P.A. Cox - Inorganic chemistry (793955), страница 56
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Carbonyl and cyanide ligands are not considered organic,although they may also be present in organometallic compounds along with other π-acceptor ligands such as phosphines.Table 1 shows a selection of the ligands found in organometallic compounds of transition metals, classified according totwo properties.• The hapticity is the number of carbon atoms bonded directly to the metal.
With some ligands this can vary; forexample, cyclopentadienyl can be η1−C5H5, η3−C5H5 or (most often) η5−C5H5 (pronounced ‘monohapto’,‘trihapto’, etc.).• The electron number is the number of electrons the ligand contributes to the metal-carbon bonding. This isuseful for applying the 18-electron (EAN) rule (see Topic H9). Ligands are taken to be neutral species even if they242SECTION H—CHEMISTRY OF TRANSITION METALSTable 1. Some organic ligands, classified according to hapticity and electron numberaUncommon bonding arrangements.are known as stable anions (e.g. C5H5, not).
For ligands of variable hapticity the electron number often variesaccordingly, but electron number is not always equal to the hapticity, as can be seen with η1 ligands, where theelectron number can vary from one to three.Structure and bondingAlkyl ligands form metal-carbon σ bonds. Often they occur in conjunction with other organic ligands or CO, but can befound on their own, as in tungsten hexamethyl (1), and in [Ti(CH2SiMe3)4] where the bulky groups are helpful instabilizing the compound. Compounds with H attached to β carbons (the nomenclature being M-Cα-Cβ-Cγ) tend to beunstable to β-hydride elimination of an alkene fragment, discussed below.
The surprising structure of (1), trigonalprismatic (D3h) rather than octahedral as found in WCl6, has been attributed to the orientation of d orbitals available forσ bonding. In an octahedral complex only two d orbitals (the eg set) can be involved, but four in the trigonal prismaticstructure. (Unlike WMe6, WCl6 also has some degree of W-Cl π bonding, which can involve the other d orbitals (t2g) inoctahedral geometry; see Topic H2.)Alkylidene and alkylidyne ligands require metal-carbon π bonding in addition to σ (see Topic C6).
This is different,however, from π complexes where bonding involves only the π orbitals of alkene or aromatic ligands. Examples arethe ethene complex [(η2−C2H4)PtCl3]− (2) found in Zeise’s salt, and the ‘sandwich compound’ ferrocene [Fe(η5−C5H5)2] (3). The Dewar-Chatt-Duncanson model of bonding in ethene complexes is shown in Fig. 1.
and isanalogous to the σ-donor-π-acceptor description of the bonding in carbonyl complexes (see Topic H9, Fig. 1). In thepresent case the ‘σ-donor’ character comes from the occupied bonding π MO of ethene (Fig. 1a), back donation(Fig. 1b. involving the empty π* antibonding MO. The relative degrees of donor or acceptor behavior depend on thecompound.H10—ORGANOMETALLIC COMPOUNDS243Fig. 1. Dewar-Chatt-Duncanson model for bonding in π complexes of C2H4.With strongly electron-withdrawing alkenes such as C2F4 or C2(CN)4 there is a large amount of back donation, whichweakens the C-C bond so that its length is similar to that of a single bond.
The geometry of the ligand then also changesfrom the planar configuration associated with sp2 hybridization, to a nonplanar form more characteristic of singlebonded sp3. The result (4) can be viewed as a metallocyclic compound with two M-C σ bonds.Bonding in sandwich compounds such as ferrocene arises through interaction of the delocalized π MOs of the ring withorbitals of the metal, and cannot be treated in a localized fashion (see Topic C6).
As in alkenes, both donor and acceptorinteractions are involved. Other ligands such as CO can be present, as in the ‘piano-stool’ structure 5 or the metalmetal bonded dimer 6.The 18-electron rule can be a useful guide to stable organometallic compounds, especially when π-acceptor ligandsare present, although it has the limitations referred to in Topic H9. Compounds 3, 5 and 6 obey this rule, but 1 withoutπ bonding ligands has an electron count of only 12. Metallocenes [M(η5−C5H5)2] are known for the 3d serieselements V-Ni, with 15–20 valence electrons, respectively.
Ferrocene (M=Fe with 18 electrons) is by far the most244SECTION H—CHEMISTRY OF TRANSITION METALSstable of these, cobaltocene (M=Co with 19 electrons) being a very strong reducing agent that easily forms the 18electron ion [Co(η5−C5H5)2]+. Compounds with more than 18 valence electrons are uncommon, and thus one canunderstand the unusual structure of [Fe(η5−C5H5)(η1−C5H5)(CO)2] (7), as two pentahapto ligands would give anelectron count of 22. Reactions of organometallic compounds often involve 16-electron intermediates formed by theloss of one ligand (e.g.
