with nucleophilic assistance AdE Attachment-Elimination AdE (C) Attachment to the C atom of bsp 2 -hybridization AdE (M) Addition oxidative to metal Nucleophilic substitution at a carbon atom in demetallation reactions of organometallic compounds occurs as a redox process: The participation of an oxidizing agent in such a stage is possible. CuCl 2, p-benzoquinone, NO 3 - and other compounds can serve as such an oxidizing agent. Here are two more elementary stages characteristic of RMX: hydrogenolysis of the M-C bond and homolysis of the M-C bond An important rule relating to all reactions of complex and organometallic compounds and associated with the principle of least motion is Tolman's 16-18 electron shell rule (Section 2). Ligands are ions or molecules that are directly bound to the complexing agent and are donors of electron pairs. These electron-abundant systems, which have free and mobile electron pairs, can be electron donors, for example: Compounds of p-elements exhibit complexing properties and act as ligands in a complex compound. Ligands can be atoms and molecules (protein, amino acids, nucleic acids, carbohydrates). The efficiency and strength of the donor-acceptor interaction of the ligand and the complexing agent is determined by their polarizability — the ability of the particle to transform its electron shells under external influence. Knest = 2 / To mouth = 1 / Knest Ligand Substitution Reactions One of the most important stages in metal complex catalysis - the interaction of the substrate Y with the complex - occurs according to three mechanisms: a) Replacement of the ligand with a solvent. This stage is usually depicted as the dissociation of the complex The essence of the process in most cases is the replacement of the ligand L with a solvent S, which is then easily replaced by a substrate molecule Y b) Attachment of a new ligand along the free coordinate with the formation of an associate followed by dissociation of the replaced ligand c) Synchronous substitution (type S N 2) without the formation of an intermediate Ideas about the structure of metalloenzymes and other biocomplex compounds (hemoglobin, cytochromes, cobalamins). Physicochemical principles of oxygen transport by hemoglobin. Features of the structure of metalloenzymes. Biocomplex compounds vary considerably in stability. The role of the metal in such complexes is highly specific: replacing it even with an element similar in properties leads to a significant or complete loss of physiological activity. 1.
B12: contains 4 pyrrole rings, cobalt ion and CN- groups. Promotes the transfer of the H atom to the C atom in exchange for any group, participates in the formation of deoxyribose from ribose. 2.
hemoglobin: has a quaternary structure. The four polypeptide chains are joined together to form an almost regular ball shape, where each chain contacts two chains. Hemoglobin- a respiratory pigment that gives blood a red color. Hemoglobin is composed of protein and iron porphyrin and carries oxygen from the respiratory system to the tissues of the body and carbon dioxide from them to the respiratory organs. Physicochemical principles of oxygen transport by hemoglobin- Atom (Fe (II)) (one of the components of hemoglobin) is capable of forming 6 coordination bonds. Of these, four are used to anchor the Fe (II) atom itself in the heme, the fifth bond is used to bind the heme to the protein subunit, and the sixth bond is used to bind the O 2 or CO 2 molecule. Metal-ligand homeostasis and the reasons for its violation. The mechanism of the toxic action of heavy metals and arsenic based on the theory of hard and soft acids and bases (HMCO). Thermodynamic principles of chelation therapy. The mechanism of the cytotoxic action of platinum compounds. In the body, the formation and destruction of biocomplexes from metal cations and bioligands (porphins, amino acids, proteins, polynucleotides), which include donor atoms of oxygen, nitrogen, and sulfur, are continuously occurring. The exchange with the environment maintains the concentration of these substances at a constant level, providing metal- ligand homeostasis.
