Yttrium Group series

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

If the material is to be separated into the cerium and yttrium groups of rare earths by precipitation of the double sulfates of the former, removal of the last trace of cerium is unnecessary since it does not seriously interfere in the fractionation of the cerium group double magnesium nitrates. It is important, however, that there be no ceriuni in a bromate series,... 

Yellow liquors containing principally yttrium-group earths with samarium will be obtained from the most soluble fractions and wUl fail to crystallize. These may be removed from the series to be combined later with other materials of similar composition obtained in the same manner. When these fractions are of sufficient volume they should be used for the preparation of bromates (synthesis 17). 

Note that the radii of the first four elements (La, Ce, Pr, Nd) are relatively large and also are more similar to each other than to the remainder of the series. This group is sometimes called the cerium group while the others are lumped as the yttrium group. Recall that the relative distributions of REs in the various ore types, described in the preceding section, seem to mirror this subgrouping scheme. Note also that the radius of Y is between Ho and Er. 

A series of structurally characterized di-yttrium(III) complexes bearing alkoxy-derivatized triazacyclononane ligands have been examined as initiators for lactone ROP.886 Both (296) and (297) are active for the polymerization of rac-LA at RT, but little control is afforded over molecular weights. Chain length distributions are broad, (Mw/Mn= 1.5-2.2) and attempts to identify the initiating group via end-group analysis have not been successful. 

Studies of the lanthanide series, usually taken to include yttrium, have been made, and complexes of the general formula [Md SOJfX examined for various R and X groups. 

The 3rd group of the Periodic Table (the 1st column within the block of the transition elements) contains the metals scandium, yttrium, lanthanum, and actinium. Lanthanum (atomic number 57) may be considered the earliest member of the family of metals, called lanthanides (general symbol Ln), forming, inside the principal transition series, an inner transition series (up to atomic number 71). Scandium and yttrium together with the lanthanides are also called rare earth metals (general symbol R). 

The first series starts at period 4, group 3 (IIIB) with the element scandium (jiSc)- "The second series starts at period 5, group 3 (IIIB), with the element yttrium ( Y). The third series starts at period 6, group 3 (IIIB) following a number of special metals. We start the third series with the element hafnium ( Hf). 

Yttrium is always found with the rare-earth elements, and in some ways it resembles them. Although it is sometimes classified as a rare-earth element, it is listed in the periodic table as the first element in the second row (period 5) of the transition metals. It is thus also classified as the lightest in atomic weight of all the rare-earths. (Note Yttrium is located in the periodic table just above the element lanthanum (group 3), which begins the lanthanide rare-earth series. 

The element yttrium (jjY), located just above lanthanum in group 3, is sometimes included in this series because its physical properties and chemical characteristics are similar to those of other elements in the series. 

In this book, the series begins with lanthanum, which is located in group 3 of period 6, following barium (j Ba). Lanthanum is not always considered as part of this series that bears its name, but it closely resembles the other elements in the series. Thus, this is a logical place to start the series given that we already include yttrium in group 3. 

Group 3 of the Periodic Table consists of the elements scandium, yttrium and either lanthanum or lutetium, depending upon the preferred arrangement of the Table. Group 3 elements have the outer electronic configuration ns2 p, and invariably their solution chemistry is that of the + 3 state. In this text, treatment of both La and Lu is carried out in Chapter 8, which deals with the f-block elements. Lanthanum and lutetium represent the first and last members of the lanthanide series. 

LANTHANIDE SERIES. The chemical elements with atomic numbers 58 to 71 inclusive, commencing with cerium t.5K)and through lutetiuni 171) frequently ate termed collectively, the Lanthanide Scries. Lanthanum, the anchor element of the series, appears in group 3h of the periodic table. Some authorities eonsider lanthanum a part of the series. Members ol the series, along with lanthanum and yttrium, are described under Rare-Earth Elements and Metals. See also Actinide Series. 

Okuda and Arnold have reported a series of 5- and 6-coordinate chiral indium complexes (e.g. 12), analogous to those previously discussed with yttrium (Table 4), which show varying activities for the polymerization of rac-lactide [71]. The homoleptic species was fluxional, interconverting between a 5-coordinate and fac- and mer-isomers of a 6-coordinate complex. It resulted in controlled polymerization and some stereoselectivity (max. P, = 0.63 and max. Ps = 0.64). MALDI-ToF mass spectrometry confirmed that the chiral alkoxide ligand was an initiating group. 

The lanthanide series of metals includes the 15 elements with atomic numbers 57-71, plus yttrium (atomic number 39). The lanthanides occur in the earth s crust at concentrations exceeding some commonly used industrial elements making the term rare earths something of a misnomer. For example, yttrium, cerium, lanthanum, and neodymium are present in the earth s crust at higher concentrations than lead. Of the 15 lanthanides, only promethium does not occur in nature - it is a man-made element. All of the lanthanides have similar physical and chemical properties. Because of similarities in their chemistry and toxicity, the characteristics of the lanthanides are often described as a group. Within the lanthanide group, however, there are differences between the toxicity of the individual lanthanide elements and their compounds. 

The chemistries of scandium and yttrium are often reported together (although yttrium is also often grouped with the lanthanides) so in this section they will be discussed together. Reference, where appropriate, will be made to the first volume in this series, Comprehensive Coordination Chemistry The Synthesis, Reactions, Properties and Applications of Coordination Compounds (CCC, 1987). Volume 3 of CCC (1987) contained a chapter entitled Scandium, Yttrium and the Lanthanides and this chapter is designed to follow part of that chapter. The articles reviewed will cover the period 1982-2001 although earlier work may be cited. Volume 2 of CCC (1987), devoted to ligands, will also be referred to in this chapter. 

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