Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 36
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Three examples are shown in 4-29.H2CPPh2Me2 SiPCy3ClIrCH3 = 78°HPCy3RNHOIrHCH2PCy3PCy34-29a4-29biPr = 76°i PrMe 2 Siα = 73.7°IrPPh 24-29cNotice that the movement from an SBP structure to a distortedTBP geometry can be described by the change of one bond angle (α)from ∼180◦ to ∼75◦ , the intermediate value of 120◦ corresponding toa regular TBP geometry (4-30). Referring to the relative positions of theligands L1 , L2 , and L3 , the SBP structure is said to be ‘T-shaped’, and thedistorted TBP to be ‘Y-shaped’.L4L4L3ML1L2L5TL4L3L1ML5L1L2TBPML3L2L5Y4-30Three questions arise:1.
Why do the diamagnetic d6 ML5 complexes not adopt a regular TBPgeometry (α = 120◦ )?2. Why are two different structures observed, ‘T-shaped’ and ‘Yshaped’?3. What are the electronic factors that favour one or other of thesestructures?Applicationsx2–y2xyyxFigure 4.8. d-block of a TBP complex (thestrongly antibonding orbital z2 is not shown).In the interests of clarity, the axial ligandslocated on the z-axis are omitted. Theelectronic occupation is given for a d6paramagnetic ML5 complex (a triplet state).xzyzL3L1ML2The answer to the first question is straightforward, if we analysethe structure of the d block of a TBP complex (Figure 4.8, where theequatorial plane xy is the plane of the page); two orbitals are nonbonding(xz and yz) and two are weakly antibonding (xy and x 2 −y2 ).Six electrons must be placed in these orbitals. It is clear that thesplitting pattern of the d block favours a paramagnetic ground state,with one electron in each of the antibonding orbitals (a triplet state,Figure 4.8).
However, this orbital arrangement is not favourable fordiamagnetic complexes (all electrons paired), and all those we havementioned are of this type. That is why none of them adopts the regularTBP structure.4.2.2.1. Correlation diagram Y ← TBP → T (σ interactionsonly)The geometries that are observed experimentally suggest that we shouldstudy the correlation diagram for the d-block orbitals as the TBP structure (α = 120◦ ) is changed either to the T-shaped (α = 180◦ ) or to theY-shaped geometry (α close to 80◦ ).
We shall suppose initially that theligands only have σ -type interactions with the metal.During this deformation, the ligands L2 and L3 move in the equatorial plane of the initial TBP (4-30). They therefore remain in one ofthe nodal planes (xy) of the nonbonding xz and yz orbitals (Figure 4.7),so the shapes and energies of these two stay constant. But substantialchanges do occur for the two other orbitals on leaving the regular TBPstructure. In the T-shaped structure, the ligands L2 and L3 are situatedyxYTBP4-31xyT‘Abnormal’ bond anglesin a nodal plane of the xy orbital, which therefore becomes nonbonding(4-31, right-hand side).
On the other hand, the antibonding interaction with L2 and L3 increases in the Y-shaped structure, leading to adestabilization of the xy orbital (4-31, left-hand side).εyεx 2 –y 2xYTBPT4-32Turning to the x 2 −y2 orbital, movement to the T-shaped structureincreases its antibonding interactions with L2 and L3 located on thex-axis, producing a destabilization (4-32, right-hand side). In contrast,these interactions are decreased, if not eliminated, in the Y-shaped structure, as these ligands are placed approximately on the nodal surfaces ofthe x 2 −y2 orbital (4-32, left-hand side).
This orbital is therefore stabilized, but not to the point of becoming nonbonding, as there is still anantibonding interaction with ligand L1 .The correlation diagram shown in Figure 4.9 groups all these results.It is striking that the deformation of the TBP to the T- or Y-shapedstructures stabilizes one of the two antibonding orbitals (xy for T, x 2 −y2for Y) but destabilizes the other. As a consequence of this lifting of theorbital degeneracy, associated with a deformation of the complex whichlowers its symmetry (D3h → C2v ), the diamagnetic state in which thetwo electrons are paired is stabilized.
This is an example of what isknown as a Jahn–Teller distortion.We are now able to understand why diamagnetic d6 ML5 complexes can adopt either the T- or Y-shaped structure, as there are threex2–y2xyyxx2–y2xyxz, yzxz, yzFigure 4.9. Schematic correlation diagram forthe four lowest-energy d orbitals, linking thecentral TBP structure to the Y- or T-shapedgeometries (left- and right-hand sides,respectively).L1ML3L2YL3L3L1L1ML2TBPML2TApplicationslow-energy d orbitals in each of them. In the first, the ground-stateelectronic configuration is ((xz)2 , (yz)2 , (xy)2 ), whereas in the second itis ((xz)2 , (yz)2 , (x 2 −y2 )2 ). The T-shaped structure does appear a littlemore favourable from the diagram, since the three occupied orbitals arenonbonding.4.2.2.2. The role of π-type interactionsThe crucial electronic factor turns out to be the π -donor or π-acceptorproperties of the ligand L1 that is trans to the angle α.
