Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 37
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Two limiting cases: Fischer carbenes and SchrockcarbenesTo clarify these points, we shall consider the carbene as an L-type ligand(4-36a). It therefore acts as a σ donor, using its lone pair described bythe nσ orbital, which interacts with an empty orbital on the metal (e.g.z2 , 4-38a). In this model, the np orbital is empty, so the carbene acquiresa π-acceptor character (single face) (4-38b). The interaction schemeis similar to that in the Dewar–Chatt–Duncanson model (Chapter 3,§ 3.4.1) used, for example, to describe ethylene complexes or molecularhydrogen complexes (§ 4.1.4).Carbene complexesee4-38a4-38bHowever, an important difference arises, since the empty p orbitalon the carbene can be strictly nonbonding (as in CH2 ), and therefore∗ , σ ∗ ) as in the precedingat low energy, rather than antibonding (πCCH2examples.
Depending on the nature of the metal, the ligands and thesubstituents on the carbene, this orbital can therefore be either higher orlower in energy than the d orbitals on the metal. To prove this point, oneneed only consider the energy of the d orbitals for the first transitionseries, which ranges from −8.5 (Sc) to −14 eV (Cu), whereas the energyof a p orbital on carbon is −11.4 eV (parameters of the extended Hückelmethod).4.3.2.1. Two interaction schemes for back-donationWhen the p orbital is higher in energy than the d orbital, we obtaina typical back-donation scheme, with the formation of a bonding MOmainly located on the metal (4-39): formally, we may consider thatthe π interaction does not oxidize the metal, since the two electronsstay mainly localized on this centre.
In this case, the description of thecarbene as an L-type ligand is therefore quite appropriate. However, ifthe p orbital is lower in energy than the d orbital, the occupied bondingMO is mainly concentrated on the carbene (4-40), so that in a formal4-394-40sense, two electrons have been transferred from the metal to this ligand.In other words, the d orbital that was occupied before the interaction isApplicationstransformed to an MO of the complex that is mainly concentrated onthe ligand: the d block therefore ‘loses’ two electrons. In this case, thedescription of the carbene as an X2 -type ligand or as a dianionic ligand(CR 2 )2− , which leads to an increase of the metal’s oxidation state bytwo units, corresponds more closely to reality.The ambiguity that we noticed previously for the electron count incarbene complexes in fact corresponds to chemical reality: the L and X2(or dianion) formulations are appropriate for the situations described in4-39 and 4-40, respectively.We shall now examine how the nature of the metal, the ligands, andthe substituents on the carbene can favour one or other of these twopossibilities.4.3.2.2.
Fischer carbenes and Schrock carbenesHow can the situation described in 4-39 be favoured, with an acceptororbital on the carbene that is clearly higher in energy than the metald orbital? We require that the d orbital is low in energy and at thesame time that the p orbital is high. The d orbital factor therefore leadsus to consider metals towards the right-hand side of the periodic classification, or at the limit in the centre, since the energy of the metalorbitals is lowered on passing from left to right in a transition series(see Chapter 1, Table 1.4).
In addition, the presence of π-acceptor ligands also leads to a lowering of the level of the d orbitals. As far asthe carbene is concerned, the energy of the empty p orbital is raised ifthe substituents are π donors, that is, if they have lone pairs (halogens,−R, NR2 , etc.). An example is shown in 4-41 for the carbene CCl2 .O−The in-phase combination of the p lone pairs on the chlorine atomsClClDestabilization of theempty orbitalpCC4-41Carbene complexesdestabilizes the empty orbital on carbon and therefore favours thesituation depicted in 4-39.
Carbene complexes that possess these characteristics are called Fischer carbenes. As examples, we may consider complexes such as [(CO)5 W==C(Ph)(OMe)], [(CO)4 Fe==C(Ph)(OMe)],[(CO)5 Cr==C(Ni Pr2 )(OEt)], or [Cp(CO)(PPh3 )Fe==CF2 ]+ . Sincethe carbene is usually considered to be an L-type ligand for the calculation of the oxidation state of the metal, these are described as complexesof W(0), Fe(0), Cr(0), and Fe(II), respectively.In this group of compounds, the carbene acts as a σ donor anda rather weak π acceptor, due to the relative energies of the d and porbitals. Overall, the electron density around the carbon of the carbeneis decreased, leading to a metal–carbon bond that is polarized M(δ − ) =C(δ + ), with an electrophilic character for the carbon centre: it is thereforelikely to be attacked by nucleophiles, and it is possible, for example, tointerconvert two carbene complexes (4-42).–MOR⬘C+OR⬘Nu–NuMNuH+CR––C+R⬘OHRR–M–Nu = H , R , R2N , RS–4-42In order to obtain the situation shown in 4-40, where the p orbitalon carbon is at lower energy than the d orbital, the conditions previously suggested for the Fischer carbenes must be reversed: the metalmust come from the left of the periodic table, and must have π-donorligands to destabilize the d orbital, instead of π-acceptors.
