Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 34
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As this gap increases, theenergy difference between the two conformations should progressivelydecrease.This preference for the perpendicular structure, which is more orless pronounced depending on the orbital energies, is an importantresult that can be generalized as follows: when two π-acceptor ligandsare present, it is preferable for each to interact with a different occupiedd orbital, as happens in the perpendicular conformation, rather thanfor both to interact with the same d orbital, as happens in the planarconformation.4.1.4.
Orientation of H2 in the ‘Kubas complex’[W(CO)3 (PR 3 )2 (η2 -H2 )]2G. J. Kubas, R. R. Ryan, B. I. Swanson, P.J. Vergamini, H. J. Wassermann J. Am. Chem.Soc. 106, 451 (1984). In the following 17 years,about 800 research articles appeared that aredevoted to studies of the structures andreactivities of molecular hydrogen complexes.COCOWR3 PPR 3COHH4-15a (exp)COPR 3W COCOR3PHH4-15bThe preferential orientation of a ligand may also be caused by the presence of non-equivalent ligands on the metallic fragment. In this section,we shall treat what is known as a ‘molecular hydrogen complex’, that is,a complex in which a molecule of dihydrogen is bound to a metal centre inthe η2 mode. This molecule behaves as an L-type ligand, thanks to thetwo electrons in its σH−−H bond.
The first example of this type of complex was characterized in 1984, by Kubas and co-workers.2 It is the d6octahedral complex [W(CO)3 (PR 3 )2 (η2 -H2 )] (4-15a), where R = i Pr,−H bond as measured by neutron diffracin which the length of the H−tion (0.82 Å) is only slightly greater than that in an isolated H2 molecule(0.74 Å).One of the remarkable aspects of the structure of this complex islinked to the non-equivalence of the ligands other than H2 (PR3 or CO).−PR 3 bonds,The dihydrogen molecule is oriented parallel to the W−−H paralleleven though at least one other conformation, that with H−−−to the W CO bonds (4-15b), could be imagined. Is there an electronicfactor that causes this conformation preference? Before we can answerthis question, we have to analyse the orbital interactions that lead to themetal-H2 bond.4.1.4.1. Orbital interactions in a d6 -[ML5 (η2 -H2 )] complexWe consider first a model d6 complex [ML5 (η2 -H2 )] in which the fivenon-H2 ligands are identical and only have σ interactions with the metal−Hcentre.
We shall limit ourselves to the conformation in which the H−−L bonds (4-16).bond eclipses two neighbouring M−Conformational problemsSAz2SSyzSAASAAFigure 4.5. Diagram of the interactionbetween the four lowest-energy d orbitals fora d6 ML5 fragment (SBP) and the σH2 and∗ orbitals of H , that enables the MO of theσH226d model complex [ML5 (η2 -H2 )] (4-16) to beconstructed.P1P2LLLHzLLWxHyH*2H2SSHMHMHHThe complex can be described as the result of interaction of an H2molecule with a d6 square-based pyramidal (SBP) ML5 metallic fragment. For the latter, we consider, as in § 4.1.2, the four lowest-energyd orbitals (three of which are doubly occupied and nonbonding, thefourth being the polarized, empty z2 orbital), and the σH2 and σH∗ 2−H bond.
Given the orbital symorbitals on H2 associated with the H−metries with respect to the planes P1 and P2 (4-16), the interactiondiagram shows two two-electron stabilizing interactions (Figure 4.5),between occupied σH2 and empty z2 on the one hand (SS symmetry),and between occupied yz and empty σH∗ 2 on the other (SA symmetry).4-16yzz2e–e–H24-17a (H2 → M)*H24-17b (M → H2 )The first interaction, which leads to a transfer of electron densityfrom dihydrogen to the metal, is of the donation type (4-17a), whereasthe second, which involves the empty orbital on H2 , is a metal →dihydrogen back-donation interaction (4-17b).CommentThis bonding scheme is completely analogous to the Dewar–Chatt–Duncanson model for the interaction of an ethylene molecule with a∗ MO being replacedmetallic centre (Chapter 3, § 3.4), the πCC and πCC∗here by the σH2 and σH2 MO.−H bond: donationNotice that both interactions weaken the H−reduces the population of the bonding orbital σH2 , and back-donationincreases the population of the antibonding orbital σH∗ 2 .
An increase ofApplications3In solution, an equilibrium is establishedfor the Kubas complex between the dihydrogen and dihydride forms, confirming that themolecular hydrogen complex is an intermediate in the oxidative addition reaction of H2 .−H distance is indeed observed, to a greater or lesser extent, inthe H−all molecular hydrogen complexes. At the same time, both the occupiedSS and SA MO are bonding between the metal and the hydrogen atoms−H bonds are being formed.
This complex may there(4-17), so M−fore be viewed as a ‘frozen structure’ along the reaction pathway that−H bond and the formation of two M−−Hleads to the rupture of the H−bonds (an oxidative addition reaction with the formation of a dihydridecomplex).34.1.4.2. Orientation of H2 in the Kubas complex−H bondWe shall consider two orientations for the H2 molecule.