We considerfirst a π-acceptor ligand such as CO. It is easy to show that the xz∗and yz orbitals, both of which are occupied, are influenced by the πCOorbitals to the same extent in the T- and Y-shaped structures: xz staysunchanged (the overlap is zero by symmetry), whereas yz is stabilized by∗ orbital that is parallel to the z-axis.a bonding interaction with the πCOIt is the third occupied orbital that makes the difference: xy (T-shapedstructure) is stabilized by a bonding interaction, but x 2 −y2 (Y-shaped∗ orbitals isstructure) stays unchanged, since its overlap with the πCOzero by symmetry (4-33).
The preference for the T-shaped structuredue to the σ interactions is therefore reinforced by the presence of aπ-acceptor ligand in the position L1 .x2–y2AxyTY4-33 (L1 = π acceptor (A))We now consider a π donor such as Cl. The same analysis isapplied, but we now find that the xy orbital of the T-shaped structure isdestabilized by an antibonding interaction with a lone pair on chlorine(4-34).x2–y2DxyY4-34 (L1 = π donor (D))TCarbene complexesThe presence of a π-donor therefore destabilizes the T-shaped structure with respect to the Y-shaped one.
Looking at the structures shownin 4-29, we do indeed notice that they all have a π -donor ligand (chloride, alkoxy, amino) trans to the acute angle α. In the case of a single-faceπ-donor ligand (amino, for example), the destabilizing effect for theT-shaped structure shown in 4-34, which therefore favours the Y-shapedstructure, can only occur if the lone pair on the donor lies in the equatorial plane, so that it can interact with the xy orbital. That is just whathappens in the complex shown in 4-29c.4.3.
Carbene complexes4.3.1. Ambiguity in the electron count for carbenecomplexes4-35a (nσ )4-35b (np )Carbene complexes, whose general formula is [Ln M==CR 2 ] and whichformally contain an M==C double bond, create a real problem for thecalculation of the metal’s oxidation state. This problem arises becausethe bent CR2 ligand possesses two nonbonding orbitals, close in energy,in which two electrons must be placed (see Chapter 1, Figure 1.5). Thelower of these is a hybrid orbital nσ (4-35a), whereas the higher is apure p orbital (if we consider the simplest example, methylene, CH2 ),np (4-35b).In the covalent model, the two electrons are placed in the lowerenergy orbital (configuration (nσ )2 ), so that the carbene is consideredas an L-type ligand which does not oxidize the metal (4-36a).
However,due to the small difference in energy between the nσ and np nonbondingMO, the ground electronic state of several carbenes (CH2 , for example)is in fact the triplet 3 [(nσ )1 (np )1 ]. In this situation, with two unpairedelectrons, it seems more logical to consider the carbene as an X2 ligand(4-36b) which oxidizes the metal by two units.4-36a (L)4-36b (X2 )4-36c (dianion)If we now consider the ionic model, the electronic octet of the carbonmust be completed, which means that we must consider the dianionicform (CR 2 )2− (4-36c). Since the two extra electrons were supplied bythe metal, we obtain the same oxidation state as that yielded by thecovalent model with an X2 ligand. In the ionic description, the dianion(CR 2 )2− is a strong π-donor, due to its doubly occupied p orbital.It is important to be aware of these different possible points ofdeparture, since the ambiguity in the calculation of the oxidationApplicationsstate appears very frequently in experimental articles about these complexes.
An example is given in 4-37, where we see that the complex[Cp2 Ta(CH3 )(CH2 )] can be described as a d2 Ta(III) complex, if CH2 isconsidered as an L-type ligand in the covalent model, or as a d0 Ta(V)complex if we use either the X2 description of the covalent model orthe ionic description. It may well seem disconcerting not to know if theelectronic configuration of this complex is d2 or d0 , since that appearsto imply that one does not know how many electrons must be placed inthe d block! Note, however, that the total number of electrons does notdepend on the model chosen (Nt = 18).CH2 = L => [Ta(L2 X) 2 (L)(X)] => Ta(III): d 2TaCH2CH3covalentCH2 = X2 => [Ta(L2 X)2 (X2)(X)] => Ta(V): d 0(CH2) 2– ; 2Cp– ; (CH3 ) – => Ta 5+ : d 0ionic4-37This apparently complicated situation (which is at least fairlycomplicated in reality) arises from the different possible choices forthe electron distribution within the CR2 ligand (L or X2 ?) or betweenthe ligand and the metallic fragment (a neutral or a dianionic ligand?).The initial distribution is in fact somewhat arbitrary no matter whichchoice is made, and the situation becomes clearer if one considers theelectronic structure of the complex as a whole, rather than that of theseparate fragments.4.3.2.
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