Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 19
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They can either bind other ligands, or exist as chains, inwhich the coordination number of the metal is higher than two, ratherthan as ML2 monomers.We should mention a few complexes in which the d block is notcompletely filled. For example, complexes with two borylamide ligands[N(Mes)(BR 2 )] are known for M=Mn (d5 ), Fe(d6 ), Co (d7 ), and Ni (d8 ),and they adopt an almost linear geometry.
These are all high-spin complexes, which is understandable given the small separation of the energylevels in the d block.2.8. Other complexes or MLn fragmentsOther structures exist for MLn complexes besides those considered sofar, though they are less common for stable species. We shall study fiveOther complexes or MLn fragmentsof them in this section: pyramidal ML3 complexes (2-74a), ‘T-shaped’ML3 (2-74b), ‘butterfly’ ML4 (2-74c), bent ML2 (2-74d), and ML (2-74e).LLMLLL2-74aL ML2-74bLLMLLL2-74cM2-74dML2-74eThese entities are interesting since they may be considered as‘fragments’ of more important complexes, such as octahedral orsquare-planar.
In this context, knowledge of their orbital structure isuseful for two reasons:1. They can be used as fragment orbitals in some complexes. For example, the main MO of the complex [(η5 cyclopentadienyl)Mn(CO)3 ] (2-76) can be constructed by interactionof the π MO of cyclopentadienyl with the d orbitals of the metallicfragment [Mn(CO)3 ] in a pyramidal geometry.2. The d orbitals of these species ‘resemble’ the orbitals of certainorganic molecules or fragments, both in their shape and their electronic occupation.
It is therefore important to be familiar with themif one wishes to establish a link between the electronic structuresof transition metal complexes and of organic molecules (the isolobalanalogy, see Chapter 5).2.8.1. Pyramidal ML3 complexesThe pyramidal ML3 structure (2-74a), with C3v symmetry, is notcommon for complexes with three ligands. But a few examplesare known, such as the d0 complexes [M(HC(SiMe3 )2 )3 ] (M=Y orLa), or the low-spin d6 complexes [M(Mes)3 ] (M=Rh or Ir) (2-75).Moreover, the pyramidal ML3 entity can be considered as a fragmentin 18-electron complexes of the type [(polyene)-ML3 ], such as [(η4 cyclobutadiene)Fe(CO)3 ], [(η5 -Cp)Mn(CO)3 ] (Cp=cyclopentadienyl)(2-76) or [(η6 -benzene)Cr(CO)3 ].MeMnOCOCRhCOMesMesMesMesMe=Me2-762-75Principal ligand fields: σ interactionsIn one approach to the derivation of the d-block orbitals of this complex, we start from an octahedral complex with the geometry indicatedin 2-77 and remove the three ligands L4 , L5 , and L6 .
A pyramidal ML3fragment is thereby obtained, in which the angles between the M–Lbonds are 90◦ . In the orientation that is chosen, the z-axis coincides withthe C3 axis of the resulting ML3 complex.zL5L4xL6yMML3L1L3L1L2L22-77The t2g (nonbonding) and eg (antibonding) orbitals of an octahedralcomplex with this orientation of the axes have already been established (§ 2.1.2.5, Figure 2.5) and are presented on the left-hand sideof Figure 2.13.
The removal of three ligands does not change the threenonbonding orbitals, since the coefficients on these ligands were zero:they remain nonbonding and degenerate in the ML3 fragment.The z2 orbital has a1 symmetry (the totally symmetric representationof the C3v point group), while the two other orbitals are degenerate bysymmetry (1e). The eg orbitals are stabilized by the elimination of halfof the antibonding interactions that were present in the octahedron.However, they are still antibonding in the ML3 fragment (Figure 2.13)and make up the degenerate orbital set 2e.
As in several of the examplesalready treated, the antibonding character of these orbitals is reduced bymixing with the px (2-78) or py (2-79) orbitals, which polarizes them inthe direction opposite to the ligands.eg2a12et2gFigure 2.13. Derivation of the d-block orbitalsfor a pyramidal ML3 complex from the dorbitals of an octahedral ML6 complex. Thehybrid s–p orbital lying above the d block isalso shown.1e1a1MMOther complexes or MLn fragments+px2-78+py2-792-80If the d block were completely filled, we would have a 16-electroncomplex.
There would therefore be an empty nonbonding orbital on themetal centre. For a trigonal-planar ML3 complex, that would be the pzorbital perpendicular to the molecular plane (2-69). But in the presentcase, it is a hybrid orbital of a1 symmetry, a linear combination of thes and p orbitals on the metal, which points in the direction opposite tothe ligands (2-80). Due to the relative energies of the nd, (n + 1)s, and(n + 1)p AO on the metal, it stays higher in energy than the 2e orbitalswhich are essentially of d character on the metal and weakly antibonding.This orbital lies lower than the pure p orbital of a trigonal-planar ML3complex, thanks to the contribution from the s orbital (εs < εp ).Notice that the structure of the d block, with three strictly nonbonding orbitals, enables us to understand why this geometry is observed forlow-spin d6 complexes, such as that represented in 2-75.2.8.2.
