Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 26
Текст из файла (страница 26)
The interaction diagramWe are now in a position to construct the diagram which describes theπ interactions in a trans-[ML4 Cl2 ] complex (Figure 3.6). We shall adoptthe notations of group theory to represent the orbitals’ symmetries. Asin the preceding examples (§ 3.1.4 and 3.2.4), we suppose that the d blockof the complex, before the interaction, is similar to that of a regular octahedral complex, with three nonbonding degenerate orbitals (xy, xz, andyz with the axes defined in 3-19) and two degenerate antibonding orbitals, x 2−y2 and z2 (left-hand side of Figure 3.6).
The first three orbitals,which formed the t2g block in the regular octahedron, have b2g (xy) andeg (xz, yz) symmetries in the D4h point group. The symmetries of theantibonding orbitals derived from the eg block in the octahedron becomeComplexes with several π-donor or π-acceptor ligandsx2–y2, z2z2 (a1g)x2–y2 (b1g)xz–*xyz–*xxz, yz (eg)xyxy (b2g)x*, y* (eu)Figure 3.6. Interaction diagram showing theperturbation of the d block of an octahedralcomplex (σ interactions only, left-hand side)by the lone pairs of two double-face π -donorligands (Cl, for example, right-hand side) intrans positions. The electronic occupationshown corresponds to a complex with a d0electronic configuration.x, y (eu)*x + xz*x + yza1g for z2 and b1g for x 2−y2 .
The occupation of these orbitals dependson the dn electronic configuration of the complex, the example given inFigure 3.6 corresponding to a d0 complex whose d block is empty. Wefind the π -type orbitals of the chlorine atoms on the right-hand side ofthe figure (3-20). The eu bonding combinations are placed slightly lowerin energy than the eg antibonding combinations, though the energydifference is only slight, due to the large separation between the twocentres. The energies of these four orbitals are therefore close to that ofa pure p orbital on chlorine.
As a result, they are placed at a lower energylevel than the nonbonding d orbitals on the metal. Notice that these πand π ∗ orbitals are all doubly occupied, and that they offer a delocalizeddescription of the four π -type lone pairs on the chloride ligands (twoper ligand).For symmetry reasons, as we have already established, the two lonepairs described by the orbitals with eu symmetry (3-20) cannot interactwith the d-block orbitals. The only interactions that are possible (S = 0)occur between the lone pairs with eg symmetry and the (xz, yz) orbitalson the metal (Figure 3.6). This interaction stabilizes four electrons fromthe lone pairs (the πx∗ + λxz and πy∗ + λyz orbitals, mainly concentrated on the Cl ligands) but destabilizes two of the three octahedralnonbonding orbitals (xz − λπx∗ and yz − λπy∗ , mainly concentrated onthe metal).
This destabilisation is larger than that produced by a singlechloride ligand ([ML5 Cl] complex, Figure 3.2) since there are now twoπ-type antibonding interactions for each of these orbitals. Two of the lonepairs, and therefore four electrons, are not affected by these interactions.π -type interactionsThe new d block of the complex, as always, contains five orbitals thatare mainly composed of the d orbitals on the metal: xy, nonbonding,xz−λπx∗ and yz−λπy∗ , π antibonding, and x 2 −y2 and z2 , σ antibonding.This example shows us that for symmetry reasons, the π interactionsare not simply additive when the number of ligands is increased.
Twolone pairs are involved in both the monochloro octahedral complex(Figure 3.2) and the trans dichloro complex, although four lone pairsare a priori available in the latter case. The difference between the twocomplexes concerns the magnitudes of the destabilization of the xz andyz orbitals of the d block and the stabilization of the lone pairs thattake part in the interactions, both of which are larger in the dichlorocompound. It must also be noted that for a given type of complex(octahedral, for example) and a given number of π -donor (or -acceptor)ligands, the symmetry properties of the complex, and therefore themetal–ligand interactions which occur, also depend on the arrangementof these ligands.
