Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 24
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In the limiting case of alow-spin d6 complex, in which the three lowest d levels are all doublyoccupied, the π interactions no longer lead to any electron transfer fromthe chloride ligand to the metal. The Cl → metal transfers in the bondingMO px + λxz and py + λyz are balanced by the metal → Cl transfers inthe antibonding MO xz − λpx and yz − λpy . In other words, the metal inthe d6 electronic configuration has no empty orbital available to acceptthe π electrons of the chloride ligand. Thus, the only consequence ofthe presence of a double-face π -donor ligand is the destabilization oftwo of the three nonbonding levels in the octahedron.3.2. π -acceptor ligands: general properties3.2.1. The nature of the π orbital on the ligandπ -acceptor ligands are characterized by the existence of at least oneempty orbital that can have a π interaction with a d orbital on the metal.If this orbital is empty, it must be fairly high in energy.
It may be:5In this description, the carbene isconsidered as an L ligand, with two electronsin the 2a1 orbital but where the p orbital of b1symmetry is empty (see Figure 1.5, Chapter 1,for the MO of a bent AH2 species). In somecases, it can also be described as an X2 ligand,with one electron in each of the a1 and b1orbitals (see Chapter 4, § 4.3).1. A p orbital on a fairly electropositive atom, such as the boron p orbitalin boryl complexes or the carbon p orbital in carbene complexes5(3-11).BHHHMHHCHempty p orbital3-11π -acceptor ligands: general properties2. A π ∗ antibonding orbital associated with a π bond between twoatoms of the ligand.
This orbital is therefore delocalized onto twocentres, but its overlap with the metal d orbital is large if it is mainlyconcentrated on the atom that is bonded to the metal. This is whathappens when the metal is less electronegative than the second centreinvolved in the double bond (Chapter 1, § 1.2.2), as is the case, forexample, for the formyl ligand HC=O (3-12).HHCMO∗ orbital3-123.2.2.
‘Single-face’ and ‘double-face’ π-acceptorsAs for π-donor ligands, we distinguish π-acceptor ligands which possessa single empty p or π ∗ orbital (single-face acceptors) from those whichhave two, pointing in two perpendicular directions (double-face acceptors). The boryl, carbene (3-11), and formyl (3-12) ligands, for example,are all in the first category.Among double-face π-acceptors, we may cite the B−H ligand, withits two nonbonding empty p orbitals (Chapter 1, § 1.5.2.2). There isanother which is particularly important in organometallic chemistry, thecarbonyl ligand, C≡O, which we shall study in this section. The principalmolecular orbitals of CO that are involved in the interaction with ametal centre are shown in Figure 3.3 (see Chapter 1, § 1.5.2.4).
They arethe three highest-energy occupied MO, the πCO bonding orbitals andthe nonbonding σC orbital that essentially characterizes the lone pair∗on the carbon atom, and the two lowest-energy empty MO, the πCOantibonding orbitals. The πCO orbitals are mainly concentrated on the∗ orbitalsoxygen, which is the more electronegative centre, and the πCO *COCFigure 3.3. Electronic structure of thecarbonyl ligand (C≡O): the three highestoccupied and the two lowest unoccupiedorbitals.COCOπ -type interactionson the carbon. The nonbonding σC orbital, which is the highest occupiedmolecular orbital (HOMO) on the ligand, forms a σ bond with themetal. As a result, the carbonyl ligand usually binds to a transition metalin the η1 mode, with a linear M−C≡O arrangement that maximizesthe σ overlap. The π interactions, that involve both the πCO bonding∗ antibonding MO, are added to this σ interaction.
ThisMO and the πCOexample illustrates a problem posed by all the ligands with a doubleor triple bond that are coordinated to the metal in the η1 mode (i.e.through one of the atoms involved in the multiple bond): using theiroccupied π orbital(s), they can act as a π donor, whereas their empty π ∗orbitals can allow them to act as a π acceptor.
Depending on the relativestrength of these two interactions, the ligand can act overall either as aπ donor or as a π acceptor.To analyse this probem in the case of the carbonyl ligand (seeAppendix C for more details), we consider a linear M−C≡O arrangement and one of the d orbitals on M (yz, 3-13) that can interact with theπ system of the carbonyl ligand. It is easy to see that this d orbital has anon-zero overlap with a pair of π and π ∗ orbitals on CO, those that areoriented parallel to the z-axis, leading to a three-orbital interaction. Thed ↔ π and d ↔ π ∗ interactions (3-13), which involve orbitals with different energies, are proportional to S2 /ε (Chapter 1, § 1.3.2). They arezxy *COS*∆*yz∆SCOMCO3-13therefore strengthened by a small orbital energy difference (ε) and bya large overlap (S).
