Tetravalent chemistry coordination compounds

The study of coordination compounds of the lanthanides dates in any practical sense from around 1950, the period when ion-exchange methods were successfully applied to the problem of the separation of the individual lanthanides,131-133 a problem which had existed since 1794 when J. Gadolin prepared mixed rare earths from gadolinite, a lanthanide iron beryllium silicate. Until 1950, separation of the pure lanthanides had depended on tedious and inefficient multiple crystallizations or precipitations, which effectively prevented research on the chemical properties of the individual elements through lack of availability. However, well before 1950, many principal features of lanthanide chemistry were clearly recognized, such as the predominant trivalent state with some examples of divalency and tetravalency, ready formation of hydrated ions and their oxy salts, formation of complex halides,134 and the line-like nature of lanthanide spectra.135... 

This chapter gives an overview on the chemistry of tetravalent lanthanide compounds, especially those of tetravalent cerium. Following a brief introduction, it covers the tetrahalides, dioxides, and other lanthanides(IV) salts. Coordination compounds of cerium in the oxidation state +4 include halogeno complexes and complexes of oxo acids, /3-diketonates and related Schiff-base complexes, as well as porphyrinates and related complexes. 

Even flien well-characterized coordination compoimds are limited to only a few classes of compounds. Notable are, e.g., halogeno complexes and complexes of oxo acids, p-diketonates and related SchifF-base complexes, as well as porphyrinates and related complexes. Two other important classes of cerium(lV) compounds, the alkoxides and amides of Ce" +, can be regarded as pjewfifo-organometallics and are discnssed together with the organocerium(IV) complexes in Tetravalent Chemistry OrganometalUc. 

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

Despite the great interest in low- and hyper-valent silicon compounds four coordinated, tetravalent silicon covers the largest part of silicon compounds in nature and in industrial applications. The chemistry of such species still yields new and extraordinary results. 

The most important difference from titanium is that lower oxidation states are of minor importance. There are few authenticated compounds of these elements except in their tetravalent states. Like titanium, they form interstitial borides, carbides, nitrides, etc., but of course these are not to be regarded as having the metals in definite oxidation states. Increased size also makes the oxides more basic and the aqueous chemistry somewhat more extensive, and permits the attainment of coordination numbers 7 and, commonly, 8 in a number of compounds. 

Coordination Number. Most common coordination numbers for tetravalent compounds are (with typical geometries in parentheses) 4 (tetrahedral), 5 (trigonal bipyramidal), and 6 (octahedral). No examples of coordination numbers higher than 6 have been characterised (cf. tin). Tetrahedral coordination predominates, and hence the structural chemistry of germanium resembles that of silicon rather than tin. 

These elements (E) have four valence electrons and are tetravalent as carbon below which they are located. The compounds ER4 thus follow the octet rule which confers them a great stability. The tetraalkyl-element complexes are almost apolar and particularly robust and inert. However, the energy of the E-C bond decreases upon going down in the column of the periodic table. Thus, the tetraalkyllead complexes are less robust towards thermolysis than the lighter analogs. They decompose between 100 and 200°C, which was eventually applied to provide their antiknock properties. The chemistry of these elements is largely dominated by the oxidation state and coordination number 4, but the oxidation state and coordination number 2 is also known for all, and its stability increases upon going down in the column of the periodic table. 

Lead cations occur in both the divalent (+2) oxidation state, which is the most stable of the group IVB elements, and in the tetravalent (H-4) oxidation state. The divalent oxidation state usually dominates the inorganic chemistry of lead, while the tetravalent state dominates its organic chemistry. The coordination numbers of its divalent compounds range from 2 to 7, while those of its tetrahedral compounds range from 4 to 8. Its stereochemistry is usually octahedral or tetrahedral. 

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