Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 39
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In the products, the (occupied) bonding and (empty) antibonding−R), and theMO associated with the new bond that is formed (R−two nonbonding MO that remain on the metal (with two electronsin total) after the departure of the two ligands.A typical correlation diagram for these MO is shown in Figure 4.11.The MO of the reactant are placed on the left. The bonding orbitals, σ (+)and σ (−) , are mainly concentrated on the ligands, whereas the antibonding orbitals, σ ∗(+) and σ ∗(−) , are mainly on the metal (Chapter 1, § 6.1).We shall not try to discover the exact shape of these MO, in particularthe contribution from the different atomic orbitals (AO) on the metal (s,p, d, or mixture?).
The representation shown here is schematic, since inreality the MO spread over the whole complex, rather than being limited to the two bonds. The important point is that there is a symmetricbonding MO (σ (+) ) and an antisymmetric bonding MO (σ (−) ), just asthere is a ‘symmetric’ (a1 ) and an ‘antisymmetric’ (b2 ) MO that describe−H bonds in a bent AH2 molecule (Chapter 1, Figure 1.5).the two A−The same remark may be made about the antibonding σ ∗(+) and σ ∗(−) .−R bonding (σR −In the products, the MO that are R−−R ) and antibond)arethelowestandhighestinenergyofthe four orbitals,ing (σR∗−−R(+)respectively.
The relative energies of σR −indicate that the−R and σALnM *R–R *(–) *(+)SLnMnSLnMnALnMALnM *(–)Figure 4.11. Schematic correlation diagramfor the reductive elimination reaction−R. Only the four[Ln MR 2 ] → [Ln M] + R −most strongly perturbed orbitals are shown. *(+)LnMS R–RApplications8If the opposite holds, the ground-stateelectronic configuration of the reactantcorrelates with a doubly excited configurationof the product.
This would be asymmetry-forbidden reaction, according toWoodward and Hoffmann.−R bond is assumed to be stronger than the M−−R bond. Among theR−two nonbonding orbitals (nA and nS ), the antisymmetric orbital nA hasbeen placed lower in energy than the symmetric nS . This is the casewhen the first is a polarized d orbital and the second a hybrid orbital ofs–p type.8The diagram in Figure 4.11 shows us that the antisymmetric orbitalσ (−) is strongly destabilized, as the two metal–ligand bonding interactions are removed.
The activation energy for the reaction is linked to thechange in energy for this orbital. In contrast, the symmetrical orbitalσ ∗(+) is strongly stabilized, as the two antibonding interactions disappear. The final point to note is the reduction of the metal centre.
Theσ (−) orbital, which is mainly concentrated on the ligands, correlateswith the nA orbital that is localized on the metal: in a formal sense, themetal has gained two electrons.−R.4.5.3. An example: d8 -[L2 MR 2 ] → d10 -[L2 M] + R −We shall consider a square-planar ML4 complex with a d8 electronicconfiguration which possesses two alkyl ligands (R) in cis positions.The reductive elimination of R2 leads to the formation of a d10 [ML2 ]complex.
An example is shown in 4-53, that involves the elimination of ethane from a complex that contains a chelating diphosphineligand.H 2CH 2CPh 2CH3PH2 CPdPh 2PPdH2 CPCH 3Ph 2+C 2H 6PPh 24-534.5.3.1. Correlation diagram for the highest-symmetrymechanism (C2v )Several different mechanisms can be considered for this reaction,depending on the way in which R2 is eliminated. We shall study here thepathway with the highest symmetry possible; the orientation of the R2unit with respect to the rest of the complex stays unchanged (4-54). C2vsymmetry is maintained along this reaction pathway, and the symmetrylabels used for the orbitals will be appropriate.In the correlation diagram, we shall represent not only the four MOdescribed in the preceding section, but also the nonbonding or weaklyantibonding orbitals in the d block and the other nonbonding orbitalson the metal.
In this way, we shall obtain a more complete descriptionThe reductive elimination reactionzLxRyLMMLRRLC2vR+4-542b22b24a12b12b14a11b23a13a11b11b12a12a1a2a21b2Figure 4.12. Correlation diagram for the MOinvolved in the reductive elimination reactiond8 -[L2 MR 2 ] → d10 -[L2 M] (bent) + R2 in themechanism where C2v symmetry is conserved.1a11a1of the electronic reorganization that is associated with this eliminationreaction.For the reactant (left-hand side of Figure 4.12), there are therefore the∗σM−−R bonding (1a1 and 1b2 ) and σM−−R antibonding (4a1 and 2b2 ) MOand, at an intermediate energy, the three nonbonding d orbitals (x 2 −y2 ,xz, and yz), together with the slightly antibonding z2 orbital and thenonbonding pz orbital (Chapter 2, § 2.2).