Violation of the existing balance leads to a number of pathological phenomena - metal-excess and metal-deficient states. An example is an incomplete list of diseases associated with a change in the metal-ligand balance for only one ion - the copper cation. Deficiency of this element in the body causes Menkes syndrome, Morfan syndrome, Wilson-Konovalov disease, liver cirrhosis, pulmonary emphysema, aorto- and arteriopathy, anemia. Excessive intake of cation can lead to a series of diseases of various organs: rheumatism, bronchial asthma, inflammation of the kidneys and liver, myocardial infarction, etc., called hypercupremias. Also known professional hypercureosis - copper fever. The circulation of heavy metals occurs partially in the form of ions or complexes with amino acids, fatty acids. However, the leading role in the transport of heavy metals belongs to proteins, which form a strong bond with them. They are fixed on the cell membranes, block the thiol groups of membrane proteins- 50% of them are enzyme proteins that disrupt the stability of the protein-lipid complexes of the cell membrane and its permeability, causing potassium to leave the cell and the penetration of sodium and water into it. A similar effect of these poisons, which are actively fixed on red blood cells, leads to disruption of the integrity of erythrocyte membranes, inhibition of the processes of aerobic glycolysis and metabolism in them in general, and the accumulation of hemolytically active hydrogen peroxide due to inhibition of peroxidase in particular, which leads to the development of one of the characteristic symptoms of poisoning with compounds this group - to hemolysis. The distribution and deposition of heavy metals and arsenic occurs in almost all organs. Of particular interest is the ability of these substances to accumulate in the kidneys, which is explained by the rich content of thiol groups in the kidney tissue, the presence of a protein in it - metallobionin, which contains a large number of thiol groups, which contributes to the long-term deposition of poisons. The liver tissue, also rich in thiol groups and containing metallobionin, is also distinguished by a high degree of accumulation of toxic compounds of this group. The deposit period, for example, of mercury can be up to 2 months or more. The release of heavy metals and arsenic occurs in different proportions through the kidneys, liver (with bile), the mucous membrane of the stomach and intestines (with feces), sweat and salivary glands, lungs, which is usually accompanied by damage to the excretory apparatus of these organs and is manifested by the corresponding clinical symptomatology. The lethal dose for soluble mercury compounds is 0.5 g, for calomel 1–2 g, for copper sulphate 10 g, for lead acetate 50 g, for lead white 20 g, for arsenic 0.1–0.2 g. The concentration of mercury in the blood is considered toxic more than 10 μg / L (1γ%), in urine more than 100 μg / L (10γ%), the concentration of copper in the blood is more than 1600 μg / L (160γ%), arsenic is more than 250 μg / L (25γ %) in urine. Chelation therapy is the elimination of toxic particles from the body, based on their chelation complexonates of s – elements. Drugs used for excretion toxic substances incorporated in the body particles are called detoxifiers. The reactions of coordination compounds always occur in the coordination sphere of the metal with the ligands bound in it. Therefore, it is obvious that in order for anything to happen at all, ligands must be able to get into this sphere. This can happen in two ways: We already got acquainted with the first method when we discussed coordination unsaturation and the 18-electron rule. We will deal with the second here. But usually there is an unspoken rule - the number of occupied coordination places does not change. In other words, the substitution does not change the electron count. Substitution of a ligand of one type for another is quite possible and often occurs in reality. Let us only pay attention to the correct handling of charges when the L-ligand is changed to the X-ligand and vice versa. If we forget about this, then the oxidation state of the metal will change, and the substitution of ligands is not a redox process (if you find or come up with a nasty example, let me know - automatic set-off right away, if I cannot prove that you are mistaken, why even in In this case, I guarantee a positive contribution to karma). With more complex ligands, there is no more difficulty - you just need to remember the rather obvious rule: the number of ligand sites (that is, the total number of ligands or ligand centers of the X- or L-types) is preserved. This follows directly from the preservation of the electron count. Here are some self-explanatory examples. Let's take a look at the last example. The starting reagent for this reaction is iron dichloride FeCl 2. Until recently, we would