Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 13
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The last two (φ3 and φ4 ), on theother hand, have maximum overlap with the ligands and are the twoantibonding eg orbitals (Figure 2.5).2.1.2.6. Weak fields and strong fieldsIn the absence of any ligand (isolated metal atom), the five d orbitals ofcourse all have the same energy. One can therefore represent the effectof complexation on the d orbitals by Scheme 2-32, where the d block ofthe complex is considered as defined earlier.
The fivefold degeneracy ofthe d orbitals is lifted, to give three nonbonding orbitals (t2g ) and twoantibonding orbitals (eg ).Principal ligand fields: σ interactionseg∆0disolated metalt2goctahedral complex (d-block)2-32The energy separation (0 ) between the t2g orbitals (nonbonding)and the eg orbitals (antibonding) depends on the strength of the σ interactions between the metal and the ligands. The value of this energy gapallows us to distinguish, in the family of octahedral complexes, strongfield (large 0 ) from weak-field complexes (small 0 ) (see Chapter 1,§ 1.6.2). Measurement of the energies of d–d transitions allows us toestimate the value of 0 in a large number of complexes, and to establish a spectrochemical series, in which the ligands are ranked according tothe strength of the field (value of 0 ) that they create:I− < Br− < Cl− < F− < OH− < O2− < H2 O < NH3−−−< NO−2 < CH3 < C6 H5 < CN < COOrganometallic complexes, which contain one or several metal–carbonbonds, are thus strong-field complexes.CommentThe value of 0 , and therefore the order of the ligands in the spectrochemical series, does not depend only on σ interactions.
We shall see in Chapter 3(§ 3.3.4.2) that the presence of π interactions can either decrease the valueof 0 (‘π-donor’ ligands such as I− or Br− ) or increase it (‘π -acceptor’ligands such as CN− or CO).2.1.3. Electronic structure2.1.3.1. d6 diamagnetic complexesegt2g2-33The notion that a complex is stable if all its bonding and nonbondingMO are doubly occupied is verified for many octahedral complexes.The six bonding MO, which form the six metal–ligand bonds, and thethree nonbonding MO (t2g ) of the d block are thus doubly occupied,giving diamagnetic complexes (all the electrons are paired) with 18 electrons. As six electrons occupy the d block, the electronic configurationof these complexes is written as d6 or (t2g )6 (2-33).If we limit ourselves to octahedral complexes with six identicalligands that only have σ interactions with the metal (the modelOctahedral ML6 complexesstudied in this chapter), examples include the complexes [Fe(H6 )]4− ,[Ru(NH3 )6 ]2+ , [Co(NH3 )6 ]3+ , [Rh(NH3 )6 ]3+ , and [Ir(NH3 )6 ]3+ whichare all 18-electron complexes, whose electronic configuration is d6 (complexes of Fe(II), Ru(II), Co(III), Rh(III), and Ir(III), respectively).
Thepresence of ligands that can have π-type interactions with the metalwill of course modify the interaction scheme shown in Figure 2.2, butwill not change it drastically. Thus, the complexes [M(CO)6 ] (M=Cr,Mo, W), [Re(CO)6 ]+ , [M(CN)6 ]5− (M=Mn, Re), [M(CN)6 ]4− (M=Fe,Os), [Co(CN)6 ]3− , [Mn(CNR)6 ]+ , [Fe(CNR)6 ]2+ , [M(H2 O)6 ]3+ (M=Co,Rh, Ir), [MCl6 ]3− (M=Rh, Ir) are also diamagnetic d6 complexes. Inother complexes, the presence of different ligands can, from the rigorous perspective of group theory, lower the symmetry of the complexconsiderably: the bond lengths are no longer all equal and the bondangles can deviate from the ideal value of 90◦ . However, the orbitalscheme of the octahedron is essentially preserved (one may speak of‘pseudo-octahedral’ symmetry), in particular as far as the main features of the d block are concerned: there are three energy levelsbelow two, even if, within each of these groups, the strict degeneracy due to symmetry has disappeared.
