Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 30
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Consider the octahedralcomplex [W(Cl)6 ], for example: according to the standard countingprocedure, its electronic configuration is d0 [W(VI)], and therefore it is a12-electron complex. But this description is substantially modified if wetake account of the π interactions. We have already seen that the threeorbitals of the t2g block are destabilized by antibonding interactions withthe p lone pairs on the chloride ligands (3-33).
In this complex, therefore,all the orbitals of the d block are antibonding, those destabilized by the σinteractions (the eg block) being higher in energy than those destabilizedby the π interactions (Figure 3.14). This antibonding character of all thed-block orbitals enables us to understand the stability of this d0 complexrather more easily.12 Moreover, though the π interactions destabilizethree orbitals in the d block, they also stabilize the three orbitals mainlyconcentrated on the lone pairs of the chloride ligands (Figure 3.14). Thet2g orbitals of the metal therefore contribute partially to the occupiedMO.
As a consequence, though it is accurate, following the normaldefinition of the d block, to describe [W(Cl)6 ] as a d0 complex, the t2gorbitals of the metal are in fact partially occupied, and the metal is muchless poor in electrons than it appears at first sight.In view of this analysis, it might seem legitimate for the π electronsthat are transferred, at least partially, to the d orbitals of the metal centreto be included in the electron count. The complex [W(Cl)6 ], previouslydescribed as a 12-electron complex (σ electrons only), thus becomesan . .
. 18-electron complex if one takes account of the six π electronsegeg antibondingd blockt2g antibondingt2gFigure 3.14. Interaction diagram showing theperturbation of the d block for an octahedral[M(Cl)6 ] complex (σ interactions only,left-hand side) by the symmetry-adapted πorbitals (t2g ) on the six double-face π -donorligands. The electronic occupation showncorresponds to a complex with a d0 electronicconfiguration ([W(Cl)6 ], for example).t2gt2gExercises13To understand this stability another way,notice that as a result of the π interactions,there is a fairly large energy gap between thehighest occupied (π bonding) and the lowestunoccupied MO (π ∗ antibonding).‘transferred’ to the metal (Figure 3.14).
It is not a coincidence if the‘18-electron rule’ is now verified with this new counting procedure:once the π interactions are considered, there are no empty nonbondingMO on the metal. To attain the normal count of 18 electrons around themetal, it is therefore sufficient for all the bonding MO (σ and π) to bedoubly occupied; this electron count allows us to understand the stabilityof a complex which at first sight is very electron-deficient.13 The stabilityof the complex [W(Cl)6 ] in an octahedral geometry contrasts strikinglywith the instability of [W(Me)6 ] in this same geometry. The latter isalso a d0 complex, but one for which the π interactions are negligible.It therefore possesses three MO based on d orbitals that are nonbondingand unoccupied, and it can be described as a ‘true’ 12-electron complex.In fact, it adopts a different geometry, which is trigonal-biprismatic; wemay conclude that the octahedral geometry of [W(Cl)6 ] is controlledby π interactions.This analysis can be extended to other ligand fields, and is not limitedto complexes whose d block is completely empty.
In the presence ofπ-donor ligands, the apparent electron deficiency is compensated byinteractions with the lone pairs on the ligands. Consider tetrahedralcomplexes such as [Ti(Cl)4 ] or [Zr(Cl)4 ], which are d0 complexes ofTi(IV) or Zr(IV), and therefore complexes with only eight electrons!However, it can readily be shown that five of the eight orbitals thatcharacterize the lone pairs on the four chloride ligands are able, bysymmetry, to interact with the d-block orbitals (Chapter 2, § 2.4.1): twohave e symmetry and three have t2 symmetry.
