Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 18
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Representation of the d-blockorbitals for an ML5 complex with a TBPgeometry, the equatorial plane (xy) beingeither perpendicular to the plane of the page(left-hand side), or in this plane (right-handside). In this latter case, for greater clarity, theaxial ligands are not shown.yzyxLeqLaxzLeqMLeqMLeqLaxLaxLaxLeqyzxLeq2.5.2.
Electronic structure2.5.2.1. Diamagnetic d8 complexese⬘e⬙2-6610For a review of the structures of ML5complexes, consult: S. Alvarez and M. LlunellJ. Chem. Soc. Dalton Trans. 3288 (2000).The most common electron count for ML5 complexes with a TBPgeometry is d8 , which corresponds to the double occupation of thedegenerate e′′ and e′ orbitals (2-66). These are therefore 18-electroncomplexes, since there are 10 additional electrons associated with thefive M–L bonds.The species [M(CO)5 ] (M=Fe, Ru, and Os) are ‘archetypal’ TBPML5 complexes. We may also cite the complexes [Mn(CO)5 ]− ,[Fe(CO)4 (CN)]− ,[Co(C=N − CH3 )5 ]+ ,[Fe(CO)4 (η2 -C2 H4 )],2[Ir(PR 3 )3 (CH3 )(η -C2 H4 )], and [Ni(P(OEt)3 )5 ]2+ . It should be notedthat the d8 electron count has already been found to be favourable forML5 complexes that adopt an SBP geometry, with the metal abovethe basal plane (§ 2.3.3.1.). The two structural types have indeed beencharacterized in the d8 − ML5 family of complexes.10 In the case of theTrigonal-planar ML3 complexes11The transformation TBP(1) → SBP →TBP(2) is nothing more than the Berrypseudorotation which allows the exchange ofaxial and equatorial ligands to take place in aTBP complex.anion [Ni(CN)5 ]3− , the SBP structure can even coexist with a slightlydistorted TBP structure when the associated cation is [Cr(en)3 ]3+(en=ethylenediamine).112.5.2.2.
Other casesThe complex [Co(dpe)2 Cl]2+ (dpe=1,2-bis(diphenylphosphino)ethane) with a d7 electronic configuration (a 17-electron low-spin complex) is also known in two structural forms: a red isomer with an SBPgeometry (the chloride ligand is in the apical position), and a greenisomer with a TBP geometry (the chloride ligand is in an equatorialposition). TBP complexes with more than 18 electrons are also known.They are weak-field complexes (the z2 orbital is occupied) with a d9electronic configuration (19 electrons), such as [CuCl5 ]3− , [CuBr5 ]3− ,or [Cu(imidazole)3 Cl2 ], or d10 (20 electrons), such as [CdCl5 ]3− or[HgCl5 ]3− .2.6.
Trigonal-planar ML3 complexesA trigonal-planar ML3 complex can be formed by removing the twoaxial ligands from a TBP ML5 complex (2-67). The d-block orbitals maytherefore be readily deduced from those established in the precedingsection.L4L3zxMyL3L1L2L2ML1L52-672.6.1. Characterization of the d blockThe variations in energy for the five d orbitals are shown in Figure 2.11.The xz and yz orbitals (e′′ ) of a TBP ML5 complex have zero coefficientson the axial ligands, as do xy and x 2 −y2 (e′ ). The removal of these twoligands does therefore not change either the shapes or the energies ofthese four orbitals (Figure 2.11).
But the z2 orbital, which was stronglyantibonding due to interactions with the axial ligands, is substantiallystabilized by their removal, becoming almost nonbonding. Only threeweak antibonding interactions with the ligands in the xy plane are left,and the amplitude of the z2 orbital in this plane is small. Note that theorbital symmetries are the same in ML5 (TBP) and ML3 trigonal-planarcomplexes, as both have D3h symmetry.Principal ligand fields: σ interactionsa1⬘e⬘e⬘x2–y2xya1⬘e⬙Figure 2.11. Derivation of the d-block orbitalsfor a trigonal-planar ML3 complex from thoseof an ML5 complex with a TBP geometry.z2e⬙yzxz2.6.2. 16-electron d10 complexese⬘a⬘1e⬙2-68The d-block for ML3 complexes with a trigonal-planar geometry thuscontains two nonbonding degenerate orbitals (xz and yz), a very weaklyantibonding orbital (z2 ), and two weakly antibonding degenerate orbitals (xy and x 2 −y2 ).