CO) from an 18-electron parent compound.Preparative methodsPreparative methods for organometallic compounds are exceedingly diverse but the following are generally useful.• Reduction of metal salt in the presence of the ligand:• Reaction of a transition metal salt with a main-group organometallic compound. C5H5 is oftendelivered as the sodium salt Na+(C5H5)−:In other cases a Grignard reagent or aluminum alkyl may often be used:• Metal vapor synthesis. Vaporizing the metal (e.g. by electron-beam heating) helps by providing the sublimationenergy required; metal atoms are then condensed in the presence of the ligand on the sides of the vessel, cooled inliquid nitrogen.
This method is good for compounds that cannot be made by other routes, or ones stable only at lowtemperatures. For example,Insertion and eliminationAmong the many reactions of organometallic compounds, ones involving insertion and elimination of ligands areimportant in applications to synthesis and catalysis. An example of a carbonyl insertion is:in which a Mn-CH3 bond is replaced by Mn-C(O)-CH3. The terminology is misleading as it is established by isotopiclabeling that the incoming CO is not the one inserted.
The first step is a reversible alkyl migration leading to a 16electron intermediate which then picks up another CO molecule as show in 8.H10—ORGANOMETALLIC COMPOUNDS245Fig. 2. Reaction steps involved in the catalytic Monsanto acetic acid process.Many other unsaturated ligands can ‘insert’ into M-C or M-H bonds; for example, alkanes as in:Such reactions are often reversible, the backwards process leading to elimination of a ligand. The reverse of alkeneinsertion is the β-hydride elimination reaction referred to above.Organometallic compounds are used widely as homogeneous catalysts in the chemical industry (see Topic J5).
Forexample, if the alkene insertion reaction continues with further alkene inserting into the M-C bond, it can form thebasis for catalytic alkene polymerization. Other catalytic cycles may include oxidative addition and reductiveelimination steps as described in Topic H9. Figure 2 shows the steps involved in the Monsanto acetic acidprocess, which performs the conversionIn the catalytic cycle on the right-hand side, the 16-electron species A undergoes oxidative addition of CH3I to form B.Carbonyl insertion then proceeds via C to give D, which regenerates A by reductive elimination of CH3COI. The organicsteps on the left-hand side of Fig.
2 can be varied to give different overall reactions, for example, convertingCH3CO2CH3 into (CH3CO)2O.Section I—Lanthanides and actinidesI1LANTHANUM AND THE LANTHANIDESKey NotesThe elementsOxidation states +3Other oxidation statesRelated topicsThe elements (sometimes called rare earths) are found together innature and are electropositive metals.
Chemistry is dominated by +3state with ions in (4f)n configurations, and is similar for all elements.A wide range of +3 compounds is formed as well as aqua ions. Theionic radius decreases gradually across the series, leading to changes insolid structures, and an increase in stability of complexes in solution.Organometallic compounds are more ionic than in the d block.Sm, Eu and Yb form many compounds in the +2 oxidation state. Withthe other elements, compounds in this state are formed only with largeanions and are often metallic. Ce, and to a lesser extent Pr and Tb,show the +4 state.The periodic table (A4)Actinium and the actinides(I2)The elementsThe lanthanides are 14 elements following lanthanum in the periodic table, and associated with the filling of theseven orbitals of the 4f shell.
The symbol Ln is often used to denote these elements collectively. Atomic configurationsare complex with electrons in 4f, 5d and 6s orbitals outside the Xe core. The first three ionization energies are relativelylow, leading to electropositive metals with chemistry dominated by the Ln3+ state in solution and in ionic solids. All Ln3+ ions have electron configurations (4f)n (see list in Figure. 1), but the 4f orbitals are highly contracted in size and do notoverlap significantly with neighboring atoms. Unlike the case with the d orbitals in the transition elements, spectra andmagnetism associated with 4f orbitals in Ln3+ compounds are very similar to those found in free gas-phase ions. Ligand fieldand chemical bonding effects associated with incomplete 4f orbitals are very small and hardly detectable in chemicaltrends.
The chemistry of all Ln3+ ions is therefore very similar and differentiated only by the gradual contraction inradius associated with increasing nuclear charge. The lanthanide contraction is also important for the transitionelements of the 5d series (see Topics H1 and H5).The oxidation states +2 and +4 are found for some elements, following the trend in ionization energies across theseries, which show patterns analogous to those found in configurations of p and d electrons (see Topics A5 and H1). Thethird ionization energy rises from La to Eu (see Fig.
1) and then a drop occurs after the half-filled shell (Eu2+, 4f7). Therise then continues to Yb, and drops at Lu because the 4f shell is filled and the electron ionized is in 5d. Fourthionization energies (which are substantially larger) show a similar pattern displaced by one element, thus rising from Ceto Gd and falling to Tb.248SECTION I—LANTHANUM AND ACTINIDESFig. 1. Ionic radius of M3+, third ionization energy I3, and number of 4f electrons in M3+ for the elements La-Lu.Promethium is a radioactive element with a half-life of 2.6 years and does not occur naturally.
The other elements,known sometimes as the rare earth elements, are always found in association, principally in the minerals monazite(LnPO4) and bastneasite (LnCO3F). The electropositive and reactive elements can be obtained by reduction of LnCl3with Ca, and are sometimes used together as ‘mischmetal’.
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