Complexes such as [M(CO)5 I]−(M=Cr, Mo), [M(CO)5 Cl] (M=Mn, Re), [Ru(CO)4 (Cl)(CH3 )],[Ir(CO)2 (PR3 )(I)(Cl)(Me)], [Mo(PR3 )4 (η2 -C2 H4 )2 ], the molecularhydrogen complex [W(PR3 )2 (CO)3 (η2 -H2 )] and even ferrocene[Fe(η5 -C5 H5 )2 ] (Chapter 1, Scheme 1-7) are all pseudo-octahedraldiamagnetic complexes, whose electronic configuration is d6 .2.1.3.2. Other cases4Or orbitals ‘derived’ from the t2g blockwhen octahedral symmetry is broken by thepresence of different ligands.There are octahedral complexes that have fewer than 18 electrons.d0 complexes in which the d block is completely empty are anextreme case: examples include [MF6 ] complexes (M=Cr, Mo),[WCl6 ], [MF6 ]− (M=V, Ta), [MCl6 ]− (M=Ta, Nb), [MF6 ]2− (M=Ti,Zr), [ZrCl6 ]2− , [Mo(OMe)6 ], and the organometallic Ti(IV) complex [Ti(PR3 )2 (Cl)3 (CH3 )].
These are very electron-deficient complexes,since formally there are only the 12 electrons that form the 6 metal–ligandbonds. However, we note that the ligands usually have one or two lonepairs that are not involved in the metal–ligand bond (halide ligands, forexample). These lone pairs play an important role in stabilizing complexes that apparently have too few electrons (Chapter 3, § 3.5). Anotherinteresting case concerns complexes in which only three electronsoccupy the d block (15-electron complexes whose electronic configuration is d3 ). In the ground state, one electron occupies each of the orbitalsin the t2g group,4 and the three electrons have parallel spin (Hund’srule).
As typical examples, we mention [Cr(NH3 )6 ]3+ , [Mo(H2 O)6 ]3+ ,[V(H2 O)6 ]2+ , [MF6 ] (M=Rh, Ir), [RuF6 ]− , [ReF6 ]2− , [MCl6 ]2−Principal ligand fields: σ interactions(M=Mn, Re), [MoCl6 ]3− and the organometallic complexes [V(CN)6 ]4− ,[TaCl2 (dmpe)2 ], [MnMe4 (dmpe)] (dmpe=dimethylphosphino-ethane).Two other striking aspects of the electronic structure of octahedral complexes may be noted for weak-field complexes (§ 2.1.2.6).
In thiscase, the energy gap between the nonbonding (t2g ) and the antibonding (eg ) orbitals of the d block is small. When there are four, five, orsix electrons to place in these orbitals we do not find them paired inthe three orbitals of lowest energy. The ground state corresponds tothe configuration in which as many orbitals of the d block as possibleare occupied by a single electron, all with parallel spin.
The exchangeenergy arising from this arrangement, together with the reduction ofthe interelectron repulsion, more than compensates the energy that isnecessary to promote one or two electrons from the t2g level to theeg (0 is small). We thus obtain a ‘high-spin’ complex, which maybe contrasted with a ‘low-spin’ complex in which as many electronsas possible are paired. Thus, the ground-state electronic configurationof high-spin d5 complexes is that shown in 2-34, with one electron3 e2 ), rather than that forin each of the five orbitals of the d block (t2gg5 ). This latter arrangethe low-spin configuration shown in 2–35 (t2gment is found for strong-field d5 complexes such as [Mn(CN)6 ]4− ,[Fe(CN)6 ]3− , [Mo(η6 -C6 H6 )]+ , or [Ir(Cl)4 (PR 3 )2 ]. Many d6 complexes are diamagnetic (electronic configuration (t2g )6 , § 2.1.3.1).
Buthigh-spin d6 complexes also exist, such as [Fe(H2 O)6 ]2+ and [CoF6 ]3− ,4 e2 ), with four unpaired electrons,whose electronic configuration is (t2ggtwo in the t2g block and two in the eg block. The fact that the destabilization of the antibonding levels in weak-field complexes is so smallallows us to understand the existence of complexes with more than 6electrons in the d block, that is, complexes with more than 18 electrons.As examples, we may mention the octahedral complexes [Ni(NH3 )6 ]2+and [Ni(H2 O)6 ]2+ , d8 complexes that have 20 electrons. It is very rareto find more than 18 electrons in organometallic complexes, as theseare strong-field complexes in which the antibonding orbitals are at highenergy.egegt2gt2g2-34 (d5 -high spin)2-35 (d5 -low spin)We shall now study other geometrical arrangements (ligand fields)that are frequently met in transition metal complexes.