This point is illustrated in Exercise 3.4, which concernsan octahedral [ML4 Cl2 ] complex in which the two chloride ligands arenow in cis positions.3.3.2. The trans-[ML4 (CO)2 ] octahedral complexThis complex has the same symmetry as the preceding one (the D4hpoint group). Since the carbonyl groups are π -acceptors, we considerthe two empty πx∗ and πy∗ orbitals on each, and these are combined inpairs to form the symmetry-adapted orbitals for the complex (3-23). The∗(+)∗(+)and πy ), which are antisymmetricbonding combinations, (πxwith respect to the inversion centre, have eu symmetry, whereas the∗(−)∗(−)and πy ), which are symmetricantibonding combinations, (πxwith respect to this operation, have eg symmetry.*(–)x*(+)y *(+)x *(–)yegeu3-23Since the overlaps between the πx∗ (or πy∗ ) orbitals are very small, dueto the large distance between the ligands, the energies of the bondingComplexes with several π-donor or π-acceptor ligands*(–)x –xz*(–)x –xzx2–y2, z2z2 (a1g)x2–y2 (b1g)*(–)*(–)x , y (eg)x*(+), y*(+)(eg)xyxz, yz (eg)xy (b2g)Figure 3.7.
Interaction diagram showing theperturbation of the d block of an octahedralcomplex (σ interactions only, left-hand side)by the π ∗ orbitals of two double-faceπ-acceptor carbonyl ligands (right-hand side)in trans positions. The electronic occupationshown corresponds to a complex with a d6electronic configuration.xz+λx*(–)yz+λy*(–)(eu ) and antibonding (eg ) combinations are close to that of a π ∗ orbitalin an isolated carbonyl ligand.
As a result, they are higher in energy thanthe nonbonding d orbitals on the metal, with the eu orbitals being veryslightly more stable than the eg (Figure 3.7, right-hand side). These fourorbitals are, of course, empty. On the left-hand side of the figure, wefind the d-block orbitals, assumed to be those of a regular octahedronwithout any π interactions. The occupation of these orbitals dependson the electronic configuration of the complex that is considered.
InFigure 3.7, we are concerned with a d6 complex. As in the precedingcomplex, only two of the four π-type orbitals on the ligands—the egorbitals—and two nonbonding orbitals on the metal (xz and yz, whichalso have eg symmetry) can interact. This interaction therefore stabil∗(−)∗(−)izes those two d-block orbitals (xz + λπxand yz + λπy , mainlyconcentrated on the metal), whereas the antibonding combinations are∗(−)∗(−)− λyz). Themainly concentrated on the ligands (πx − λxz and πy∗(−)new d block therefore consists of the following orbitals: xz + λπxand∗(−)yz + λπy , which are π-bonding, xy (nonbonding), and x 2−y2 and z2 ,which are σ -antibonding.In the case of a d6 complex, four electrons from the d block arestabilized.
The stabilization is larger than that produced in the complexwith just one carbonyl ligand (Figure 3.5), since there are two π -bondinginteractions in each orbital stabilized instead of only one.3.3.3. Construction of the d-block orbitals ‘by hand’The relative energies of the π -type ligand orbitals and the metal d orbitalscontrol the nature of the two interactions that occur in the d block: (i) anπ -type interactionsantibonding interaction with the occupied orbital of a π-donor, whichdestabilizes the d orbital; (ii) a bonding interaction with the emptyorbital of a π-acceptor which stabilizes the d orbital.
The shape of theperturbed d orbitals can easily be obtained when there is a single ligandwith a π system: one only needs to combine the d orbital and the π-typeorbital with which it can overlap, in a bonding manner for an acceptorbut in an antibonding sense for a donor. When there are two ligandsof this type, we need first to construct the symmetry-adapted π MO forthe complex, and then decide which of these could interact with thed orbitals. Once this is done, the same rules are applied for the perturbation of the d orbitals by the symmetry-adapted orbitals on the ligands:an antibonding mixing with destabilization for π -donors, but a bondingmixture with stabilization for π-acceptors (Figures 3.6 and 3.7).
In thetwo complexes we have studied, the shapes of the symmetry-adaptedorbitals are obvious (§ 3.3.1 and 3.3.2), but it can be much more difficultto obtain them in other cases (see, for example, Chapter 6, § 6.6.6 andExercise 6.13). We now consider whether it is always necessary, for eachtype of complex (octahedral, TBP, square-planar, etc.) and for each typeof substitution, to determine the symmetry-adapted combinations ofligand orbitals before being able to discover the shapes of the perturbedd orbitals. In principle, it is indeed necessary to work in stages, alongthose lines, but we are now going to show, starting from the two preceding examples, how the main thrust of the information can be obtainedmore quickly.3.3.3.1. The trans-[ML4 Cl2 ] and trans-[ML4 (CO)2 ]complexes revisitedThe d ↔ π interactions due to a single Cl ligand were studied in themonochloro complex ([ML5 Cl], Figure 3.2).
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
