Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 32
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The first of these interactions involves two electrons;it constitutes the donation interaction of the Dewar–Chatt–Duncansonmodel (Chapter 3, § 3.4.1.2) for the metal–olefin bond in this complex.It is easy to see that the overlap between these orbitals does not dependon the orientation of the olefin, due to the cylindrical symmetry of theempty orbital on the metal centre (4-4a and b). Things are essentiallythe same for the second interaction: the overlap occurs mainly with thelobe of the d orbital that points towards the π orbital, and this lobe alsohas cylindrical symmetry with respect to the metal–olefin axis (4-5a andb).
We may conclude that since the interactions which involve the πorbital of the ethylene ligand are identical in the two conformations,they cannot contribute to a pronounced energetic preference for oneof them.4-4a4-4b4-5a4-5bWe now consider the interaction of the empty π ∗ orbital with theorbital of the same symmetry on the metal fragment. In the perpendicular conformation, the nonbonding orbital with SA symmetry isinvolved, but the antibonding AS orbital is concerned in the coplanarconformation (4-2). In both cases, this is a two-electron stabilizing interaction that leads to a transfer of electron density from the metal to theligand. This is therefore the back-donation interaction in the Dewar–Chatt–Duncanson model (Chapter 3, § 3.4.1.2).
As the orbital involvedon the metallic fragment is not the same in the two conformations,the strength of this interaction depends on the conformation considered. Nowthe energetic stabilization created by an interaction is proportional tothe square of the overlap and inversely proportional to the energy difference between the orbitals (Chapter 1, § 1.3.2). The antibonding π ∗ApplicationsSAASAS∆ EcFigure 4.1. Comparison of the back-donationinteractions (d → π ∗ ) in the perpendicular(on the left) and coplanar (on the right)conformations of a d8 -[ML4 (η2 -ethylene)]complex (4-1). The energy levels andsymmetries of the d orbitals involved in thetwo cases are given in the centre of thediagram.SA∆ EpMMMorbital is higher in energy than the d-block orbitals, which are nonbonding or, in one case, weakly antibonding (4-2 and Figure 4.1).
Theenergy separation between the interacting orbitals is therefore smallerin the coplanar conformation, since, in this case, it is the highest-energyoccupied orbital on the metallic fragment that is involved. Moreover,the overlap involves a d orbital that is polarized towards the π ∗ orbitalin the coplanar conformation (Sc ), but a pure d orbital in the perpendicular case (Sp ). Therefore, Sc > Sp , (4-6). In summary, a smaller energyseparation and a larger overlap favour the back-donation interaction inthe planar structure.4-6a (Sp )4-6b (Sc )The greater electronic stabilization which follows (Ec > Ep ,Figure 4.1) leads to a preference for this conformation, which is indeedadopted in all known complexes of the type d8 -[ML4 (η2 -olefin)].
The barrier to olefin rotation measured by nuclear magnetic resonance (NMR)is of the order of 10–15 kcal mol−1 .4.1.2. d6 -[ML5 (η2 -C2 H4 )] complexes: staggered or eclipsedconformation?An octahedral complex with an ethylene ligand may adopt either theconformation in which the carbon–carbon bond eclipses the neighbour−L bonds (4-7a), or the staggered conformation shown in 4-7b.ing M−This latter should be more stable on steric grounds. But experimentally,the eclipsed conformation is observed for complexes with a d6 electronicconfiguration, such as [Mo(PR 3 )5 (η2 -C2 H4 )]—hence the interest in thisconformational problem.Conformational problemsP1P1zP2P2yxMM4-7a (eclipsed)SASS4-84-7b (staggered)The two conformations have C2v symmetry, and as in the precedingexample, we shall use the two planes of symmetry P1 (yz) and P2 (xz) toanalyse the symmetries of the orbitals on the ethylene fragment (π andπ ∗ ) and of those on the d6 metallic fragment ML5 .
For the latter, weshall consider the four lowest-energy d orbitals (three doubly occupiednonbonding and the empty polarized z2 orbital, see Chapter 2, § 2.3.1).To pass from the eclipsed conformation (4-7a) to the staggered(4-7b), the orientation of the ethylene molecule has been fixed and arotation of 45◦ applied to the ML5 fragment.
The orbitals of the ethylene fragment are therefore as shown in 4-8 for both conformations,whereas the reorientation of the metallic fragment leads to a change inthe name and the symmetry of one of the its nonbonding orbitals (4-9aand b): xy (AA) becomes x 2 −y2 (SS) (Chapter 2, § 2.1.2.4 and 2.1.2.5).z2SSz2SSAAASSASSASSAxyxzyzx2–y2xzyz4-9a (eclipsed)4-9b (staggered)In this system, the donation interaction of the Dewar–Chatt–Duncanson model involves the occupied π orbital and the empty z2orbital, both with SS symmetry, in the two conformations.
As thez2 orbital has cylindrical symmetry, the overlap between π and z2 doesnot depend on the orientation of the ethylene ligand, so this interaction cannot lead to any conformational preference. The back-donationinteraction involves the empty π ∗ orbital and the occupied yz orbital,both with symmetry SA, in the two conformations. This secondApplicationsSA2e–2 e–2 e–2 e–SSSSSASSSA4 e–SSMMFigure 4.2. Interactions between the π andπ ∗ orbitals on ethylene (in the centre) and theorbitals of the same symmetry on the d6 -ML5fragment in eclipsed (left) and staggered(right) conformations.interaction is therefore also identical in the two conformations. So inthis system, the conformational preference does not arise from eitherof the stabilizing interactions that have been invoked to describe themetal–ligand bond.
If we consider the symmetries of the various orbitals in the eclipsed conformation, no additional interaction is possible,other than those we have already described. However, in the staggeredstructure, there is an interaction between the occupied π orbitals andx 2 −y2 , as both have SS symmetry. This destabilizing four-electron interaction, between the π orbital on the olefin and one of the nonbonding dorbitals on the metal (Figure 4.2), exists only in the staggered conformation; it is therefore the origin of the observed preference for the eclipsedconformation. The experimental barrier to olefin rotation is of the orderof 10 kcal mol−1 .The electronic structure of the two conformations can be describedmore completely by constructing the MO that result from the interaction of the fragment orbitals. Those on ethylene are placed in themiddle of Figure 4.3, while those of the metallic fragment are on the left(eclipsed) or on the right (staggered conformation).
In the interests ofclarity, only the doubly occupied MO of the full complex are shown. Theback-donation interaction between the SA orbitals produces the samestabilization in both conformations. There is a stabilizing interaction(donation) that involves the SS orbitals in the eclipsed conformation (lefthand side). For the staggered conformation, the two-electron stabilizinginteraction and the four-electron destabilizing interaction describedabove (Figure 4.2) are represented by a three-orbital interaction scheme(SS symmetry) that involves four electrons.The lowest-energy orbital in each conformation of the complexis mainly concentrated on the π orbital of ethylene.
This is a bondingmetal–ligand orbital that does not belong to the d block. The three otheroccupied MO are mainly or entirely concentrated on the metal d orbitals.They are the MO derived from the t2g block of a regular octahedralSAFigure 4.3. Construction of the MO for theeclipsed and staggered conformations of thed6 -[ML5 (η2 -ethylene)] complex, byinteraction between the π and π ∗ orbitals onethylene (centre) and the orbitals of thed6 -ML5 fragment (on the left for the eclipsedconformation, on the right for the staggered).SSSSSAASAASAASSSSSMMMMConformational problemscomplex, the degeneracy being partially or completely lifted by thepresence of the ethylene ligand.
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