Yves Jean - Molecular Orbitals of Transition Metal Complexes (793957), страница 43
Текст из файла (страница 43)
The example above illustrates this point nicely:independently of the energetic order of the fragment orbitals and theinitial electronic occupation, the occupied and empty MO in the twodimers have analogous symmetry properties (5-15).* CC* MM*CC*MM MMCCMM CCC 2 H4[Fe 2 (CO) 8 ]5-15The analogy is not limited to molecules that are formed by the combination of two fragments. For example, the complex [Os3 (CO)12 ] canbe considered as the combination of three [Os(CO)4 ] fragments withC2v geometry. Each of these is of course isoelectronic to [Fe(CO)4 ],and therefore isolobal to the organic fragment CH2 .
The [Os3 (CO)12 ]complex is therefore an isolobal analogue of cyclopropane (5-16). Eachmetallic centre is bound to two neighbours by a single bond, as inApplicationsthe organic analogue. Other complexes, which appear to be very different, also have this property. The fragment [CpRhL] is isoelectronicto [CpCoL], and therefore isolobal to the [Fe(CO)4 ] and CH2 fragments (5-11). As a result, the complex [Cp2 Rh2 (CO)2 (µ-CH2 )] can bedescribed as a ‘two-thirds inorganic’ cyclopropane (5-16).CpCCOsCRhOsCOsRhCp[Os3 (CO)12 ]C3 H6[Rh 2 (Cp)2 (CO)2 (µ-CH2 )]5-165.3.2.
Conformational problemsSeveral conformational properties that are well known in organicmolecules reappear in inorganic complexes or in mixed organic/inorganic systems. The ethylene molecule provides a good example;it adopts a planar geometry that allows the formation of two bondsbetween the carbon atoms (a σ bond and a π bond).
In the ‘orthogonal’ structure, where the two methylene groups lie in perpendicularplanes, only the σ bond would be preserved. We now turn to thecarbene complex [Fe(CO)4 (CH2 )] in a TBP geometry, with the carbenein an equatorial position. Since the CH2 and [Fe(CO)4 ] fragments areisolobal, this complex is an isolobal analogue of ethylene, C2 H4 . Twolimiting conformations can be imagined, with the CH2 ligand either perpendicular to the equatorial plane of the complex, or in this plane. Theanalogue of the first of these conformations is indeed a planar moleculeof ethylene (5-17a), but the second corresponds to a strongly destabilizedstate of ethylene in the orthogonal geometry (5-17b).
However,LLLFeHCCHLLFeCHHL5-17aCHLLHHHHCCHH5-17bHThe isolobal analogythe similarity in electronic structures does not necessarily imply thatthe barriers to rotation (5-17a → b) will be comparable in the organicmolecule and in the organometallic complex. About 64 kcal/mol areneeded to twist ethylene from its planar to its orthogonal structure, butthe rotation of the carbene ligand in a complex of d8 [ML4 (CR 2 )] typeneeds only about 15–20 kcal/mol (see Exercise 5.8).In the same way, we can predict (or rediscover) the conformationalpreferences of several ethylene complexes. If we replace the carbene inthe preceding example by ethylene, still in an equatorial position, weobtain the complex [Fe(CO)4 (C2 H4 )] which is isolobal to cyclopropane.The conformation shown in 5-18a is isolobal to cyclopropane in its moststable geometry, with three carbon atoms on a tetrahedral environment,whereas that shown in 5-18b is isolobal to a ‘deformed’ cyclopropane,in which the coordination around one of the carbon atoms is squareplanar.
The first of these conformations is therefore more stable: incontrast to the carbene, ethylene ‘prefers’ to lie in the equatorial planeof the complex. In a simple way, we come to the same conclusionconcerning the conformational preference as we did in the precedingchapter (§ 4.1.1), when we examined the problem starting from theinteraction diagrams between fragment orbitals.LLLFeC5-18aLLLLFeC5-18bL5.4. LimitationsThe isolobal analogy draws out similarities in the electronic structuresof organic and inorganic molecules that can easily escape our attention.As a result of these similarities, we can discover resemblances in thenumber and nature of bonds, molecular geometries, and sometimeseven reactivities.
But analogies must not be pushed too far, and therecan be significant differences between analogous molecules.One of the limitations concerns the kinetic stabilities of analogousspecies. For example, in the analogous series of compounds ethylene,carbene complexes of iron tetracarbonyl and its dimer, [Fe2 (CO)8 ](5-19), the first two are known, but the last is unstable and to date itLimitationshas only been observed in an inert matrix at very low temperatures.
Inother words, the analogy with a very stable organic molecule is not aguarantee that the inorganic species will be stable.CHCOHHCOHOCCFeOCCCOFeFeCOOCOCHCOHCOCOCO5-19Another limitation concerns the capacity of some ligands, such as thecarbonyl group, to coordinate in either terminal or bridging positions(µ-type coordination) in polynuclear complexes. Consider the bimetallic complex [CpFe(CO)2 ]2 , for example, (5-20b). Each [CpFe(CO)2 ]fragment is analogous to the methyl radical (5-11), so the complex isisolobal to ethane (5-20a). However, it is known in two different forms:[CpFe(CO)2 ]2 , where all the carbonyl ligands are in terminal positions,and [CpFe(CO)(µ-CO)]2 , in which two carbonyl ligands are bridging(5-20c), and this latter form is the more stable.
