P.A. Cox - Inorganic chemistry (793955), страница 51
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A selection of oxides and halides of the elements Sc-Cu. X represents any halogen unless specified. Oxidation states are shown in mixedvalency and ternary compounds.Low oxidation states (e.g. +2) are of limited stability for the early elements. The unusual metal-rich compoundSc2Cl3 has a structure with extensive Sc-Sc bonds. Compounds such as TiOx and VOx are nonstoichiometric (see below)and are also stabilized by metal-metal bonding using d electrons. With Cu the +1 oxidation state is stable in compoundssuch as Cu2O and CuCl, but CuF is not known, presumably because the larger lattice energy of fluorides makes thisunstable with respect to disproportionation to Cu and CuF2. The differential stability of oxidation states with differenthalogens is also shown by the existence of CuI but not CuI2.The existence of several stable oxidation states gives rise to the possibility of mixed valency compounds wherean element is present in different oxidation states.
Thus the compounds M3O4 with M=Mn, Fe, Co, have both MII andMIII states present. Many oxides also show nonstoichiometry where a continuous range of composition is possible.For example, ‘TiO’ is really TiOx where x can vary continuously over a wide range, and ‘FeO’ does not actually existbut is approximately Fe0.9O (and thermodynamically unstable below 550°C). Such nonstoichiometric compounds arebetter described by phase diagrams than by simple stoichiometric formulae, which can be misleading.Halide and oxide structuresA majority of halides and oxides have the structures expected for largely ionic compounds, with the metal inoctahedral coordination (see Topic D3, especially Fig.
1). Common oxide structures are rocksalt (e.g. MnO,NiO), corundum (see Topic G5, e.g. Cr2O3, Fe2O3) and rutile (e.g. TiO2, CrO2). Most MF2 compounds have therutile structure, other dihalides forming layer (CdCl2 and CdI2) types. Many ternary oxides and halides also follow thispattern; for example, the LaMO3 compounds formed by all elements of the series (M=Sc—Cu) have the perovskitestructure (see Topic D5).The 3d4 ions Cr2+ and Mn3+ and the 3d9 ion Cu2+ are subject to Jahn-Teller distortions (see Topic H2). Forexample, CuO does not have the rocksalt structure, but one with four close Cu—O neighbors and two at longerdistance; similar tetragonally distorted coordination is found in most other simple compounds of Cr2+ and Cu2+.
(Notethat CrO is unknown.)Tetrahedral coordination is also sometimes found. In high oxidation states (e.g. molecular TiCl4, polymericCrO3 and in complex ions such asand MnO4−) this can be understood in terms of the small size of thetransition metal ion. However, tetrahedral (zinc blende) structures are also found in CuI halides such as CuCl. As Cu+ has222H4—3D SERIES: SOLID COMPOUNDSthe 3d10 configuration this appears to be typical post-transition metal behavior as seen, for example, with Zn2+, andmust involve some degree of covalent bonding (see Topics D4 and G4).Some ternary and mixed-valency oxides have the spinel structure where metal ions occupy a proportion oftetrahedral and octahedral holes in a cubic close-packed lattice (see Topic G5).
Examples include M3O4 with M=Mn,Fe, Co. The distribution of M2+ and M3+ ions between the tetrahedral and octahedral sites shows the influence of ligandfield stabilization energies (see Topic H2). In Fe3O4, Fe2+ (3d6) has an octahedral preference whereas Fe3+ (3d5) hasnone, and this compound has the inverse spinel structure where Fe2+ is octahedral and Fe3+ is present in bothoctahedral and tetrahedral sites. In Co3O4 the low-spin 3d6 ion Co3+ has a very strong octahedral preference and thenormal spinel structure is found with all Co3+ in octahedral sites and Co2+ tetrahedral.
Mn3O4 is also based on the normalspinel structure, but with a tetragonal distortion as expected for the sites occupied by Mn3+ (3d4).Other binary compoundsSulfides are formed by all elements and have structures different from oxides. Many MS compounds (which are generallynonstoichiometric) have the NiAs structure. TiS2 and VS2 have layer (CdI2) structures, but later disulfides containions (e.g. FeS2 with the pyrites and marcasite structures; this is a compound of FeII not FeIV).
