P.A. Cox - Inorganic chemistry (793955), страница 52
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AgF2 (d9) is an exceptionalthough sometimes the stoichiometry is misleading, AgO being a mixed valency compound, AgIAgIIIO2. Even electronconfigurations are favored by the large ligand field splittings found in these series, giving low spin states, with the d6octahedral and d8 square-planar arrangements being particularly favorable (see Topic H2).Aqueous chemistryUnlike elements of the 3d series, 4d and 5d elements have little simple aqueous cationic chemistry. The main exceptionsare Y3+ and La3+ (see lanthanides, Topic I1), and Ag+, which forms some soluble salts (AgF, AgNO3). The aqua Ag+ ionshows strong class b complexing behavior, with an affinity for ligands such as NH3, I− and CN− comparable with Cd2+ inthe next group (see Topics E3 and G4).
Some other aqua cations can be made, but they are extensively hydrolyzed andpolymerized (e.g. Zr4+, Hf4+), strongly reducing (e.g. Mo3+), or have a very high affinity for other ligands (e.g. Pd2+)and are difficult to prepare in uncomplexed form.Table 1. A selection of oxides and halides of elements from the 4d and 5d series. M represents either of the two elements from the corresponding group,and X any halogen unless exceptions are specified.Numerous complexes are, however, formed, the most stable with early groups being ones with F− and oxygen donorligands, and in later groups ones with softer ligands such as heavy halides and nitrogen donors.
This trend is similar to thatfound in the 3d series but is more marked. The most commonly encountered solution species for later elements arechloride complexes such as [PdCl4]2−, [PtCl6]4− and [AuCl4]−.H5—4D AND 5D SERIES225Oxoanions are formed by elements of groups 5–8, examples beingand. They are invariablyless strongly oxidizing than their counter-parts in the 3d series. MoVI and WVI, and to a lesser extent NbV and TaV, formextensive series of polymeric oxoanions: isopolymetallates such as [Mo6O19]2− and [Ta6O19]8− are mostly based on metaloxygen octahedra sharing corners and edges (see Topic D3); heteropolymetallates such as thephosphopolymolybdate ion [PMo12O40]3− incorporate other elements, in this case as a tetrahedral PO4 group.Solid structuresLarger ionic radii compared with the 3d series elements often lead to higher coordination numbers (see Topics D3 andD4).
ZrO2 and HfO2 can adopt the eight-coordinate fluorite structure as well as a unique seven-coordinate structureknown as baddeleyite (cf. TiO2, rutile). ReO3 is the prototype of a structure with six-coordination, and is adopted also(in slightly distorted form) by WO3, in contrast to CrVI, which is tetrahedral. MoO3 and WO3 form extensive series ofinsertion compounds known as oxide bronzes (see discussion of NaxWO3 in Topics D5 and D7). In halides, highercoordination often leads to polymeric forms for compounds MX4 and MX5 where the corresponding 3d compounds aremolecular.Compounds of elements in low oxidation states very frequently have extensive metal-metal bonding.
Sometimesthis acts to modify an otherwise normal structure, as in NbO2, MoO2 and WO2, which have the rutile form distorted bythe formation of pairs of metal atoms. Often the structures are unique. For example, MoCl2 contains [Mo6Cl8]4+clusters formed by metal-metal bonded octahedra with chlorine in the face positions (see 2; only one of eight Clshown). Complex halides often show metal-metal bonding, such as in [Re2Cl8]2− (3) where all four d electrons of ReIIIare paired to form a quadruple bond.Later elements tend to show coordination geometries that are specific to certain low-spin electron configurations (seeabove and Topic H2). d6 compounds are invariably octahedral, d8 nearly always square planar (e.g. in PdCl2 4 and PdO;a rare exception is PdF2, which, like NiF2, has the octahedral rutile structure with two unpaired electrons per Pd).
Thed10 configuration often has a tendency to linear two-coordination (cf. HgII, Topic G4). Although AgF, AgCl and AgBrhave the rocksalt structure some other AgI compounds such as Ag2O have two-coordination, and it is normal for AuI;for example, AuCl has a chain structure with a linear Cl-Au-Cl arrangement.Section H—Chemistry of transition metalsH6COMPLEXES: STRUCTURE AND ISOMERISMKey NotesCoordinate number andgeometryNomenclatureIsomerismRelated topicsClassical or Werner complexes have a metal in a positive oxidationstate coordinated by donor ligands.
The coordination number andgeometry are determined by size and bonding factors, octahedral andtetrahedral being common for 3d ions, and square-planarcoordination for some d8 ions. Polynuclear complexes can havebridging ligands and/or metal-metal bonding.H2O and NH3 ligands are called aqua and ammine respectively. Thenames of anionic ligands ends in -o, and of anionic complexes in -ate.Either the oxidation state or the overall charge on the complex isspecified.The study of isomerism depends on kinetic factors limiting the rate ofinterconversion, and in the 3d series is confined to complexes of CrIIIand CoIII Coordination, linkage, geometrical, and optical isomerismare possible.DescribinginorganicComplex formation (E3)compounds (B5)Ligand field theory (H2)Coordination number and geometryTransition metal complexes are cationic, neutral or anionic species in which a transition metal is coordinated by ligands.A classical or Werner complex is one formed by a metal in a positive oxidation state with donor ligands such asH2O, NH3 or halide ions.
Ligands with strong π-acceptor properties are discussed in Topic H9.The coordination numbers (CN) observed in complexes range from two (e.g. [Ag(NH3)2]+) to nine (e.g. [ReH9]2−; see Topic H5, Structure 1). The commonest geometries for 3d ions are octahedral (CN=6, e.g.