We shall generally limit ourselves to the characterization of the ‘structure’ of thed block, that is, the shape and the relative energy of the five orbitalsSquare-planar ML4 complexesthat make it up. Knowledge of this structure will enable us to anticipate the (or those) electronic configuration(s) that will be particularlyfavoured for a given family of complexes (e.g. d6 for octahedral diamagnetic complexes). Moreover, the occupied d orbitals are in general thehighest-energy occupied orbitals of the complex (nonbonding or weaklyantibonding orbitals), and they therefore play an important role in problems linked to the geometrical structure and the reactivity of complexes(see Chapter 4).2.2.
Square-planar ML4 complexesIn a square-planar ML4 complex, the metal is placed in the centre of asquare whose corners are occupied by the four ligands. One can therefore consider, at least formally, that a square-planar complex is formed byremoving two ligands from an octahedral complex, for example, thosesituated on the z-axis (2-36). To establish the structure of the d block,it is convenient to start from the results already obtained for octahedralcomplexes.L6L1L2L4ML3zxyL1 ML2L5L4L32-362.2.1. Characterization of the d blockWe consider first the three nonbonding orbitals of the octahedron (xy,xz, and yz), where the representation chosen is given in Figure 2.3.
Theseorbitals are nonbonding in the octahedron, as the coefficients for the sixligands are zero by symmetry. Removal of two of these ligands thereforehas no effect on either the shape or the energy of these three orbitals,which stay pure nonbonding d orbitals in the square-planar complex(2-37a–c). The x 2−y2 orbital is antibonding in the octahedron, becauseof its interactions with the four ligands in the xy plane. In contrast, thecoefficients on the ligands located on the z-axis are zero by symmetry.Removal of these two ligands therefore has no influence on the shape andenergy of this orbital, which remains antibonding in the square-planarcomplex (2-37d). The only d orbital which is modified on passing fromthe octahedron to the square plane is the antibonding z2 orbital.
Nowthe two main antibonding interactions in the octahedron concern theligands placed on the z-axis. Removal of these ligands therefore leadsPrincipal ligand fields: σ interactionsto a substantial stabilization of this orbital, though it remains weaklyantibonding due to the small interactions with the four ligands placedin the xy plane (2-37e).xyxy(a)xzxz(b)yzyz(c)x2–y2x2–y2(d)z2z2(e)ML4ML62-37These changes in energy for the orbitals of the d block are illustratedin Figure 2.6, where the symmetry of the orbitals in the square-planarcomplex is also indicated (point-group symmetry D4h ). Notice that whilethe three nonbonding orbitals xy, xz, and yz are degenerate from theenergetic point of view, only two of them (xz and yz) are degenerate bysymmetry (eg representation whose dimension is 2).The main difference between the d blocks of octahedral and squareplanar complexes concerns the number of nonbonding or weaklyantibonding orbitals: there are three in the former but four in thelatter.x2–y2 (b1g)egz2 (a1g)t2gFigure 2.6.
Derivation of the orbitals of the dblock for a square-planar ML4 complex fromthose of an octahedral ML6 complex.xy (b2g)xz(eg)yzSquare-based pyramidal ML5 complexes2.2.2. Electronic structure for 16-electron d8 complexesz2xyyzxz2-38pz2-39The favoured electronic configuration for a square-planar diamagneticcomplex ML4 involves the double occupation of the four low-energyorbitals of the d block (2-38). There are indeed many diamagneticcomplexes with a d8 electronic configuration, such as [Pd(NH3 )4 ]2+ ,[M(CN)4 ]2− (M=Ni, Pd), [PtCl4 ]2− , [PtHBr(PR 3 )2 ], [AuBr4 ]− , Vaska’scomplex [Ir(CO)(Cl)(PPh3 )2 ], and Wilkinson’s hydrogenation catalyst[Rh(Cl)(PPh3 )3 ].If we include the four doubly occupied MO that form the four metal–ligand bonds (these are not represented in Figure 2.6), we find that thetotal number of electrons in a d8 complex is 16.
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