Group trends lanthanide contraction

The computational bond-length variations in Table 4.53 exhibit the expected periodic trends. Most noticeably, third- and second-series elements for groups 4, 6, and 10 exhibit similar bond lengths, i.e., the post-lanthanide contraction with respect to the ordinary increase of atomic size with increasing Z. 

Niobium (formerly called columbium) and tantalum are Transition Metals having a considerable affinity for oxygen donor groups they are thus called oxophilic see Oxophilic Character). They occur as mixed-metal oxides such as columbites (Fe/Mn)(Nb/Ta)206 and pyrochlore NaCaNb206p. Their discovery in minerals extends back to the beginning of the nineteenth century, when they were believed to be identical and called tantalum. Rose showed that at least two different elements were involved in the minerals, and named the second one niobium. Their separation was resolved around 1866, especially by Marignac. These metals often display similar chemical behavior as a result of nearly identical atomic radii (1.47 A) due to the lanthanide contraction see Periodic Table Trends in the Properties of the Elements)... 

Two trends are apparent in Table 2.8. Firstly, in going from Sc to to La there is a stepwise increase in radius as principal shells of electrons are added. However, in all of the other groups the increase between the first and second row is not repeated in the second to third row. This is a result of the lanthanide contraction and the filling of the 4f subshell between La and Hf. This reflects the poor screening of 4f electrons one by another, leading to an increase in and a decrease in radius. Secondly, for metals in the same oxidation state, there is a d-block contraction across the rows as a result of the increase in Z... 

In this section, we review the results of calculations and comparison with experiments for lanthanide hydrides. Although YH and ScH are strictly not lanthanide or rare-earth compounds, it is important to investigate these compounds as well to compare and contrast these compounds with their periodic analog, namely, LaH. For this reason we include some discussions on Sc and Y compounds. With the intent of demonstrating the effect of the lanthanide contraction, we also include the results of HfH and HfHj and compare them with ZrH and ZrHj. In each section, we briefly describe with acronyms the method of computations. The reader is referred to section 2.1.2 for their meanings. We also include the periodic trends within a group and emphasize any anomalies. 

Any explanation for the lack of a smooth trend in down the group first requires a decision as to which member is out of line. It was argued by King " that W(CO)g has an especially strong M—C bond because the lanthanide contraction makes the covalent radius of W smaller than expected (Cr, 1.25 A Mo, 1.36 A W, 1.37 A), with the result that the 5d and maybe 4/ orbitals give much better n back bonding. More recent theoretical studies indicate that this argument is overly simplistic. 

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