have said: "It's just salt, where does the coordination chemistry?" But we will no longer allow ourselves such ignorance. In the chemistry of transition metals, there are no “just salts”; any derivatives are coordination compounds to which all reasoning about electrons, d-configuration, coordination saturation, etc. is applicable. Iron dichloride, as we are used to writing it, would turn out to be a Fe (2+) complex of the MX 2 type with the d 6 configuration and the number of electrons 10. Not enough! Fine? After all, we have already figured out that ligands are implicit. To make a reaction, we need a solvent, and for such reactions it is most likely THF. The dissolution of the crystalline iron salt in THF occurs precisely because the donor solvent takes up free spaces, and the energy of this process compensates for the destruction of the crystal lattice. We would not be able to dissolve this “salt” in a solvent that does not provide metal solvation services due to the Lewis basicity. In this case, and in a million others, solvation is simply a coordination interaction. Let us write, just for definiteness, the result of solvation in the form of the FeX 2 L 4 complex, in which two chlorine ions remain in the coordination sphere in the form of two X ligands, although most likely they are also displaced by donor solvent molecules with the formation of a charged complex FeL 6 2+... In this case, it is not so important. And so and so, we can safely assume that we have an 18-electronic complex both on the left and on the right. If we remember organic chemistry, then there were two mechanisms of substitution at a saturated carbon atom - SN1 and SN2. In the first, the substitution took place in two stages: the old substitute first left, leaving a vacant orbital on the carbon atom, which was followed by a new substituent with a pair of electrons. The second mechanism assumed that departure and arrival were carried out simultaneously, in a coordinated manner, and the process was a one-step process. In the chemistry of coordination compounds, it is quite possible to imagine something similar. But a third possibility appears, which the saturated carbon atom did not have - first we attach a new ligand, then we unhook the old one. It immediately becomes clear that this third option is hardly possible if the complex already has 18 electrons and is coordinatively saturated. But it is quite possible if the number of electrons is 16 or less, that is, the complex is unsaturated. Let us immediately recall the obvious analogy from organic chemistry - nucleophilic substitution at an unsaturated carbon atom (in an aromatic ring or at a carbonyl carbon) also proceeds first as the addition of a new nucleophile, and then the elimination of the old one. So, if we have 18 electrons, then the substitution proceeds as elimination-addition (fans of “smart” words use the term dissociative-associative or simply dissociative mechanism). Another way would require the expansion of the coordination sphere to a count of 20 electrons. This is not absolutely impossible, and such options are sometimes even considered, but it is definitely very unprofitable and every time a suspicion of such a path requires very strong evidence. In most of these stories, researchers eventually come to the conclusion that they have overlooked or left out something, and the associative mechanism has been rejected. So, if the original complex has 18 electrons, then first one ligand must leave, then a new one must come in its place, for example: If we want to introduce into the coordination sphere a hapto-ligand occupying several places, then first we must free them all. As a rule, this occurs only under sufficiently severe conditions, for example, in order to replace three carbonyls with η 6 -benzene in the chromium carbonyl, the mixture is heated for many hours under pressure, from time to time releasing the released carbon monoxide. Although the scheme depicts the dissociation of three ligands with the formation of a very unsaturated complex with 12 electrons, in reality the reaction most likely occurs in stages, one carbonyl is removed, and benzene enters the sphere, gradually increasing the haptiness, through the stages minus CO - dihapto - minus one more CO - tetragapto - minus one more CO - hexagapto, so that less than 16 electrons are not obtained. So, if we have a complex with 16 electrons or less, then the ligand substitution most likely proceeds as an addition-elimination (for those who like thoughtful words: associative-dissociative or simply associative): a new ligand comes first, then the old one leaves. Two obvious questions arise: why does the old ligand leave, because 18 electrons is very good, and why not do it the other way around in this case, as in 18-electron complexes. The first question is easy to answer: each metal has its own habits, and some metals, especially from the late