Moreover, toensure that the p orbital on the carbene is as low in energy as possible,its substituents cannot be π donors: CH2 itself is a good candidate, and,more generally, alkyl substituents are suitable (the term ‘alkylidene’is often used for a carbene substituted by alkyl groups). The complexes [Cp2 (CH3 )Ta==CH2 ] and [CpCl2 Ta==C(H)(CMe3 )] are typicalexamples. As we have already noted, the metal in this group of complexes is usually considered to be oxidized by two units by the alkylideneligand, so the two examples mentioned above contain Ta(V).Carbene complexes that possess these properties are called Schrockcarbenes. The π interaction shown in 4-40 is accompanied by a substantialtransfer of electron density from the metal to the carbon atom.
Thistransfer more than outweighs the σ donation (ligand → metal), sothat the metal–carbon bond is polarized in the sense M(δ + )==C(δ − ).Schrock carbenes therefore possess a nucleophilic carbon, and give, forexample, addition products with Lewis acids such as AlMe3 (4-43).Applications+Cp2MeTaAlMe3HH−C+AlMe 3Cp 2 MeTaCHH4-43In summary, Fischer carbenes (electrophiles) and Schrock carbenes(nucleophiles) correspond to two different possibilities for the orbitalsthat participate in the π interaction (4-39 and 4-40).
We often distinguish between them for the calculation of the metal’s oxidation state:the carbene ligand is considered to be of L-type for the first category,but as an X2 ligand (or CR 2−2 ) for the second. The characteristics thatwe have suggested for each category are, of course, only general indications. Some complexes may be found on the border between the twogroups. For example, the complex [Cp2 (CH3 )Ta==CH2 ] is, as expected, a nucleophilic carbene (Schrock-type), but the tungsten complex[Cp2 (CH3 )W==CH2 ]+ , which has exactly the same ligands, is electrophilic (Fischer-type).
The presence of a positive charge on the metallicfragment, which stabilizes the metal’s d orbitals and thereby favours thesituation shown in 4-39, provides a plausible explanation of this difference in behaviour, despite the low energy of the nonbonding p orbitalon the carbon atom.4.4. Bimetallic complexes: from a singleto a quadruple bondIn organic chemistry, we know that two carbon atoms can be linkedby a single bond (σ ), a double (σ + π), or a triple bond (σ + 2π).The increase in bond order is accompanied by a decrease in interatomic−C), through 1.34 (C==C) to 1.20 Å (C≡≡C).distance, from 1.54 Å (C−From the orbital point of view, these different bond orders correspondto the occupation of one, two, or three bonding MO between the twocarbon atoms (4-44), it being clear that the corresponding antibondingMO are empty.
The orbital that characterizes the σ bond has cylindricalsymmetry about the internuclear axis in each case, whereas the orbitalsthat characterize the π bond(s) have a nodal plane (4-44).It is natural to wonder how many bonds, of what type, can existbetween the two metal centres in bimetallic complexes. The characteristics presented above will reappear in the description of metal–metalbonds, but the presence of d-type orbitals introduces a significant newfeature.Bimetallic complexes: from a single to a quadruple bond1H3 CCH3H2CCH2HCCH24-444.4.1.
σ , π, and δ interactionsWe shall consider the interactions between the d orbitals on two metalliccentres, initially ignoring any influence of the ligands on these orbitals. Eachinteraction leads to the formation of a bonding and an antibonding MO,but only the first of these, written (+) , is shown in 4-45. The interactionbetween the z2 orbitals (4-45a) is of σ type; the axial overlap betweenthe two orbitals has cylindrical symmetry around the internuclear axis(z). The MO xz(+) and yz(+) (4-45b) have the same characteristics asthe π orbitals of acetylene (4-44); they are bonding, and they possess anodal plane that passes through the nuclei.δxyzz 2 (+)4-45axz (+)xy (+)yz (+)(x2–y2)(+)4-45b4-45cThe novel feature compared to organic systems arises in the interactions between the pairs of orbitals xy and x 2 −y2 .
In each case, thebonding MO that are formed have two nodal planes, instead of just onefor the π MO: these are the xz and yz planes for the xy(+) orbital, andthe planes that bisect the x-and y-axes for the (x 2 −y2 )(+) orbital (4-45c).These are described as δ interactions.ApplicationsThe overlaps involved depend on the type of interaction. The axialoverlap Sσ is larger than or comparable to the lateral overlaps Sπ . However, the Sδ overlap is much smaller than these, only about a third or aquarter of Sπ .4.4.2. M2 L10 complexesLL MLLConsider a bimetallic complex of the type M2 L10 (4-46), whose MO maybe constructed by the interaction of two monometallic fragments ML5with an SBP geometry.On each fragment, we shall consider the four lowest-energyd orbitals, as in the preceding examples that concerned this fragment(§ 4.1.2, 4.1.4, and 4.2.1.2): three are nonbonding (xy, xz, and yz) andone is weakly antibonding (z2 ).
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