The H−−CO bonds (4-18a), or parallel to the W−−PR3is either parallel to the W−bonds (4-18b), the latter conformation being found experimentally.COCOPR 3W COCOR3PHzyCOxPR 3W COR3 PHHH4-18a4-18bAs in the preceding examples (§ 4.1.2 and 4.1.3), the donation interaction does not depend on the orientation of the dihydrogen molecule, dueto the cylindrical symmetry of the z2 orbital (4-19a). The back-donationinteraction involves either the yz or the xz orbital on the metallic fragment, in the first or second confirmations, respectively (4-19b and c),and the σH∗ 2 MO on dihydrogen. If all the ligands other than H2 areidentical, the xz and yz orbitals are degenerate and there is therefore nodifference between the two back-bonding interactions represented in 419b and c.
Both sketches therefore represent the eclipsed conformationof a d6 [ML5 (η2 -H2 )] complex.z2yz*H2*H2H24-19axz4-19b4-19cIn the Kubas complex, the presence of carbonyl ligands (π acceptors)and phosphines (which may, in a first approximation, be considered asConformational problemspure σ -donors), lifts the degeneracy of the nonbonding d orbitals. Aswe have shown in Chapter 3 (§ 3.3.3.2), yz is stabilized by three bonding∗ , xy by two but xz by only one (4-20); as a result,interactions with πCOthe energies of these orbitals follow the order εyz < εxy < εxz .xzxyyz4-20There is an important consequence of this energetic nonequivalence: in this complex, the back-donation interaction dependson the conformation that is adopted. In structure 4-18a, σH∗ 2 interactswith the lowest of the three d orbitals (yz), but with the highest (xz)in 4-18b.
Since the σH∗ 2 orbital is strongly antibonding, the energy gap(ε) between it and any of the three d orbitals is large. We may thereforeassume that the stabilization created by the interaction is proportionalto S2 /ε. The energetic factor (ε) favours conformation 4-18b, sincethe energy gap between the interacting orbitals is smaller (Figure 4.6).When we consider the overlap, it is important to realize that yz is delocalized into three carbonyl groups, but xz into only one (4-20). SinceMO are always normalized, the coefficient of the d orbital is larger∗ ) than for yz(+3π ∗ ), thereby favouring structure 4-18bfor xz(+1πCOCO(Sσ ∗ −xz > Sσ ∗ −yz , Figure 4.6). So both factors, energy gap and over−H is parallel to the W−−PR3lap, favour the conformation in which H−bonds.We note, in conclusion, that the conformation adopted by H2 in theKubas complex is controlled by the back-donation interaction (4-15).zH*yHx22S–yz*S–xz*xzxyyzCOFigure 4.6.
The back-donation interaction in−Hconformations 4-18a (on the left, H−−CO bonds) and 4-18b (onparallel to the W−−H parallel to the W−−PR3 bonds)the right, H−of the complex [W(CO)3 (PR 3 )2 (η2 -H2 )].COPR3OCWCOR3PR3 PHPR3COWOCHHHApplicationsBut since there is a large energy gap between the orbitals involved inthis interaction, the conformational preference is rather weak, and thebarrier to rotation of the H2 molecule is only about 2–3 kcal mol−1 .4.2. ‘Abnormal’ bond anglesThe geometry of some complexes may be very different from one’s initialthoughts, particularly for bond angles. We shall study two examples,to see which electronic factors may cause what appears to be a largedistortion from an ‘ideal’ structure.4.2.1.
Agostic interactions4.2.1.1. IntroductionRLn MLn MCH4-21aCH4-21bRIn carbene complexes [Ln M==CHR], the most common geometry is−C−−R angles−C−−Hα and M−trigonal-planar about the carbon, with M−◦close to 120 (4-21a). But a large structural change occurs in some−C−−Hα angle decreases to about 80◦ , whereas thecomplexes: the M−−C−−R angle increases very considerably, to 160–170◦ (4-21b).
TheM−deformation that is observed can therefore be described as a pivoting of−Hα bond closer to the metalthe alkylidene ligand, which brings the C−centre.The same phenomenon can be observed with an alkyl ligand. For−C−−H1 angleexample, the methyl group can pivot, decreasing the M−from the expected value of 109◦ (4-22a) to about 90◦ (4-22b).HLnMCH14-22a4M. Brookhart, M. L. H. Green J.Organomet. Chem. 250, 395 (1983).HLnMCHHH14-22bAll these complexes are electron-deficient (Nt < 18). The deformation that is observed has therefore often been interpreted as theconsequence of the pressure to complete the valence shell of the metal,even if a substantial geometrical distortion is necessary, by introducing−H bond.an additional interaction with a C−−H bond with its two electrons intoThe group which brings the C−the vicinity of the metal centre has been called ‘agostic’ by Brookhartand Green.4 These complexes can be considered as ‘frozen’ structures on−H bond, in the samethe pathway for the insertion of a metal into a C−way that molecular hydrogen complexes were treated on the oxidativeaddition pathway that leads to a dihydride (§ 4.1.4).‘Abnormal’ bond angles4.2.1.2.
An agostic methyl group: the complex−(CH2 )2 −−PMe2 )(CH3 )][Ti(Cl)3 (Me2 P−PMe 2ClMe 2PClTiCCl = 93.7°H14-235This is an acceptable model for aqualitative analysis based on changes inoverlaps, on the occupation of differentorbitals, etc. But if more accurate calculationswere to be undertaken for a quantitative study,then it would be necessary to study either thereal complex, or a much less simplified model,such as [Ti(Cl)3 (PH3 )2 (CH3 )].−PMe2 )(CH3 )]−(CH2 )2 −The structure of the complex [Ti(Cl)3 (Me2 P−obtained by neutron diffraction reveals an agostic methyl group, as the−C−−H1 angle is only 93.7◦ (4-23).
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