ones, with almost completely filled d-shells, prefer 16-electron counting and the corresponding structural types, and therefore throw out an extra ligand, returning to their favorite configuration. Sometimes the space factor still interferes with the matter, the already existing ligands are large and the additional one feels like a bus passenger at rush hour. It's easier to get off and walk on foot than to suffer like that. However, you can shove out another passenger, let him take a walk, and we'll go. The second question is also simple - in this case, the dissociative mechanism would first have to give a 14-electron complex, and this is rarely beneficial. Here's an example. For a change, we will replace the X-ligand with the L-ligand, and we will not get confused in the oxidation states and charges. Once again: upon substitution, the oxidation state does not change, and if the X-ligand is gone, then the loss must be compensated for by the charge on the metal. If we forget about this, then the oxidation state would decrease by 1, and this is not true. And one more oddity. A metal-pyridine bond was formed due to the lone pair on nitrogen. In organic chemistry, in this case, we would definitely show a plus on pyridine nitrogen (for example, during protonation or the formation of a quaternary salt), but we never do this in coordination chemistry either with pyridine or with any other L-ligands. This is terribly annoying for everyone who is used to the strict and unambiguous system of drawing structures in organic chemistry, but it will take some getting used to, it is not that difficult. And there is no exact analogue of SN2 in the chemistry of coordination compounds, there is a distant one, but it is relatively rare and we do not really need it. We could not talk about the mechanisms of ligand substitution at all, if not for one extremely important circumstance that we will use a lot: ligand substitution, be it associative or dissociative, necessarily presupposes dissociation of the old ligand. And it is very important for us to know which ligands leave easily and which ones leave poorly, preferring to remain in the coordination sphere of the metal. As we will soon see, in any reaction, some of the ligands remain in the coordination sphere and do not change. Such ligands are usually called spectator ligands (if you do not want such simple, “unscientific” words, use the English word spectator in the local transcription spectator, spectator ligand, but, I beg you, not spectator - it's unbearable!). And a part is directly involved in the reaction, turning into reaction products. Such ligands are called actors (not actors!), That is, acting. It is quite clear that the ligand-actors need to be easily introduced and removed into the coordination sphere of the metal, otherwise the reaction will simply get stuck. But it is better to leave ligands-spectators in the coordination sphere for many reasons, but at least for such a banal one as the need to avoid unnecessary fuss around the metal. It is better that only ligands and actors and in the required quantities can participate in the required process. If there are more available coordination sites than necessary, unnecessary ligand-actors can settle on them, and even those that will participate in side reactions, reducing the yield of the target product and selectivity. In addition, spectator ligands almost always perform many important functions, for example, provide the solubility of complexes, stabilize the correct valence state of the metal, especially if it is not quite usual, help individual steps, provide stereoselectivity, etc. We are not decoding yet, because we will discuss all this in detail when we get to specific reactions. It turns out that some of the ligands in the coordination sphere should be firmly bound and not prone to dissociation and substitution by other ligands. Such ligands are usually called coordinatively stable
... Or simply stable, if it is clear from the context that we are talking about the bond strength of the ligands, and not about their own thermodynamic stability, which just does not bother us at all. And ligands that easily and willingly enter and exit, and are always ready to give way to others, are called coordination labile