The compound CuS isparticularly complicated, having apparently CuI and CuII present as well as S2− and.Hydrides, nitride and carbides are known for some of the elements. Some have simple stoichiometry and structure,such as TiN and TiC with the rocksalt structure. Many are nonstoichiometric with metallic properties, and some can beregarded as interstitial compounds with the nonmetal atom occupying sites between metallic atoms in the normalelemental structure.Elements: occurrence and extractionThe decreasing electropositive character of the elements across the series is shown in the typical minerals they form, andin the methods required to extract them (see Topics B4 and J2).
Early elements are found in oxide or complex oxideminerals (e.g. TiO2, CrFeO3) and are known as lithophilic, whereas later elements are found mainly in sulfides (e.g.NiS) and are called chalcophilic. Iron forms the dividing line in this trend, and is found both as Fe2O3 and FeS2.Reduction of later elements is relatively easy, as sulfides may be roasted to form oxides and then reduced with carbon.For example, iron, a major structural metal, is produced in blast furnaces by reduction of Fe2O3:However, early transition metal oxides cannot be reduced in this way, because they form stable carbides (e.g.
TiC) and/or because the temperature required for reduction by carbon is too high. The Kroll process for manufacture of Tiinvolves first making TiCl4,which is then reduced by metallic magnesium. Titanium is widely used as a lightweight structural metal; althoughpotentially very reactive towards water and air it forms a very inert protective TiO2 film.Section H—Chemistry of transition metalsH54d AND 5d SERIESKey NotesOxidation statesAqueous chemistrySolid structuresRelated topicsHigher oxidation states are more stable than in the 3d series, and lower onesless common. The group oxidation state is found up to group 8. 4d and 5delements of early groups are very similar; in later groups higher oxidationstates occur in the 5d series.Very few simple aqua cations are found, but many complexes are known,increasingly dominated by softer ligands for later elements.
High oxidationstates form oxoanions that are less strongly oxidizing than corresponding 3dspecies, and that form extensively polymerized structures.Larger ions formed by early elements have high coordination numbers.Many compounds show extensive metal-metal bonding. Later ions have lowcoordination numbers related to specific electron configurations.Introduction to transition3d series: aqueous ions (H3)metals (H1)3d series: solid compounds (H4)Oxidation statesTable 1 shows the main binary oxides and halides formed by transition elements of the 4d and 5d series. Comparisonwith the corresponding information for the 3d series (Topic H4, Table 1) shows a similar pattern, with early elements inthe series forming states up to the group maximum (ZrIV, NbV, etc.) where all valence electrons are involved inbonding. The principal difference is that this trend persists further in the lower series, the compounds RuO4 and OsO4having no counterpart with the 3d element iron.
Following group 8, the highest oxidation state shown in Table 1remains higher than ones in 3d elements, as in RuF6, IrF6, PtF6 and AuF5. In addition to oxides and halides, highoxidation states are sometimes found with surprising ligands, such as in the ion [ReH9]2− (1), which is formally ahydride complex of ReVII.224SECTION H—CHEMISTRY OF TRANSITION METALSA counterpart to the stability of higher oxidation states is that lower ones (+2, +3) are less often found than in the 3dseries.For the earlier groups the patterns of 4d and 5d behavior are so similar that the corresponding elements (Zr, Hf, etc.)are placed together in Table 1.
but in later groups high oxidation states become slowly less stable in the 4d comparedwith the 5d series. This tendency is especially marked with Pd, Pt, Ag and Au. The factors underlying the differencesfrom 3d elements, and the general similarity of the two lower series, are discussed in Topic H1. The slow divergencebetween 4d and 5d series arises because increasing nuclear charge across the series has more effect on ionization energiesof 4d orbitals than on the larger 5d.Another trend apparent from Table 1 is the preponderance of oxidation states with even rather than odd electronconfigurations in later groups; these include PtIV, AuV (d6), PdII, AuIII (d8) and AgI (d10).
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