[M(H O) ]2+) and2 6tetrahedral (CN=4, e.g. [MCl4]2−). As in solid compounds, higher coordination numbers are often found with thelarger 4d and 5d ions. Other coordination geometries may be dictated by bonding arrangements depending on the delectron number (see Topics H2 and H5).The relative preference for octahedral or tetrahedral coordination is partly steric, but ligand field effects can also playa role. Ions with the d3 and low-spin d6 configurations (e.g.
Cr3+ and Co3+, respectively) have a large octahedral ligandfield stabilization energy and are notably resistant to forming tetrahedral complexes. Square-planar complexes wouldnever be predicted in preference to tetrahedra on steric grounds alone. They are commonly found, however, with 4d8and 5d8 ions such as Pd2+ and Pt2+ where the pattern of ligand field splitting is favorable if its magnitude is large enoughfor spin-pairing to occur.
The correspending 3d8 ion Ni2+ gives square-planar complexes only with strong-field ligandsSECTION H—CHEMISTRY OF TRANSITION METALS227such as CN−; otherwise octahedral or sometimes tetrahedral coordination is found. With the d9 or high-spin d4configuration a distorted octahedral geometry is often found with only four ligands strongly attached. This is common withCu2+, as in [Cu(NH3)4]2+, where two weakly bound water molecules are also present.Low coordination numbers are often found with post-transition metal ions having the d10 configuration (seeTopic G4).
This is also true for the d10 ions Cu+, Ag+ and Au+, which form many linear complexes with CN=2 (e.g. [AuCl2]−, isoelectronic to HgCl ).2Polynuclear complexes contain more than one metal atom. Sometimes these may be held by bridging ligands,as in [(RuCl5)2O]4− (1). In other cases metal-metal bonds may be present, as in [Re2Cl8]2− (see Topic H5, Structure3). Metal-metal bonding is commoner in the 4d and 5d series than with 3d elements, although binuclear compounds ofCrII are known; for example, [Cr2(CH3CO2)4] (2), which has bridging acetate groups (only one shown explicitly) and aquadruple Cr-Cr bond formed by all remaining valence electrons of the 3d4 ions.NomenclatureThe naming of coordination compounds is introduced in Topic B5.
Some further examples will illustrate the principlesinvolved.• [Ni(H2O)6]2+, hexaaquanickel(II) ion; [Cu(NH3)4]2+, tetraamminecopper(II) ion. The terms aqua and ammine areused for water and ammonia ligands. Other neutral ligands are referred to by their normal (molecular) name.Sometimes the prefixes bis, tris, ...
are used where normal form (bi, tri, ...) could cause confusion with the ligandname; for example, [Co(H2O)3(CH3NH2)3]3+, tris(methylamine)triaquacobalt(III) ion.• [CoCl4]2− tetrachlorocobaltate(II), [Fe(CN)6]3− hexacyanoferrate(3-). For anionic ligands the normal ending -ide isreplaced by -o. Names of anionic complexes end in -ate, and are sometimes based on Latin rather than Englishnames of the metallic element (see Topic B5, Table 1). Either the oxidation state of the metal atom or the totalcharge on the complex is specified.• [CoCl(NH3)5]Cl2, pentaamminechlorocobalt(III) chloride.
Coordinated ligands are shown in square brackets, othersare assumed to be separate in the structure. Anionic ligands are usually written before neutral ones in the formula,but after them in the name.• [(RuCl5)2O]4− (1), µ-oxo-bis(pentachlororuathenate)(4−). The Greek letter µ (‘mu’) is used to denote bridgingligands.IsomerismIsomers are compounds with the same (molecular) formula but different structure. When several isomers exist, one maybe thermodynamically more stable than the others, or there may be an equilibrium between them (see Topic B3). Thusthe isolation and study of individual isomers depends on kinetic factors that limit the rate of interconversion.
Such228H6—COMPLEXES: STRUCTURE AND ISOMERISMkinetic inertness is associated with only a few ions (see Topic H7) and most examples of isomerism involve complexesof CrIII, CoIII and PtII.Ionization isomerismThis is best shown by an example. ‘CrCl3.6H2O’ exists in four solid forms, which dissolve in water to give differentspecies:The different isomers all contain an octahedral CrIII complex but the coordinated ligands are different; for example, inthe first case the three Cl− ions are present in the crystal lattice of the solid compound but are not directly bound to themetal.Linkage isomerismA few ligands are ambidentate, meaning that they can coordinate through alternative ligand atoms. Examples arenitrite(which can bind through N or O) and thiocyanate SCN− (S or N).
The nomenclature N-nitrito and Onitrito is recommended for complexes where in formulae the ligand atom is underlined,andrespectively (although the nonsystematic names nitro and nitrito are also used forcomplexes).Geometrical isomerismThe fact that a tetrahedrally coordinated compound MX2Y2 has only one possible isomer was historically important inestablishing the structure of carbon compounds. When the coordination is square planar there are two possibilities,known as the cis (3) and trans (4) forms.
Geometrical isomers occur also in octahedral complexes: with MX2Y4 the twoisomers are also called cis (5) and trans (6), and for MX3Y3 the terms mer (7 from ‘meridional’) and fac (8 from‘facial’) are used.SECTION H—CHEMISTRY OF TRANSITION METALS229Geometrical isomerism can also refer to the possibility of different coordination geometries, although these are ratherrare. Square-planar or tetrahedral coordination is, in principle, possible with CN=4, and an example with CN=5occurs with [Ni(CN)5]3−, which can adopt shapes approximating either to a trigonal biyramid (the normally expectedshape; see Topic C2) or a square pyramid (9).Optical isomerismWhen a species cannot be superimposed on its mirror image the two forms are known as enantiomers or opticalisomers.
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