Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 23
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Thus, a halogen linked to a metal centre is a ‘double-face’π -donor.Analogous interactions occur when the p orbitals are replaced by twoπ-bonding orbitals. This is the case when a ligand with a triple bond (oneσ and two π ) binds to the metal in the η1 mode, as shown in 3-6 forthe nitrile ligand. As we have already noted for ‘single-face’ π donors,the π-donor character of the ligand increases as the orbital coefficient becomes larger on the atom bound to the metal.
This thereforeπ -type interactionsrequires the more electronegative atom to be bound to the metal in theη1 mode.MHMHNNCCHH3-63.1.3. Perturbation of the d orbitals: the generalinteraction diagramWe now wish to discover how the orbitals of the d block that we havepreviously obtained, by analysing only the σ interactions (Chapter 2), aremodified by the presence of a π donor. These MO, either nonbonding orantibonding, are mainly concentrated on the d orbitals of the metal, asthis is a rather electropositive element. The orbital involved on a π -donorligand characterizes either a lone pair located on a very electronegativecentre, or a π bond. In all cases, the p or π orbitals of a π-donor ligand arelower in energy than the d-block orbitals obtained by considering only theσ interactions.This relative position of the orbital energies has important consequences for the way in which the d block is perturbed by interactionwith a π donor.
We shall consider the simplest case, where a nonbonding doubly occupied ligand p orbital interacts with a nonbondingmetal d orbital that is assumed to be empty (Figure 3.1). Due to therelative energies of the interacting orbitals, their bonding combinationis mainly concentrated on the ligand, whereas the largest coefficientin the antibonding combination is on the metal. Which of these twoorbitals is the one that we shall consider a member of the d block?To make our choice, we use the same criterion as that previously adopted when examining σ -type interactions (see, for example, Chapter 2,new orbital in the d-blockXdMFigure 3.1.
Sketch of the interaction betweenthe doubly occupied orbital on a π-donorligand (X) and an empty d orbital on the metalcentre.pe–π-donor ligands: general properties§ 2.1.2.4): it is the molecular orbital that is mainly concentrated on themetal d orbital, that is, the antibonding combination of the interactingorbitals. The bonding molecular orbital, as we have seen, is mainlyconcentrated on the ligand.Rule: the π interaction between a d-block orbital and the orbital of a πdonor leads to a destabilization of the d-block orbital, by mixing with theligand orbital in an antibonding sense.e–e–3-7ClLLML3-8LLFigure 3.1 also enables us to obtain further insight into the conceptof a π -donor ligand.
Before the interaction, the d orbital on the metal isempty but the p orbital localized on the π donor contains two electrons.After the interaction, these electrons occupy the bonding molecularorbital which is partially delocalized onto the metal. This delocalizationof the occupied MO on to the two centres results in a partial electrontransfer from the ligand to the metal centre, a result which is quite consistent with the label of π donor attached to the ligand. The interactionis stabilizing, since it involves two electrons (Chapter 1 § 1.3.2).However, the d orbital on the metal centre is not necessarily alwaysempty.
In fact, the occupation of this orbital depends on the dn electronicconfiguration of the complex, and it is quite possible for it to be doublyoccupied. In that case, the π interaction is destabilizing on balance,since it involves four electrons, and moreover the electron transfer fromthe ligand to the metal in the bonding MO is now compensated byelectron transfer from the metal to the ligand in the antibonding MO(3-7): overall, there is no longer any electron transfer between the twocentres.In this case, therefore, the ligand can no longer play the role of aπ donor, since there is now no empty d orbital on the metal into whichsome of its electron density can be transferred.
However, this ligandis still described as a π donor, to indicate that it possesses at least oneπ-type orbital that is capable of electron transfer to a metal centre, solong as the latter can act as an acceptor. Notice that the rule given aboveabout the consequences of the interaction with a π-donor ligand on thed block is valid for any dn electronic configuration of the complex: sincethe ligand orbital is at lower energy than the d orbital with which itinteracts, the new d orbital, whether it is empty or occupied, is alwaysdestabilized by an antibonding interaction with the ligand π orbital.3.1.4. A first example: the octahedral complex [ML5 Cl]Consider an octahedral complex with one Cl ligand (or any otherhalogen) which is a double-face π -donor (§ 3.1.2) (3-8); the five otherligands only have σ interactions with the metal.π -type interactions2In particular, the orbitals derived fromthe eg block of the regular octahedron, wherewe find the antibonding σ -type interactionswith the ligands, are no longer degenerate inthe lower-symmetry complex [ML5 Cl].In order to construct the d block of this complex, we start from theorbitals that were obtained by only considering σ interactions, then weperturb them by introducing the π interactions.