These five orbitals are sufficiently low in energyto be doubly occupied. As a result, the large majority of these complexes have a d10 electronic configuration (2-68), the simplest no doubtbeing [PdH3 ]3− . We may also cite the tricarbonyl complexes [M(CO)3 ](M–Ni, Pd, or Pt), as well as [Pt(Ph3 P)3 ], [Pt(η2 -ethylene)(PR 3 )2 ], or[Ni(η2 -ethylene)3 ], complexes in which the oxidation state of the metalis zero. There are also complexes of the metals from groups 11 (oxidation state I), such as [Cu(CN)3 ]2− , [Cu(SR)3 ]2− , or [Au(PPh3 )2 Cl], and12 (oxidation state II), such as [HgI3 ]− and [Hg(SR)3 ]− .These are all 16-electron complexes (six for the bonds and ten in thed block).
The ‘lack’ of two electrons compared to the 18-electron rulearises because a nonbonding orbital on the metal remains empty. As inthe case of square-planar ML4 complexes, this is the p orbital perpendicular to the molecular plane (2-69), which, although nonbonding, is toohigh in energy to be occupied.2-692.7. Linear ML2 complexesThere are many ways in which a linear ML2 complex may be obtainedfrom the complexes already studied. For example, four coplanar ligandscan be removed in an octahedral complex (2-70).zL1xL6L3ML5yL1ML4L2L22-70Linear ML2 complexesegz2 (g)t2gFigure 2.12. Derivation of the d-block orbitalsfor a linear ML2 complex from the d orbitalsof an octahedral ML6 complex.xzyz(g)xyx2–y2(g)2.7.1. Characterization of the d blockConsider the orbitals in the d block of an octahedron (Figure 2.12, lefthand side).
Removal of the four ligands L3 –L6 has no effect on thethree nonbonding t2g orbitals which have zero coefficients on all theligands. The antibonding x 2 −y2 orbital in the eg block has antibondinginteractions with the four ligands L3 –L6 , but zero coefficients on L1and L2 . After the removal of the ligands L3 –L6 , this orbital becomesstrictly nonbonding (Figure 2.12). There are, therefore, four nonbondingorbitals in the d block of linear ML2 complexes, whose symmetries areπg (xz, yz) and δg (x 2 −y2 , xy) in the D∞h point group. These orbitalsare therefore degenerate in pairs by symmetry, but in fact all four havethe same energy (accidental degeneracy).CommentNotice that the π-type orbitals have just a single nodal plane that containsthe internuclear axis, whereas the δ orbitals have two.We have not yet considered the second orbital in the eg block ofthe octahedron, z2 .
It is stabilized by the elimination of the four weakantibonding interactions in the xy plane, but this stabilization should berather small, as the two principal antibonding interactions, along thez-axis, are still present.In fact, this fifth orbital is not very high in energy since it is polarizedby mixing with the s orbital of the metal (s and z2 have the same symmetry, σg , in the D∞h point group). As in the previous examples whichinvolved polarization by a p orbital (see also Appendix A, § 2.A3), thesign of the s orbital is such that its interaction with the ligand orbitals isbonding (2-71).
The effect of this mixing is to increase the amplitude ofthe z2 orbital in the xy plane, perpendicular to the internuclear axis, andto decrease it along this axis. The antibonding character of the polarizedPrincipal ligand fields: σ interactionsz2 orbital is therefore substantially reduced by the participation of the sorbital, so that its energy is not very high (Figure 2.12).S >0+=S >0z2polarized z 2s2-712.7.2. Electronic structuregg, g2-72pxpy2-73In the very large majority of linear (or quasi-linear) ML2 complexes,the five orbitals of the d block are doubly occupied (d10 electronicconfiguration, 2-72). The most common examples involve the elementsof group 11 (Cu, Ag, Au), where the metal is in the oxidation stateI.
Anions such as [Cu(Me)2 ]− , [Cu(Ph)2 ]− , [Cu(Mes)2 ]− (Mes =2, 4, 6−Me3 C6 H2 ), [Ag(CN)2 ]− , [Ag(C(SiMe3 )3 )2 ]− , [Au(C6 F5 )2 ]− ,or [AuCl2 ]− , cations such as [Cu(NH3 )2 ]+ , [Ag(NH3 )2]+ , or[Ag(CO)2 ]+ , and neutral complexes such as [AuMe(PMe3 )] or[AuCl(Mes)] are all known. There are also examples from group 12,such as [Hg(CN)2 ].Although the d block is full, these complexes have only 14 electrons.This electronic deficiency is linked to the presence of two p orbitals onthe metal (2-73) which remain empty, even though they are nonbonding,since they are too high in energy to be occupied. The small number ofligands, combined with this electron deficiency, makes these complexesvery reactive.
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