‘T-shaped’ ML3 complexes12There are also a few planar complexes inwhich two of the three L–M–L angles arelarger than 120◦ (Y-shaped structures). Thepreference for T- or Y-shaped structures willbe analysed in greater detail in Chapter 4. Forfurther information on the structures of ML3complexes, the reader may consult: S. AlvarezCoord. Chem. Rev. 193–195, 13 (1999).There are a few rare ML3 complexes that adopt a non-trigonal planargeometry, in which one L–M–L angle is substantially larger than 120◦(C2v symmetry).12 This type of structure is sketched in 2-81, where theangle L1 –M–L3 has the limiting value of 180◦ , justifying the name ofa T-shaped structure.
The diamagnetic d8 complexes [Rh(PPh3 )3 ]+ (282) and [Ni(Mes)3 ]− are typical examples. The T-shaped entity ML3 isalso interesting as it can be used as a fragment in monometallic complexes such as Zeise’s salt, [Pt(Cl)3 (η2 -ethylene)]− (2-83), or bimetalliccomplexes of the type [Pd2 L6 ]2+ (2-84).L3L2ML12-81P(Ph)3180°+(Ph)3 P Rh159°P(Ph)32-82Principal ligand fields: σ interactions2+ClCl−ClPdPd2-83Pd2-84The orbitals of the d block may be derived from those of a squareplanar complex (§ 2.2.1), by removing one of the ligands (2-85).zxL3L2 ML1yL3L4L2 ML12-85Changes to the shapes and energies of the orbitals are shown inFigure 2.14.The three strictly nonbonding orbitals in the square-planar complex(with b2g and eg symmetries in the D4h point group) are not affected bythe removal of a ligand.
They become the a2 , b1 , and b2 nonbondingorbitals of the ML3 fragment. The z2 orbital (a1g ) is very weakly stabilized (1a1 ), due to the disappearance of a small antibonding interaction,while the x 2 −y2 orbital, which is antibonding towards the four ligandsin the square-planar complex (b1g ), is stabilized and becomes the 2a1orbital in the ML3 complex. This stabilization is enhanced by mixingwith the py orbital which polarizes the orbital in the opposite directionto the ligand L2 (2-86).
But overall, this orbital stays fairly high in energy,due to the three antibonding interactions that are still present in the xyplane, along the directions of the lobes of x 2 −y2 .b1g2a1a1g1a1egb1b2gFigure 2.14. Derivation of the d-block orbitalsof a T-shaped ML3 complex from the dorbitals of a square-planar ML4 complex.MMb2a2Other complexes or MLn fragments+=2-86The T-shaped ML3 fragment is thus characterized by the presenceof four low-energy d orbitals, instead of the five present in the trigonalplanar geometry (§ 2.6.1). As a result, the stable complexes mentioned atthe beginning of this section, such as [Rh(PPh3 )3 ]+ (2-82), are diamagnetic and have a d8 electronic configuration, whereas the d10 complexes,which are far more numerous, adopt a trigonal-planar geometry (§ 2.6.2).2.8.3.
‘Butterfly’ ML4 complexesIn the ‘butterfly’ ML4 complex sketched in 2-74c, one of the L–M–Langles is 180◦ , where the two bonds form the body of the butterfly. The angle between the other two bonds, which form the (folded)wings of the butterfly, is much smaller. More generally, ML4 fragmentsare described as ‘butterfly’ when they have C2v symmetry, which onlyimplies the existence of two planes of symmetry and a C2 -axis (2-87). Itis therefore possible for the two angles mentioned to have very variablevalues, so long as they are not equal (the symmetry would then be D2d ,as for the intermediate points along the deformation shown in 2-53).This ML4 structure, adopted by some complexes such as [Fe(CO)4 ] or[Ir(H)2 (P(t Bu)2 Ph)]+ (2-88), can also be regarded as a fragment in theanalysis of the electronic structure of other species, such as the carbenecomplex [Fe(CO)4 (CR 2 )] (2-89) or the bimetallic complex [Fe2 (CO)8 ](2-90).+P1LLP2LMHC288°HL2-87COOCOCFeCO2-89P(t Bu) 2 PhIr174°P(t Bu) 2 Ph2-88OCOCCRCOCORFeFeCOCOCOCO2-90Principal ligand fields: σ interactionsA butterfly ML4 fragment can be obtained by the removal of twocis ligands in an octahedral complex (L5 and L6 in 2-91).
The L1 –M–L2and L3 –M–L4 angles are then 90◦ and 180◦ , respectively. The d orbitalsthat we shall establish are adapted to this particular geometry, so somechanges may be anticipated if these angles have different values (see, forexample, Exercise 2.9).zL2L3ML1xyL3L6L2L5L1ML4L42-9113In the character table for the C2v group(Chapter 6, § 6.2.5.1), one does indeed findthat the symmetries of the xy, xz, and yzorbitals are a2 , b1 , and b2 . However, the nameof the orbital in a given symmetry is not thesame as that we have just indicated.
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