The d block derivedfrom these σ interactions will be assumed identical to that alreadyestablished for a fully octahedral complex (ML6 ), that is, split into twodegenerate groups (t2g and eg , Chapter 2, § 2.1.2.4). This is of course asimplification, since in an [ML5 Cl] complex, the σ interaction createdby one of the L ligands is different from that involving the Cl.23.1.4.1. Perturbation of the d blockP1P2zClLLMLLxL3-9yBy examining the symmetry properties of the d orbitals of the regularoctahedron as well as those of the π donor, we can predict which oneswill be perturbed by the π interactions. In the case of the [ML5 Cl]complex (3-8), it is convenient to use the symmetry planes xz (P1 ) andyz (P2 ) (3-9).The xy orbital is antisymmetric with respect to the two planes (AA),xz is symmetric with respect to P1 but antisymmetric with respect toP2 (SA), while yz is antisymmetric with respect to P1 but symmetricwith respect to P2 (AS).
The antibonding orbitals, x 2−y2 and z2 , aresymmetric with respect to both planes (SS). On the chlorine atom, thelone pairs (px and py ) have SA and AS symmetries, respectively (3-10).xyxzyzx2–y2z2AASAASSSSSp x (Cl)p y (Cl)SAAS3-10We can now construct the interaction diagram between the orbitalsof the d block and those of the π-donor ligand (Figure 3.2), by combiningorbitals with the same symmetry, px with xz, and py with yz.π-donor ligands: general propertiesnew d blockx2–y2, z2x2–y2, z2xz – pxyz – pyxz, yzxyFigure 3.2. Interaction diagram showing theperturbation of the d block of an octahedralcomplex (σ interactions only, left-hand side)by the two orbitals of a double-face π -donorligand (Cl, for example, right-hand side).
Theelectronic occupation shown corresponds to ad0 electronic configuration.34These expressions are not normalized.Notice that in the point group of thiscomplex (C4v ), the degenerate MO havee symmetry, while xy, x 2−y2 , and z2 have b2 ,b1 , and a1 symmetries, respectively.xypx + xzp x , pypx + yzThese two interactions are equivalent in the sense that they involveorbitals that are related by a rotation of 90◦ : xz and yz on the onehand, px and py on the other. They therefore lead to the formation oftwo degenerate bonding MO and two antibonding MO that are alsodegenerate. As we showed in § 3.1.3, the antibonding MO (xz − λpxand yz − λpy , mainly concentrated on the metal (λ < 1)),3 are theones which belong to the d block of the [ML5 Cl] complex.
The presenceof a chloride ligand therefore lifts the degeneracy of the three initialorbitals in the t2g block of the octahedron, destabilizing two of themwhile leaving the third unchanged.The two remaining orbitals in the d block (x 2 −y2 and z2 ) are identicalto the initial MO derived by considering only the σ interactions, since,by symmetry, they are not involved in the π interactions.4 The twolowest-energy MO (px + λxz and py + λyz) are mainly localized onthe chlorine and characterize the two p lone pairs that are stabilizedby a bonding interaction with the d orbitals of the same symmetry.The occupation of the d block naturally depends on the exact nature ofthe [ML5 Cl] complex; Figure 3.2 corresponds to a complex with a d0electronic configuration.3.1.4.2.
The influence of the electronic configuration on theπ -donor character of the Cl ligandThe interaction scheme given in Figure 3.2 does not depend on thedn electronic configuration of the [ML5 Cl] complex: starting from thed block of the octahedron, two of the three nonbonding orbitals willalways be destabilized while the three other orbitals are unaffected.However, the electron transfer resulting from the π interactions doesdepend on the electronic occupation of the d block.π -type interactionsIn a d0 complex (Figure 3.2), the two lone pairs px and py are partiallydelocalized on to the metal after the interaction (bonding MO px + λxzand py + λyz), and each interaction results in some electron transferfrom the ligand to the metal.
Since all the other orbitals are empty,there is no transfer in the opposite sense: in an [ML5 Cl] complex witha d0 electronic configuration, the chloride does indeed play the roleof a double-face π -donor. A d2 complex behaves in the same way: thetwo electrons in the d block occupy the nonbonding xy orbital after theinteraction, and therefore remain completely localized on the metal.For the d3 –d6 electronic configurations, the xz and yz orbitals, whichwere nonbonding before the interaction, are progressively filled, leadingto some electron transfer in the opposite direction, from the metal tothe ligand. So these orbitals, which were localized on the metal, becomepartially delocalized onto the chloride ligand after the π interaction(antibonding MO xz − λpx and yz − λpy ).
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