, or simply labile, and here, fortunately, there are no ambiguities. This is probably the most striking example of the fact that in the coordination sphere a very unstable molecule can become an excellent ligand, and by definition it is coordinatively stable, if only because if it dares to leave the warm and cozy sphere outside, nothing good awaits it (at the cost the way out is just the energy of antiaromatic destabilization). Cyclobutadiene and its derivatives are the most famous examples of anti-aromatics. These molecules exist only at low temperatures, and in a highly distorted form - in order to get as far as possible from antiaromaticity, the cycle is distorted into an elongated rectangle, removing delocalization and maximally weakening the conjugation of double bonds (otherwise this is called the Jahn-Teller effect of the second kind: degenerate system, and cyclobutadiene-square is a degenerate biradical, remember the Frost circle - it is distorted and reduces symmetry to remove the degeneracy). But in the complexes, cyclobutadiene and substituted cyclobutadienes are excellent tetragapto ligands, and the geometry of such ligands is exactly a square, with the same bond lengths. How and why this happens is a separate story, and not nearly as obvious as it is often presented. It should be understood that there is no reinforced concrete fence with barbed wire and guard towers between the areas of labile and stable ligands. Firstly, it depends on the metal, and in this context, ZhMKO works well. For example, late transition metals prefer soft ligands, while early transition metals prefer hard ones. For example, iodide is very tightly attached to the d 8 atoms of palladium or platinum, but rarely enter the coordination sphere of titanium or zirconium in the d 0 configuration at all. But in many metal complexes with less pronounced features, iodide manifests itself as a completely labile ligand, easily giving way to others. All other things being equal: Let's do it again, only from the other side In the coordination sphere, metals are preserved (are coordinatively stable) as a rule: The latter condition looks strange, but imagine a complex that has many different ligands, among which there are no unconditionally stable ones (no chelators and polyhapto ligands). Then the ligands in the reactions will change, relatively speaking, in the order of relative lability. The least labile and will remain the last. This trick takes place, for example, when we use palladium phosphine complexes. Phosphines are relatively stable ligands, but when there are many of them, and the metal is rich in electrons (d 8, d 10), they give way, one after the other, to ligand-actors. But the last phosphine ligand usually remains in the coordination sphere, and this is very good from the point of view of the reactions in which these complexes are involved. We will return to this important issue later. Here is a fairly typical example, when from the initial coordination sphere of the palladium phosphine complex in the Heck reaction, only one remains, the “last” phosphine. This example brings us very close to the most important concept in transition metal complex reactions - the ligand control concept. We will discuss it later. When replacing some ligands with others, it is important not to overdo it with the reactivity of the incoming ligand. When we deal with the reactions of organic molecules, it is important for us to deliver exactly one molecule of each of the reagents to the coordination sphere. If instead of one there are two molecules, there is a high probability of side reactions involving two identical ligands. A loss of reactivity is also possible due to saturation of the coordination sphere and the impossibility of introducing into it other ligands necessary for the expected process. This problem occurs especially often when strong anionic nucleophiles, for example, carbanions, are introduced into the coordination sphere. To avoid this, less reactive derivatives are used, in which, instead of an alkali metal cation, which causes a high ionicity of the bond, less electropositive metals and metalloids (zinc, tin, boron, silicon, etc.) are used that form covalent bonds with the nucleophilic part ... The reactions of such derivatives with transition metal derivatives give ligand substitution products, in principle, just as if the nucleophile were in the anionic form, but due to reduced nucleophilicity with fewer complications and without side reactions. Such ligand substitution reactions are commonly called transmetallation, in order to emphasize the obvious circumstance that the nucleophile seems to change metals from a more electropositive to a less electropositive one. Thus, this name contains an element of unpleasant schizophrenia - we seem to have already agreed that we will look at all reactions from the point of view of a transition metal, but suddenly we broke off again and are looking at this reaction and only this reaction from the point of view of a nucleophile. We'll have to endure, this is how the terminology developed and is so accepted. In fact, this word goes back to the early chemistry of organometallic compounds and to the fact that the action of lithium or organomagnesium compounds on halides of various metals and metalloids is one of the main methods for the synthesis of any organometallic, primarily intransient, and the reaction that we are now considering in The chemistry of coordination compounds of transition metals is simply a generalization of the old method of organometallic