Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 35
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It is an essentially octahedral comTi−plex, in which the oxidation state of titanium is +4. The electronicconfiguration is thus d0 , so the d block is completely empty! This complex is therefore severely electron-deficient, as Nt = 12 (the electronsassociated with the six metal–ligand bonds).To simplify the problem, we shall analyse the model octahedral complex [Ti(H)5 (CH3 )]2− , in which all the ligands except CH3 have beenreplaced by hydrogen atoms.
The two negative charges are necessary toobtain a d0 electronic configuration. Although this simplified complexhas no experimental reality, it does possess the essential characteristics ofthe real complex 4-23: the same number of ligands, the same electronicconfiguration, and the presence of a methyl ligand that may be subjectto an agostic distortion.5We shall consider two structures in turn: the non-agostic case (4-24a)−C−−H1 angle is 109◦ , and an agostic version (4-24b)in which the Ti−−−−−where the Ti C H1 angle is close to 90◦ . To pass from one structureto the other, the methyl group is pivoted about the carbon atom, as inthe experimental structure 4-23.2–HH TiHHHH1CH14-24a (non-agostic)2–HHHTiHHC4-24b (agostic)We shall analyse the interactions between the methyl group and themetallic centre by decomposing the complex [Ti(H)5 (CH3 )]2− into two−fragments, CH−3 and [TiH5 ] .
In the first, which is a pyramidal AH3system, the lone pair on carbon is described by a hybrid orbital thatis essentially nonbonding (see Chapter 1, Figure 1.4). This MO is thehighest-energy occupied orbital; we shall refer to it as nCH3 , and at leastinitially, it is the only orbital that we shall consider on this fragment (Figure 4.7, right-hand side). The metallic fragment is of the ML5 type (SBP),where the metal is located in the base of the pyramid. As in the examplestreated in § 4.1.2 and 4.1.4, we shall consider only the four lowest-energyd orbitals on this fragment, three of which are nonbonding (xy, xz,Applicationsz2zyxxyxzyznCHHFigure 4.7. Orbital interaction diagram for−the CH−3 and [TiH5 ] fragments in thenon-agostic structure 4-24a.HHTi3HHHCHHand yz) and one antibonding (z2 ).
Since the electronic configuration of[TiH5 ]− is d0 , these four orbitals are empty (Figure 4.7, left-hand side).We consider first the non-agostic structure 4-24a, in which the local−C bond. The orbitalC3 axis of the methyl group coincides with the Ti−interaction diagram is presented in Figure 4.7. The orbital nCH3 cannotinteract with any of the nonbonding orbitals on the metallic fragment,since it is of σ -type and located in one of the nodal planes of the xy,xz, and yz orbitals.
However, its overlap with z2 is very large, sincethese two orbitals both point along the z-axis and they are polarizedtowards each other. Their interaction produces a bonding MO, which isdoubly occupied and mainly concentrated on the ligand; it characterizesthe σTi−−C bond. The antibonding combination makes up one of theantibonding MO of the d block in this octahedral complex (the z2 orbital).The pivoting of the methyl group (4-24b) moves its local C3 axis, sothat the nCH3 orbital is no longer oriented along the z-axis (4-25). Theoverlap between nCH3 and z2 therefore decreases, as does the electronicstabilization associated with this interaction. If this were the only factorto consider, this motion would therefore be energetically unfavourable,and no agostic distortion would be expected.z2SnCH34-25‘Abnormal’ bond anglesHowever, there is another significant change in the orbital interactions: the overlap between nCH3 and yz, which was zero in thenon-distorted structure, becomes substantial in the agostic structure(4-26).
Since the yz orbital is empty (a d0 fragment), a new stabilizinginteraction is created which now favours the agostic structure. Notethat no interaction can take place between nCH3 and xy or xz (S = 0),whatever the geometry.yzSnCH3S≠0S=04-26Two factors have therefore been revealed; one is unfavourable forthe distortion, but the other favours it. How, then, can we explain whythe second factor dominates the first? We need to consider the relativeenergies of the orbitals concerned. The energy gap ε between nCH3and yz (nonbonding) is much smaller than that between nCH3 and z2(antibonding; see Figure 4.7). Now the electronic stabilization increasesif the difference in energy between the interacting orbitals decreases.The agostic distortion therefore progressively replaces the (nCH3 ↔ z2 )interaction by a stronger one (nCH3 ↔ yz).
