Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 28
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Strong-field and weak-field complexesegeg∆E + t2g∆E∆E + -donorst2 g-acceptorsFigure 3.8. Perturbation of the d block (in thecentre, σ interactions only) of an octahedralcomplex ML6 by the π interactions withdouble-face π -donor ligands (L = Cl, forexample) on the left, and double-faceπ -acceptor ligands (L = CO, for example) onthe right.The energetic consequences of the π interactions for the d-block orbitals in the [MCl6 ] and [M(CO)6 ] complexes are shown in Figure 3.8. Ifeverything else is equal, the σ interactions in particular, the presenceof π-donor ligands decreases the energy separation (E) between thet2g and eg orbitals (weak field), whereas π-acceptor ligands increasethis separation (strong field).
This trend is consistent with the spectrochemical series presented below, which, from measurements of the d–dtransition energies (t2g → eg ), ranks ligands according to the strength ofthe field that they create (Chapter 2, § 2.1.2.6). It is clear that π -donorligands, such as the halogens, are found at the beginning of the series(weak field), whereas π-acceptor ligands, such as carbonyl or cyanide,are at the end (strong field). For example, E decreases from 26,600to 15,060 cm−1 passing from [Cr(CN)6 ]3− (six π -acceptor ligands) to[CrF6 ]3− (six π-donor ligands), even though these two complexes havethe same d3 electronic configuration.−I− < Br− < Cl− < F− < OH− < O2 < H2 O < NH3−−−< NO−2 < CH3 < C6 H5 < CN < PR 3 < CO(3.1)Notice, however, that the π interactions are not the only factorwhich influences the t2g −eg energy separation.
We must remember thatthe σ interactions create the primary d-orbital splittings, and that as theseinteractions become stronger (phosphine ligands PR3 , metal–carbonbonds in organometallic complexes, for example), the antibonding eglevels are pushed to ever higher energy.π complexes: the example of ethylene3.4. π complexes: the example of ethyleneIn all of the preceding examples, the ligands that possess a π systemare bound to the metal centre by a single atom (coordination mode η1 ).The situation is different when several ligand atoms are bound in anequivalent way to the metal centre. In particular, this is the case for πcomplexes (Chapter 1, § 1.1.1.3), in which the π system of the ligand‘points’ in the direction of the metal, rather than being perpendicular tothe metal–ligand bond.
The description of the metal–ligand bonds nowrequires us to take all the π orbitals of the ligand into account, bothoccupied and empty, as well as the orbitals of appropriate symmetry onthe metal. As an example, we shall first consider the simplest complexfrom this family, that in which a molecule of ethylene is bound to themetal in the η2 mode.8J. Chatt, L. A. Duncanson J. Chem. Soc.2939 (1953); M.J.S. Dewar Bull.
Soc. Chim. Fr.18, C79 (1951).xz3-35yz3-363.4.1. Orbital interactions: the Dewar–Chatt–Duncansonmodel83.4.1.1. σ or π interactions?We consider first the π-bonding orbital on ethylene. It can interact witha metal orbital, for example, z2 (3-35). The resulting overlap is intermediate between an axial overlap, normally associated with a σ interaction,and a lateral overlap which corresponds to a π interaction.
Notice thatthere is no nodal surface that is common to the two orbitals, and thattheir overlap is not changed by a rotation of the ligand around the zaxis. As a consequence, even though the ligand orbital is of π type, themetal–ligand interaction in which it is involved has the characteristicsof an ordinary σ interaction (see 3-17a, for example). The π ∗ orbitalcan also interact with a metal d orbital, for example, xz (3-36). As inthe ‘traditional’ π interactions described in preceding sections, the twoorbitals have a common nodal plane (yz), and a rotation of the ligandaround the z-axis decreases their overlap, to the extent that it is completely eliminated for a rotation of 90◦ .
The interaction that involves theπ ∗ orbital can therefore be described as a π interaction.3.4.1.2. The Dewar–Chatt–Duncanson modelEthylene behaves as an L-type ligand thanks to its doubly occupiedπ orbital. It can transfer electron density to the metal (a donation interaction) by interaction with an empty orbital on the metal centre, whosesymmetry is suitable (z2 , for example, 3-37a). This is a stabilizing twoelectron interaction, which stabilizes the π level of the ligand. Due to therelative energies of the two initial orbitals, the occupied MO is mainlyconcentrated on the ligand (a bonding orbital).π -type interactionsThere is a second interaction that involves the empty antibondingπ ∗ orbital and the metal d orbital with the same symmetry (xz), whichis lower in energy than the π ∗ orbital (3-37b).