chemistry, from which it all grew. Remetallation is both similar to conventional substitution and not. It looks like - if we consider a non-transition organometallic reagent to be just a carbanion with a counterion, that is, a carbon-non-transition metal bond is ionic. But this view seems to be true only for the most electropositive metals - for magnesium. But even for zinc and tin, this idea is very far from the truth. Therefore, two σ-bonds and four atoms at their ends enter into the reaction. As a result, two new σ-bonds are formed and four atoms are bonded to each other in a different order. Most likely, all this occurs simultaneously in a four-membered transition state, and the reaction itself has a consistent character, like very many other reactions of transition metals. The abundance of electrons and orbitals for literally all tastes and all kinds of symmetries makes transition metals capable of simultaneously maintaining bonds in transition states with several atoms. In the case of remetalling, we get a special case of a very general process, which is simply called σ-bond metathesis. Do not confuse only with the real metathesis of olefins and acetylenes, which are complete catalytic reactions with their own mechanisms. In this case, we are talking about the mechanism of re-metallization or another process in which something similar occurs. Reactions of substitution, addition or elimination of ligands, as a result of which the coordination sphere of the metal changes. In a broad sense, substitution reactions are understood as the processes of substitution of some ligands in the coordination sphere of a metal by others. Dissociative (D) mechanism. In the limiting case, the two-stage process proceeds through an intermediate with a lower coordination number: ML6<->+ L; + Y - "ML5Y Associative (A) mechanism. A two-stage process, characterized by the formation of an intermediate with a large coordination number: ML6 + Y =; = ML5Y + L Mutual exchange mechanism (I). Most metabolic reactions proceed by this mechanism. The process is one-step and is not accompanied by the formation of an intermediate. In the transition state, the reagent and the leaving group are bound to the reaction center, enter its nearest coordination sphere, and in the course of the reaction one group is displaced by the other, two ligands are exchanged: ML6 + Y = = ML5Y + L Internal mechanism. This mechanism characterizes the process of ligand substitution at the molecular level. 2. Features of the properties of lanthanides (Ln) associated with the effect of lanthanide compression. Ln 3+ compounds: oxides, hydroxides, salts. Other oxidation states. Examples of the reducing properties of Sm 2+, Eu 2+ and the oxidizing properties of Ce 4+, Pr 4+. The monotonic decrease in atomic and ionic radii when moving along a series of 4f-elements is called lanthanide compression. I am. It leads to the fact that the radii of the atoms of the 5d transition elements of the fourth (hafnium) and fifth (tantalum) groups following the lanthanides are practically equal to the radii of their electronic analogs from the fifth period: zirconium and niobium, respectively, and the chemistry of heavy 4d and 5d metals has a lot in common. Another consequence of f-compression is the closeness of the ionic radius of yttrium to the radii of heavy f-elements: dysprosium, holmium, and erbium. All REEs form stable oxides in the +3 oxidation state. They are refractory crystalline powders that slowly absorb carbon dioxide and water vapor. Oxides of most elements are obtained by calcining hydroxides, carbonates, nitrates, oxalates in air at a temperature of 800-1000 ° C. Form oxides M2O3 and hydroxides M (OH) 3 Scandium Hydroxide Amphoterine Only Oxides and hydroxides dissolve easily in acids Sc2O3 + 6HNO3 = 2Sc (NO3) 3 + 3H2O Y (OH) 3 + 3HCl = YCl3 + 3H2O Only scandium compounds are hydrolyzed in aqueous solution Cl3 ⇔ Cl2 + HCl All halides are known in the +3 oxidation state. All are refractory. Fluorides are poorly soluble in water. Y (NO3) 3 + 3NaF = YF3 ↓ + 3NaNO3
Coordinated ligand reactions
Reactions of organometallic compounds
Instability constant:
Cytochromes- complex proteins (hemoproteins) that carry out a stepwise transfer of electrons and / or hydrogen from oxidized organic substances to molecular oxygen in living cells. This forms an energy-rich ATP compound.
Cobalamins- natural biologically active cobalt organic compounds. The structural basis of K. is the corrine ring, which consists of 4 pyrrole nuclei, in which nitrogen atoms are bonded to the central cobalt atom.Any combination of ligands of any type can be substituted
Substitution involving hapto-ligands
Substitution, addition and dissociation of ligands are closely and inextricably linked
Stable and labile ligands
Cyclobutadiene as a ligand
Coordination labile ligands
Coordination stable ligands
Remetalling
How does the re-metallization work?
