P.A. Cox - Inorganic chemistry (793955), страница 55
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In ligands known as π acceptors or π acids a donor-acceptor interaction also happens in the reversedirection. If a ligand has empty orbitals of π type symmetry with respect to the bond axis (see Topic C4) these may actas acceptors for electrons in filled metal orbitals of the correct symmetry. This is known as back donation. Thesimplest and commonest π acid ligand is carbon monoxide CO. This acts as a a donor in the normal way, through theoccupied lone-pair orbital centered on carbon (the 3σ MO; see Topic C5).
The π antibonding orbital can also interact withfilled d orbitals to give the π-acceptor interaction (Fig. 1). The combination of σ-donor and π-acceptor interaction issometimes described as synergic, as the electron flows in opposite directions facilitate each other.Evidence for the π-acceptor interaction comes from various sources.• CO and related ligands stabilize very low oxidation states of transition elements, often zero (see below).
πacceptor interactions remove electron density from a metal atom and make possible a lower oxidation state than iscommonly found with ligands such as water and ammonia.238SECTION H—CHEMISTRY OF TRANSITION METALSFig. 1. Bonding in CO complexes showing (a) σ overlap of CO lone-pair with empty metal d orbital, and (b) overlap of CO π* with occupied metal dorbital.• Partial occupation of the π antibonding orbital in CO weakens the bond. This is most easily seen from the bondstretching frequency measured by IR spectroscopy (see Topics B7, C8). CO stretching frequencies in carbonylcompounds are nearly always less than in free CO, and also decrease in a sequence such aswhere the availability of metal electrons for back donation is increasing. (A few CO complexes, e.g.
BH3CO(Topic F3) and Au(Cl)CO, have stretching frequencies slightly higher than in the free molecule, indicating that littleor no π interaction is taking place in these cases.)π-acceptor properties in other ligands may be judged by their ability to stabilize low oxidation states in a similar way toCO, or by their effect on the CO stretching frequency when placed in the same complex. Two π-acceptor ligands in atrans configuration (see Topic H6) will compete for the same d orbitals.
Placing a strong π acceptor trans to CO willtherefore lessen the availability of electrons for back-bonding and so the CO stretching frequency will be higher thanotherwise. On this basis the following order of π-acceptor strength has been deduced for some ligands:π back-bonding with phosphines is generally assumed to involve valence expansion on the phosphorus. As expected, thestrength increases with the electronegativity of the attached groups.
Although nitrogen ligands such as pyridine (whereN is part of an aromatic ring system) are π acceptors, amines R3N are not, as nitrogen cannot expand its valence shell(see Topic F1).Binary carbonylsCO forms binary neutral compounds with most transition metals, and some anionic and cationic species. Table 1 showscompounds from the 3d series.
Some of these compounds can be obtained by direct reaction of the metal and CO athigh pressure. The Mond process for the purification of nickel depends on the formation of nickel tetracarbonyl Ni(CO)4 in this way, followed by its thermal decomposition to deposit metallic nickel. For earlier elements in the seriesreductive carbonylation is required, with a compound (generally a halide) reduced in the presence of CO at highpressure. Polynuclear carbonyls are formed naturally for some elements (Mn, Co); in other cases, such as Fe where themononuclear carbonyl Fe(CO)5 is stable, polynuclear compounds can be made from it by photolysis or controlledH9—COMPLEXES: Π ACCEPTOR LIGANDS239Table 1. Binary carbonyls and ions formed by 3d series elementspyrolysis.
Binary carbonyls are volatile compounds, often very toxic, and thermodynamically not stable in the presenceof oxygen but often with considerable kinetic stability, especially for metals later in the series.In mononuclear carbonyls CO is invariably attached to the metal through carbon giving a linear M—C—Oarrangement. Polynuclear carbonyls have relatively short distances between metal atoms indicative of metal-metalbonds.
CO can then bond in either a terminal or a bridging mode, the former bonded to one metal as in Mn2(CO)10 (1) and the latter attached to more than one metal as in Co2(CO)8 (2). In larger clusters formed by some elements,triply bridging CO is also possible. Terminal and bridging CO may be distinguished by IR spectroscopy, as bridginggroups show a characteristically lower stretching frequency.Many compounds are known containing CO in conjunction with other ligands, which may include π acceptors such asphosphines, and/or a bonding ligands.
For example, there is a series of compounds Mn(CO)5X, where X=H, halogenor an alkyl group.The 18-electron ruleA great majority of stable carbonyls obey the 18-electron rule (sometimes called the effective atomic number(EAN) rule). To use this rule one first counts the number of valence electrons in the neutral atom, equal to the groupnumber (thus both s and d electrons are included; see Topic H1), then adds two electrons for the lone-pair of eachattached CO. For example, in Fe(CO)5, the group number of Fe is eight; five COs make 18. The EAN calculation startswith the actual atomic number (Fe=26). Adding two electrons for each CO makes an EAN=36, which is the noble gasconfiguration of Kr.