The overlap term clearly favours the interaction with∗ (S∗ > S , due to the relative sizes of the coefficients on carbon inπCOππ∗ and πthe πCOCO MO). Moreover, it can be shown (see appendix) that thed orbital is closer in energy to the antibonding orbital than to the bondingπ -acceptor ligands: general propertiesorbital (επ∗ < επ , 3-13) for almost all the transition elements. In thethree-orbital interaction scheme shown in 3-13, both factors, overlap andenergy difference, therefore favour the d ↔ π ∗ interaction.
We maytherefore simplify the diagram, by neglecting the relatively weak interaction(d ↔ π) and retaining only the dominant interaction (d ↔ π ∗ ). This latterinteraction, which involves an empty orbital on the carbonyl ligand,therefore gives it π-acceptor character.In this simplified model, we therefore consider that the π interac∗ orbitals; it istions with the carbonyl ligand involve only the two πCOthus a double-face π-acceptor (3-14).3-143.2.3.
Perturbation of the d orbitals: the generalinteraction diagram6This point is treated in detail in theappendix for the carbonyl ligand.When the unoccupied orbital on the acceptor ligand is a π ∗ antibondingMO, its energy level is almost always higher than that of the nonbonding or weakly antibonding orbitals in the d block.6 The same remarkmay be made about a nonbonding p orbital on a very electropositivecentre, such as boron in boryl complexes (3-11). But the relative orbitalenergies are less clear when the centre bearing the empty p orbital ismoderately electronegative, such as carbon in the carbene complexes[Ln M−CR2 ] (3-11).
They then depend on the nature of the metal andof the substituents on the carbene ligand (see Chapter 4, § 4.3). At thisstage of our analysis, we shall only consider the more common casewhere the energy level of the p or π ∗ orbitals on the π -acceptor ligandis higher than that of the nonbonding or weakly antibonding orbitals inthe d block.As an example, we consider a π -acceptor ligand whose nonbondingp orbital interacts with a doubly occupied d orbital (Figure 3.4). Dueto the relative energies of the initial orbitals, the bonding combinationis mainly concentrated on the metal and the antibonding combinationon the ligand.
As in the preceding examples, the new d-block orbitalis mainly concentrated on the metal, that is, in this case, the bondingcombination of the initial orbitals.Rule: the π interaction between a d-block orbital and the orbital of aπ -acceptor ligand leads to a stabilization of the d-block orbital, by mixingwith the ligand orbital in a bonding sense.Figure 3.4 shows the resulting π electron transfer involving theligand and the metal. Before the interaction, the two electrons werelocalized in the metal d orbital. After the interaction, these electronsoccupy the bonding molecular orbital which is partially located onπ -type interactionsMpXdnew orbital in the d blockFigure 3.4.
Sketch of the interaction betweena doubly occupied d orbital and theunoccupied orbital of a π acceptor (X).e–the ligand. This delocalization is thus accompanied by some electrontransfer from the metal to the π-acceptor ligand.3.2.4. A first example: the octahedral complex [ML5 CO]P23.2.4.1. Interaction diagramP1OCzLMLyxLLL3-15 x* (CO) *y (CO)SAAS3-16We consider an octahedral complex with one carbonyl ligand, a doubleface π -acceptor, and five other ligands that only have σ interactions withthe metal (3-15).To construct the d block of this complex, we shall follow the sameprocedure as that used for a complex with a π donor (§ 3.1.4): we startfrom the orbitals for an ideal octahedral ML6 complex in which thereare only σ interactions, then we perturb these orbitals by ‘switching on’the π interactions with the carbonyl ligand.We have already established (see 3-10) the symmetries of the metal dorbitals with respect to the planes of symmetry P1 and P2 (3-15): xy (AA),xz (SA), yz (AS), x 2−y2 and z2 (SS).
Following the analysis developed in∗ orbitals are treated. They are§ 3.2.2, only the empty antibonding πCOoriented parallel to the x- and y-axes, and have SA and AS symmetries,respectively (3-16).We can now construct the diagram for the interaction between theorbitals of the d block and the πCO orbitals on the carbonyl ligand(Figure 3.5), by combining the πx∗ and xz orbitals, with SA symmetry,and the πy∗ and yz orbitals, with AS symmetry.
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