Despite what might have−C bond.been expected, this distortion reinforces the Ti−At this point in our analysis, we must note that the interpretation ofthe agostic distortion we have suggested does not really match the notionthat it is due to an additional interaction between a hydrogen atom, or a−H bond, and the metal centre. Notice, however, that a weak M. . . HC−bonding interaction does appear in the MOs represented in 4-26 for theagostic structure (right-hand side). Moreover, one of the bonding MO−H1of CH3 (often referred to as πCH3 ), that mainly characterizes the C−bond, does have an increased interaction with the empty yz orbital, dueto the substantial overlap between 1sH1 and yz (4-27).
This interactionyzSCH34-27Applicationsleads to a transfer of electron density from the ligand to the metal,−H1 bond (loss of bonding electrons) but creating anweakening the C−−H bond (gain of bonding electrons). This factor adds toincipient Ti−the one previously discussed, further stabilizing the agostic structure.To conclude, the important point is that the agostic distortion occursbecause there is a low-energy, empty orbital (yz in our example) in the nondistorted complex that can be used to make the distortion favourable.Both characteristics are important: (i) empty, because if it were occupied,the two-electron stabilizing interactions shown in 4-26 and 4-27 (righthand sides) would become repulsive four-electron interactions, whichwould therefore oppose the distortion.
We are reminded of the needfor an electron-deficient metal, that was noted at the beginning of thissection; (ii) low in energy, so that the favourable factor described in4-26 dominates the unfavourable factor described in 4-25. This aspectis also very important, since no agostic distortion of the methyl group isobserved, for example, in the complex [Ti(Cl)3 (CH3 )], even though it isextremely electron-deficient (formally, only eight electrons!). It can beshown that the empty orbital that could be used in an agostic distortionis in fact too high in energy in this complex to be useful; the three−C−−H angles remain equal.Ti−4.2.2. d6 ML5 complexes: a ‘T-shaped’ or ‘Y-shaped’geometry?d6 ML5 complexes are electron-deficient species (16e) that often appearas reaction intermediates.
They are indeed obtained from octahedrald6 ML6 complexes (18e) by the loss of a ligand, which is the first stagein many organometallic reactions. Despite this high reactivity, many ofthem have been isolated and geometrically characterized.For a long time, the only coordination mode known for these complexes was the SBP. The group VI (Cr, Mo, W) pentacarbonyl complexeswere the first to be observed, and many others with this geometry werelater characterized with different metals (Ru(II), Rh(III), Ir(III), etc) andvery varied ligands (alkyl, silyl, phosphine, halide, etc.).
In all these complexes, the angle between two basal transoid ligands lies between 160◦and 180◦ ; three examples are shown in 4-28.COCOOCOCMoCO4-28aPh3 PClPPh3ClRuPPh34-28bPiPr3COOC WOCPiPr34-28c‘Abnormal’ bond angles6The first two structures of this type werediscovered in 1986: H. Werner, A. Hohn, M.Dziallas Angew. Chem. Int. Ed. Engl. 25, 1090(1986); M. D. Fryzuk, P. A.
McNeil, R. G. BallJ. Am. Chem. Soc. 108, 6414 (1986).There is nothing surprising about this coordination mode for diamagnetic d6 ML5 systems. The d block of an SBP complex, with themetal either in the basal plane of the pyramid or close to it, containsthree nonbonding orbitals that are derived from the t2g block of theoctahedron (see Chapter 2, § 2.3.1). This environment is therefore idealto accommodate three pairs of electrons. Subsequently, however, farmore surprising structures were observed.6 These complexes adopt thegeometry of a distorted trigonal bipyramid, where one of the equatorialangles is only some 75-80◦ .
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