The only difference between the 18-electron and the EAN count is that the latter includes coreelectrons and so gives a different count for the three series: 36 (Kr core) for 3d, 54 (Xe core) for 4d and 86 (Ra core)for 5d (see Topic A4).All the mononuclear species except V(CO)6 in Table 1 satisfy the 18-electron rule. The bi- and tri-nuclear species doalso if (i) the two electrons in a metal-metal bond are counted as contributing to the valence shells of both metal atomsconcerned, and (ii) a bridging CO contributes one electron to each metal.
Monomeric Mn and Co carbonyls would havean odd number of electrons and dimerize in consequence. V(CO)6 is exceptional as a stable radical with 17 valenceshell electrons, presumably because it is sterically impossible for it to dimerize without losing one CO ligand. It does,however, readily form the 18-electron anion [V(CO)6]−.240SECTION H—CHEMISTRY OF TRANSITION METALSWhen other ligands are present it is normal in 18-electron counting to assume covalent rather than ionic bonding. InMn(CO)5X, where X=H, Cl or CH3, Mn and X therefore contribute one electron each to the Mn—X bond, and X isregarded as a one-electron ligand even if it is a halogen.One can make a connection between the 18-electron rule and ligand field theory by noting that a d6 octahedralcomplex has 18 valence electrons.
π-acceptor ligands provide strong fields and hence low-spin configurations (seeTopic H2) thus making the d6 octahedral combination extremely favorable. In general, the 18-electron configurationwith π-acceptor ligands provides a large gap between the highest occupied MO (HOMO) and the lowest unoccupiedMO (LUMO). Without the stabilization of the lower-energy set of d orbitals provided by a π-acceptor ligand theHOMO-LUMO gap is not so large, and the 18-electron rule does not generally apply to complexes with weak-fieldligands.
Even with π-acceptor ligands it can break down under some circumstances.• With elements early in the transition series that contribute few electrons themselves it may be sterically impossibleto coordinate enough ligands to achieve the 18-electron count. V(CO)6 is an example.• For later elements (group 9 onwards) there is a tendency towards lower electron counts (see below).16-electron complexesA square-planar complex of a d8 ion, such as [Ni(CN)4]2−, has a valence electron count of 16 rather than 18. Similar 16electron complexes are formed by other elements in groups 9, 10 and 11, for example Vaska’s compound Ir(CO)(PPh3)2Cl (3).
Some 16-electron complexes (especially in the 3d series) can readily add another ligand to form a fivecoordinate 18-electron complex such as [Ni(CN)5]3−. Another important reaction is known as oxidative addition,where a molecule X—Y adds by cleavage of the bond to form an 18-electron complex that can be regarded as d6octahedral:X—Y can be a simple molecule such as H2 or HCl, or an organic compound. Vaska’s compound also reacts with O2 toform Ir(CO)(PPh3)2(O2)Cl (4). In this case, O2 remains intact on coordination, although the bond lengthens, suggestingthat 4 can be regarded as a complex with a bidentate peroxo ligand (see Topic F7).The reverse of oxidative addition is reductive elimination.
Such reversible processes are important in many catalyticcycles involving transition metal compounds (see Topic H10, Fig. 2. and Topic J5).Section H—Chemistry of transition metalsH10ORGANOMETALLIC COMPOUNDSKey NotesLiquid classifictionStructure and bondingPreparative methodsInsertion andeliminitionRelated topicsOrganic ligands for transition metals are classified by their hapticity(the number of bonded atoms) and by the number of electrons theyprovide in bonding. Sometimes but not always these numbers areequal.Compounds with metal-carbon σ bonds may be unstable toelimination reactions; some have unexpected structures.
πcomplexes including sandwich compounds are formed by interactionof metal d orbitals with π electrons in the ligand. The 18-electronrule can be useful for rationalizing differences of structure or stability.Methods include reduction of metal compound in the presence of theligand, reaction with a main-group organometallic compound, andmetal vapor synthesis.Carbonyl and alkene groups may insert into metal-carbon bonds; thereverse process gives elimination of a ligand.
Together with oxidativeaddition and reductive elimination steps, these reactions form the basisfor many catalytic applications.Inorganic reactions andComplexes: π acceptorsynthesis (B6)ligands (H9)Industrialchemistry:catalysts (J5)Ligand classificationOrganometallic compounds with metal-carbon bonds are formed by nearly all metallic elements, but those of transitionmetals show a diversity without parallel in main groups.
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