Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 38
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These orbitals are not involved in the−L bonds in each fragment, and are therefore availdescription of the M−able to form bonding and antibonding MO between the two metalliccentres.There is a very strong σ interaction between the z2 orbitals, sincethe two fragment orbitals are polarized towards each other, two π-typeinteractions between the xz and yz orbitals, and a weaker δ-typeinteraction between the xy orbitals (4-47).LLMLLLL4-46z2 interactionxz interactionsz2z2∗yz∗xyxy, xz, yz interactionxy, xz, yz4-47MMMMFigure 4.10. Interaction scheme for the dorbitals on two ML5 fragments (SBP) to formthe MO of a bimetallic complex M2 L10 .The interaction diagram for these fragment orbitals is shown inFigure 4.10, where it is clear that the σ interaction is stronger than theπ interactions, which in turn are larger than the δ interaction.Bimetallic complexes: from a single to a quadruple bondSince the xy, xz, and yz orbitals are initially at the same energy,there is no ambiguity in the energetic order of the π and δ MO: επ <εδ < εδ ∗ < επ ∗ .
Moreover, the highest-energy MO (not shown) is σ ∗ .However, it is less easy to be sure of the position of the σ bondingMO. Although the interaction is the strongest of those concerned, theorbitals involved are the highest in energy (z2 ). The energy separationbetween the nonbonding and antibonding levels in each fragment, whichis variable from one complex to another, is thus an important parameter.The ordering shown in Figure 4.10, where the σ orbital is placed betweenthe π and δ levels, corresponds to a fairly small energy separation in thefragments. The number and the nature of the metal–metal bonds clearlydepend on the electronic occupation of these orbitals.We shall examine first the complex [Re2 (CO)10 ].
It may be considered to be formed from two [Re(CO)5 ] fragments, whose electronicconfiguration is d7 (Re(0)). There are therefore 14 electrons in all tobe placed in these MO, which leads to the electronic configuration(π)4 (σ )2 (δ)2 (δ ∗ )2 (π ∗ )4 (4-48a). The interactions of π and δ type therefore do not contribute to bonding between the metallic centres, sinceboth the bonding and antibonding MO are occupied. However, the σorbital is occupied, whereas its antibonding σ ∗ counterpart is empty.We can therefore conclude that there is a single bond between the twometallic centres, of σ type, in the complex [Re2 (CO)10 ].******M4-48aMMM4-48bApplicationsIt is clear that to increase the bond order, the number of electrons mustbe reduced, so that fewer antibonding MO are occupied.
In the complex[Re2 (Cl)8 (H2 O)2 ]2− , each monometallic fragment [Re(Cl)4 (H2 O)]−has a d4 electronic configuration (Re(III)). There are therefore eightelectrons to be placed in the MO of this complex, which leads to the electronic configuration (π)4 (σ )2 (δ)2 (4-48b). As only the bonding orbitalsare occupied, there is a quadruple bond between the two metal centres.There are two π bonds, one σ bond, and a new entity that does notexist organic chemistry: a δ bond.The change from a single to a quadruple bond is accompanied by avery substantial shortening of the Re–Re bond, whose length decreasesfrom 3.04 Å in [Re2 (CO)10 ] to 2.22 Å in [Re2 (Cl)8 (H2 O)2 ]2− .4.4.3.
The [Re2 (Cl)8 ]2− complex: a staggered or aneclipsed conformation?7F. A Cotton Inorg. Chem. 4, 334 (1965)The most famous example of a quadruply bound bimetallic complexis [Re2 (Cl)8 ]2− , whose structure inspired Cotton to propose, for thefirst time, the existence of a δ bond, in addition to σ and π bonds,between two metallic centres.7 The Re–Re distance is very short (2.24 Å),a value close to that found in the complex [Re2 (Cl)8 (H2 O)2 ]2− alreadystudied above; most strikingly, the complex is observed to have an eclipsedstructure (4-49a) rather than the staggered conformation (4-49b) thatwould have been expected.
It is also important to note that this complexis diamagnetic.ClClReReClClClzClClClyRexClCl4-49a (exp)ClClClClReClCl4-49bAs a first approximation, we can construct the MO for this complexby considering the interaction of two monometallic [ReCl4 ]− frag−Re−−Cl anglesments with a square-planar geometry (in fact, the Re−are 103.7◦ ).We consider first the eclipsed conformation (4-49a).