In this way, 10 π electronsare ‘transferred’ to the metal, so the complexes may be considered tohave 18 electrons.Exercisesπ interactions in an ML4 complex3.11. Indicate the shapes and the relative energies of the fourlowest-energy d-block orbitals for a square-planar ML4 complex(σ interactions only).2. How are these shapes and energies modified in the complexes?(a) [PtCl4 ]2−(b) [Ni(CN)4 ]2−3. Repeat the question for Wilkinson’s catalyst, [Rh(PPh3 )3 Cl]; forsimplicity, the triphenylphosphine ligands may be considered tobe pure σ donors.π -type interactionsπ interactions in an ML5 complex (TBP)3.21. Indicate the shapes and the relative energies of the four lowestenergy d-block orbitals for an ML5 complex in a TBP geometry(σ interactions only).2. How are the shapes and energies of these orbitals modified ifa carbonyl group is in an axial position (1a), or an equatorialposition (1b)? (for the first structure, show that certain overlapsthat are non-zero by symmetry are in fact very small, so they maybe neglected).OCM1aClM C1bOMM2a2bCl3.
Deduce the more favourable substitution site for π interactionsin diamagnetic d4 and d8 complexes.4. Repeat question 2 for complexes substituted by a chloride ligandin an axial (2a) or equatorial position (2b).5. Deduce the more favourable substitution site for π interactionsin diamagnetic d4 and d8 complexes 2a and 2b.π interactions in an octahedral ML6 complex3.31. By writing a Lewis structure, show that C≡N is an X-type ligand.2. Assuming that the order of increasing energy for the MO is thesame in CN as in C≡O,(a) indicate the nature of the MO that has a σ interaction with ametal centre;(b) give the shape and the occupation of the π-type MO;(c) indicate whether this ligand is a π acceptor or a π donor;(d) indicate whether the CN ligand binds to the metal throughcarbon or nitrogen in the η1 coordination mode.3.
Give the shapes and relative energies of the three lowest-energyd orbitals in an octahedral complex of the type [M(CN)6 ].4. In which of the two complexes [Fe(CN)6 ]4− and [Fe(CN)6 ]3− arethe Fe−CN bonds shorter?5. Is this result predictable just from the oxidation state of the metalin the two complexes?Exercises3.4Consider an octahedral complex [ML4 Cl2 ] in which the Cl ligandsare in cis positions: the x-axis bisects the Cl−M−Cl angle, as shownbelow.LLMzLyxClClL1.
Identify the two planes of symmetry in the complex (P1 and P2 ).2. Give the shapes of the three nonbonding orbitals for a regularoctahedral complex in which the ligands only have σ interactionswith the metal. Indicate their symmetry properties with respectto P1 and P2 .3. Construct the four symmetry-adapted orbitals that characterizethe π -type lone pairs on the Cl ligands, and give their symmetrieswith respect to P1 and P2 .4. How many lone pairs on the ligands are involved in π interactions?5.
Place these seven orbitals (three d orbitals on the metal and fourlone-pair orbitals) on an energy-level diagram, and justify therelative positions:(a) of the ligand and metal orbitals;(b) of the four different ligand orbitals.6. Construct the interaction diagram, and indicate which of theresulting MO belong to the d block.7. How many electrons are there around the metal according to thetraditional count in the case of a d0 complex?8. How many electrons are there once the π transfers are taken intoaccount?Pentagonal bipyramidal ML7 complex3.5Among the different geometries that can be adopted by an ML7complex, the pentagonal-bipyramid (PBP), with two axial ligands(L1 and L7 , on the z-axis) and five equatorial ligands (L2 –L6 , in thexy plane), is one of the most common.L3L2L1ML6 L7zL4L5xyπ -type interactions1. How many of the d orbitals are strictly nonbonding? Give theshapes of these orbitals.2.
Consider PBP complexes with a d4 electronic configuration inwhich there is one carbonyl ligand, in either an axial (L1 ) or anequatorial position (choose the position L2 ).(a) analyse the π-type interactions in each isomer;(b) deduce the favoured site for a carbonyl ligand.The use of group theory3.6Solve Exercise 3.1 (question 2) by making use of the results of Exercise 6.13 (Chapter 6) on the symmetry-adapted orbitals for ML4complexes with a π system on the ligands, and the character tablefor the D4h point group (Table 6.18).3.71. Give the shapes and relative energies of the five d-block orbitalsfor a trigonal-planar ML3 complex (σ interactions only).2. Consider the complex [Ni(CO)3 ] in this geometry.
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