A structure of this type,with bridging hydrogen atoms, is of course unknown for ethane, theorganic analogue of this complex.HHOCOCHCHCCpFeHCOCOCpHFeFe5-20aCpOCOCCpFeCO5-20bCO5-20cIn metallic complexes, the bridged and non-bridged structures areoften very close in energy, and small changes can favour one geometry or the other. Thus, in the trimetallic complex [Os3 (CO)12 ],which is analogous to cyclopropane (5-21a), all the carbonyl ligandsare terminal.
However, in the isoelectronic iron complex, two carbonylOCCO OCCOOsCOCOOsOCCOCOOCOsCOCOCO5-21a(CO) 3FeFeCOCOCOFe(CO)3OCCO5-21bligands bridge the two metallic centres, giving the isomer[Fe(CO)4 (Fe(CO)3 (µ-CO))2 ] (5-21b). As in the previous example, aThe isolobal analogybridged structure of this type cannot be seriously imagined for theorganic analogue, but that does not prevent the complex 5-21b, likethe complex 5-21a, from being an isolobal analogue of cyclopropane.Exercises5.11. Give an organic and a bimetallic isolobal analogue of the complex[Mn(CO)5 CH3 ].2.
Suggest an orbital interaction diagram that is suitable for thedescription of the bond between the metallic centre and themethyl group in this mixed complex.5.21. By using the isolobal analogy with an organic molecule, predictthe number and nature (σ or π ) of the metal–metal bond(s) in thebimetallic complex [CpRh(CO)]2 .2. Suggest a structure for this compound.5.31. How many carbonyl ligands (n) are necessary on each metalcentre for the bimetallic complex of osmium (2) to be an isolobalanalogue of cyclobutane (1)?H2 CC H2H2 CH2CCH2(CO)nOs1C H2Os(CO)n22.
Indicate the geometrical arrangement of the ligands around themetal centres.5.41. Show that the trimetallic complex [(CO)4 Fe(Pt(CO)L)2 ](L = PR 3 ) is an isolobal analogue of cyclopropane.(CO)4 FePt(CO)LPt(CO)LExercises2. Deduce the positions of the ligands around each metalliccentre.5.5In the complex below, [Cr(CO)4 (BH4 )]− , the borohydride ligand isbound to the metal centre in the η2 mode, that is, with two bridginghydrogen atoms.COHOCHBCrHOCHCO1. Give an organic isolobal analogue of the fragment [Cr(CO)4 ]− .2. Give a boron-based analogue of this fragment.3. Which boron-based compound is the isolobal analogue of the fullcomplex?Deduce (or rediscover) its geometrical structure.5.6Use the isolobal analogy to predict the more stable conformationsfor the following ethylene complexes (Cp = η5 -cyclopentadienyl).COPtCOPtorCOCO(b)(a)CpRhCpRhorCOCO(b)(a)5.71.
Show that the complex [Ir4 (CO)12 ] (1) is an isolobal analogue oftetrahedrane (2).RC(CO) 3Ir(OC) IrIr(CO)33RCCRIr(CO)CR123The isolobal analogy2. What can be said about the complex [(η3 -cyclopropenyl)Co(CO)3 ]3 and the bimetallic complex [µ-(η2 -alcyne)(Co(CO)3 )2 ]4, in which the acetylenic bond is perpendicular to the metal–metal bond?Co(CO)3CRRCCR(OC)3 CoCo(CO)3CRCR343. Give another limiting representation for the structures ofcomplexes 3 and 4.5.8Examine the analogies described in Scheme 5-17. Does the doublebond really disappear on passing from structure a to structure b for(i) the organic molecule and (ii) the organometallic complex? Elements of group theory andapplicationsExamination of the geometries of isolated molecules shows that thereare four types of symmetry elements: reflection planes, axes of rotation,inversion centres, and improper axes of rotation.
A symmetry operationmoves a molecule from an initial configuration to another, equivalentconfiguration, either by leaving the position of the atoms unchanged, orby exchanging equivalent atoms. The ideas of symmetry element andsymmetry operation are of course very closely linked, since a symmetryoperation is defined with respect to a given element of symmetry, and,conversely, the presence of a symmetry element is established by thepresence of one or more symmetry operations that are associated withthat element.6.1. Symmetry elements and symmetry operations6.1.1. Reflection planesH1OyH4C2C1 zH2 H3H66-1H5Consider the dimethylether molecule (O(CH3 )2 ) in the conformation−O−−C2shown in 6-1: the hydrogen atoms H1 and H4 lie in the plane C1 −(the plane of the paper), but the atoms H2 , H5 and H3 , H6 are ‘above’and ‘below’ this plane.The xz plane, which is perpendicular to the plane of the paper−O−−C2 , is a symmetry element of theand which bisects the angle C1 −molecule, written σv .
Reflection in this plane leads to a configurationthat is equivalent to the initial one: the position of the oxygen atom(located in the σv plane) stays unchanged, and the positions of thepairs of equivalent atoms (C1 , C2 ), (H1 , H4 ), (H2 , H5 ), and (H3 , H6 )are interchanged (6-2).H1H4OC1H2H3C2H6H4v (xz)H5H1OC2H5 H6C1H3 H26-2Elements of group theory and applicationsThe yz plane (written σv ′ ), which contains the three heavy atoms,is also a symmetry elements of the molecule, since reflection in thisplane maintains the positions of the atoms O, C1 , C2 , H1 , and H4 , andexchanges the pairs of hydrogen atoms (H2 , H3 ) and (H5 , H6 ) (6-3).H1H4OC1H2 H3C2H6v (yz)H5H1H4OC2C1H3 H2H5H66-3If the operation of reflection in either of these planes is performedtwice (an operation which is written σ 2 ), a configuration is obtained thatis identical to the initial one.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