If this latter orbital isdoubly occupied, this interaction is stabilizing, and it leads to a transferof electron density from the metal to the ligand. This is therefore a backdonation interaction, where ethylene plays the role of a π acceptor, usingits empty π ∗ orbital. The doubly occupied orbital, mainly concentratedon the metal, is part of the d block of the complex; it can be described asa metal d orbital that is stabilized by a bonding interaction with the π ∗orbital on ethylene.electronselectrons3-37a3-37bThese two stabilizing interactions taken together constitute theDewar–Chatt–Duncanson model of the bond between an olefin anda metal centre.3.4.2.
Electronic structure of a d6 complex[ML5 (η2 -C2 H4 )]3.4.2.1. The complete interaction diagramP2P1LLLMzLLx3-38yConsider a pseudo-octahedral d6 complex of the type [ML5 (η2 -C2 H4 )](3-38), where the five L ligands are supposed to have only σ interactionswith the metal centre. If this complex is decomposed into a d6 fragmentML5 , with a square-base pyramidal (SBP) geometry, and an ethylenefragment, the interaction between the orbitals on the two fragmentsenables us to analyse the electronic factors that are at the origin of theethylene–metal bond.On the metallic fragment ML5 , we consider the four low-energyd orbitals in this type of structure (Chapter 2, § 2.3.1), together with the πand π ∗ orbitals on the ethylene ligand.
The π orbital is doubly occupied,as are the three strictly nonbonding d orbitals of the d6 fragment ML5 .π complexes: the example of ethyleneSSSA* (SA)z2 (SS)AS, AAxy (AA)xz (AS)yz (SA)SA (SA)SSFigure 3.9. Diagram for the interactionbetween the d orbitals of a d6 ML5 fragmentand the π and π ∗ orbitals on an ethyleneligand (the Dewar–Chatt–Duncanson model).9In the C2v point group of the complex,the symmetry labels are a1 (SS), a2 (AA), b1(AS), and b2 (SA).Given the orbitals’ symmetry properties with respect to the planes P1and P2 that are defined in 3-38, we obtain the interaction diagram shownin Figure 3.9.
The π orbital (SS) is stabilized by a bonding interactionwith the polarized z2 orbital (SS) on the metallic fragment. The doublyoccupied yz orbital (SA) is stabilized by a bonding interaction with theπ ∗ orbital (SA). The two other d orbitals (xy (AA) and xz (AS)) are notaffected by the interaction.9 The two stabilizing interactions, donation(π → z2 ) and back-donation (yz → π ∗ ), are thus indeed those thatwere described in a general way in the preceding section.The MO of the complex that are shown in Figure 3.9 can thereforebe described in the following ways: (i) the SS bonding MO is a bondorbital; (ii) the SA weakly bonding MO and the AS and AA nonbondingMO are the three orbitals that are derived from the t2g block of a regularoctahedron.
If they are doubly occupied, the electronic configurationis d6 ; (iii) the SA antibonding MO is essentially antibonding on theethylene ligand; (iv) lastly, the SS antibonding MO, largely composedof the z2 orbital, is mainly antibonding for the ethylene ligand and thetrans ligand L.
In fact, it is one of the orbitals that are derived from theantibonding eg block of a regular octahedron. The other orbital, x 2−y2 ,does not appear on this diagram since we did not consider it on theinitial ML5 fragment, as its energy is too high.3.4.2.2. A molecular ethylene complex ora metallacyclopropane?The donation interaction (π → z2 ) reduces the electron densityin the π-bonding orbital of the ethylene ligand. The carbon–carbonπ -type interactionse–3-39e–3-403-41a3-41b10There is, however, a difference in therelative magnitudes of the coefficients on themetal and on the ligand in the orbital whosesymmetry is written SA.
In the molecularethylene complex, this orbital is concentratedmostly on the metal, whereas in themetallacyclopropane, it is mainly on theligand. We shall return to this point in § 4.5 ofChapter 4, which is devoted to the mechanismof oxidative addition.bond is therefore weakened (3-39). Now the back-donation interaction(yz → π ∗ ) transfers electron density into the π ∗ antibonding orbital,which also weakens this bond (3-40).
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