The interactionscheme for the fragment orbitals is similar to that shown in Figure 4.10for two SBP ML5 fragments which also have an eclipsed conformation.The only change in the d orbitals of the fragments is the lowering inenergy of the z2 orbital, which is almost nonbonding in a square-planarML4 complex (see Scheme 4-11), whereas it is antibonding in an SBPML5 complex (Figure 4.10). As a consequence, the strong interactionBimetallic complexes: from a single to a quadruple bondbetween the z2 orbitals leads to a σ MO that is now lower in energythan the π orbitals. The energetic ordering of the MO is therefore:εσ < επ < εδ < εδ ∗ < επ ∗ < εσ ∗ . Since each fragment has a d4electronic configuration (Re(III)), there are eight electrons to be placedin these MO.
In this diamagnetic complex, the ground-state electronicconfiguration (σ )2 (π)4 (δ)2 is thus characteristic of a quadruple bondbetween the two metallic centres (4-50a).*d(a)(b)4-50a (eclipsed)4-50b (staggered)We now turn to the staggered conformation. One monometallicunit has thus been rotated by 45◦ around the z-axis with respect to theother (4-49b). This has no consequence for the σ interaction, due to thecylindrical symmetry of the orbitals concerned (z2 ). The π interactionsinvolve two degenerate orbitals on each centre that are concentrated intwo perpendicular planes, (as in the ML5 fragments, 4-47).
The sum ofthe two π interactions is not changed by the rotation of one fragment,as the reduction of the overlap with one of the degenerate orbitals iscompensated by the increase in overlap with the other. But an importantchange does occur for the δ interaction: the overlap between the xyorbitals in the eclipsed conformation disappears in the staggered form,since the two orbitals that are now concerned (xy(1) and (x 2 −y2 )(2) ) havedifferent symmetries (with respect to the plane of the paper, for example)(4-51).
There are therefore two nonbonding d levels in this conformation(4-50b).xy (2)xy (1)xy (1)S≠0(x2–y2)(2)S=04-51ApplicationsIn the diamagnetic ground state, movement from the eclipsed tothe staggered confirmation therefore leads to the rupture of the δ bond,which explains why the first structure (quadruply bound) is more stablethan the second (‘only’ a triple bond). However, if we consider the tripletstate 3 δδ ∗ , the staggered conformation with two nonbonding d electronsis more stable than the eclipsed (one electron in a δ bonding orbital, onein an antibonding δ ∗ ).
As a result, the movement from the diamagneticground state to the first excited triplet state is accompanied by a changein conformation for the complex (eclipsed → staggered).4.5. The reductive elimination reactionIn this last example, we shall study a chemical reaction. In general, problems concerning reactivity are more difficult to treat than structuralproblems. A reaction is accompanied by a complete electronic reorganization, involving the breaking and making of bonds, and several vitalfactors, such as the relative stabilities of the reactants and products,are not easy to obtain from qualitative analyses of electronic structure.However, some worthwhile information can be obtained, usually froma correlation diagram that relates the MO in the reactants to those in theproducts. We thus obtain a description of the electronic reorganizationthat is associated with the reaction under study, in terms of molecularorbitals.4.5.1.
Definition−R ′ (R and R ′ are X-type ligands suchThe elimination of the molecule R −as H, alkyl, halogen, etc.) is a decomposition mode that is frequentlyfound for organometallic complexes [Ln MRR ′ ] (4-52).[Ln MRR⬘]reductive[Ln M] + R–R⬘elimination4-52The removal of two X-type ligands leads to a decrease of two unitsin the metal’s oxidation state; the electronic configuration changesfrom dn to dn+2 .
The metal is therefore reduced, which explains thename ‘reductive elimination’. The opposite reaction is called ‘oxidativeaddition’.4.5.2. Simplified model for the reaction−R[Ln MR 2 ] → [Ln M] + R−Four electrons are intimately involved in the reorganization of the bonds.−RIn the reactant, these are the electrons associated with the two M−The reductive elimination reactionbonds that will be broken. In the products, they are the two elec−R bond and the two electrons that remain on thetrons in the new R−reduced metal. In the simplest description of this reaction by MO theory, we shall consider only the orbitals associated with these electrons,that is:1. In the reactant, two (occupied) bonding MO that describe thebonds that are to be broken, and the two corresponding (empty)antibonding MO.2.
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