The bonding MO are doublyoccupied, as are the four d orbitals (d8 ). For the products (right-hand side∗of Figure 4.12), we consider the σR −−R and σR −−R MO (1a1 and 2b2 ), the22five nonbonding (x −y , xz, and yz), or weakly antibonding (z2 and,at slightly higher energy, xy) d orbitals as well as the two nonbondingorbitals pz and s−py (2b1 and 4a1 ) (Chapter 2, § 2.8.4). In the ground-stateelectronic configuration, the σR −−R MO and the five d orbitals are doublyoccupied (d10 ).
C2v labels are used for the symmetries of these orbitals.ApplicationsThe first conclusion that we may draw from this correlation diagramis that the elimination reaction is symmetry-allowed according to thismechanism: the ground-state electronic configuration of the reactantcorrelates with the ground state of the products. Moreover, the energychanges of the four ‘principal’ MO (solid lines) are the same as thosegiven in the simplified diagram of the previous section (Figure 4.11). Inparticular, notice the destabilization of the 1b2 orbital and its transformation from a ligand orbital in the initial complex into a d-block orbital ofthe ML2 complex.4.5.3.2.
Use of the correlation diagram: influence of the natureof the metalExperimentally, it has been shown that the reductive elimination ofR2 is easier for a nickel complex than for the isoelectronic palladiumcontaining species. This result may be readily understood from thecorrelation diagram in Figure 4.12.Pd1b2Ni1b2[ML2][ML2R2]4-55As we have already noted, the activation energy of this reaction ismainly caused by the destabilization of the 1b2 orbital. In the reactant,this MO is mainly concentrated on the ligands, but in the products it isa metal-based orbital, since it is part of the d block of the [ML2 ] complex.
On moving from palladium to nickel, the energy of the d orbitals islowered appreciably, from −9.58 to −12.92 eV (see Table 1.4, Chapter 1).This lowering in energy stabilizes the 1b2 orbital, only weakly in thereactant (small coefficient on the metal) but strongly in the product (a dorbital). This analysis shows us that the destabilization of the 1b2 orbitalis smaller for a nickel complex than for one containing palladium (4-55).The activation energies follow the same trend, and the greater tendency for a nickel complex to undergo reductive elimination is therebyexplained.Exercises4.6. Principal references used for each section (§)of this chapterReferencesT.
A. Albright, J. K. Burdett, M.-H. Whangbo, Orbital Interactionsin Chemistry, John Wiley & Sons, New York (1985), chapter 19.5(§ 4.5).T. A. Albright, R. Hoffmann, J. C. Thibeault, D. L. Thorn J. Am.Chem. Soc. 101, 3801 (1979) (§ 4.1.1, 4.1.2, and 4.1.3).C. Bachmann, J. Demuynck, A. Veillard J. Am. Chem. Soc. 100, 2366(1978) (§ 4.1.3).M. Bénard. J. Am. Chem. Soc. 100, 2354 (1978) (§ 4.4).J. K.
Burdett, T. A. Albright Inorg. Chem. 18, 2112 (1978) (§ 4.1.3).F. A. Cotton, R. A. Walton, Multiple Bonds Between Metal Atoms, Wiley,New York (1982) (§ 4.4).R. H. Crabtree, The Organometallic Chemistry of Transition Metals, JohnWiley & Sons (1988), chapter 11 (§ 4.3).O. Eisenstein, Y. Jean J. Am.
Chem. Soc. 107, 1177 (1985) (§ 4.2.1).R. J. Goddard, R. Hoffmann, E. D. Jemmis J. Am. Chem. Soc. 102, 7667(1980) (§ 4.2.1 and 4.3).P. J. Hay J. Am. Chem. Soc. 104, 7007 (1982) (§ 4.4).Y. Jean, O. Eisenstein, F. Volatron, B. Maouche, F. Sefta J. Am. Chem.Soc. 108, 6587 (1986) (§ 4.1.4).F. Mathey and A. Sevin, Chimie Moléculaire des Eléments de Transition, LesEditions de l’Ecole Polytechnique (2000), chapter 3 (§ 4.3).I. E.-I. Rachidi, O. Eisenstein, Y. Jean New J. Chem. 14, 671 (1990) (§ 4.2.2).J.-F.
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