Group trends metallic radii

All complexes mentioned above were highly effective single-component catalysts for the ROPs of e-caprolactone (Scheme 8) and rac-lactide (Scheme 14) without the need of an activator. The metal radius was influential to the catalytic activity. Both polymerization catalysis rates decreased in the trend of 96 >99 >98 >97, in agreement with the decrease in metal ion radii (La > Nd > Sm > Y). The investigation of polymer end group showed that the polymer chain growth was initiated by allyl transfer to monomer [77]. 

ElO.lO Recall from Sections 1.7 and 3.10(a) that the size of an ion is not really a fixed value rather it depends largely on the size of the hole the ion in question fills in a larger hole the ion will appear bigger than in the smaller one. We have also to recall periodic trends in cation variation. All hydrides of the Group 1 metals have the rock-salt structure (see Section 10.6(b) Saline hydrides). This means that in all Group 1 hydrides H" has a coordination number 6 and is surrounded by cations in an octahedral geometry. In other words, H occupies an octahedral hole within the face-centred cubic lattice of cations. The sizes of cations increase down the group (i.e., r L ) < KNa" ) < r(K )< tfCs"")) as a consequence both the sizes of octahedral holes and the apparent radius of H" will increase in the same direction. 

It is possible to explain these trends in terms of the electron configurations of the corresponding atoms. Consider first the increase in radius observed as we move down the table, let us say among the alkali metals (Group 1). All these elements have a single s electron outside a filled level or filled p sublevel. Electrons in these inner levels are much closer to the nucleus than the outer s electron and hence effectively shield it from the positive charge of the nucleus. To a first approximation, each inner electron cancels the charge of one pro-... 

Metallic and nonmetallic properties are related to the number of valence electrons and the radius of an atom. Within a period, as the metallic properties decrease from left to right, the nonmetallic properties increase. Within a group as the metallic properties increase, the nonmetallic properties decrease from top to bottom. If the above trends are considered, francium, Fr, would be expected to have most metallic properties. However, since Fr is a radioactive element, not all of its properties have been determined yet. 

Within a given group of the periodic table, the first ionization energy decreases with increasing atomic number. This is related to the increase in atomic radius and the decreasing attraction of the nucleus for the increasingly distant outermost electron. It should be mentioned that this trend is not uniformly noted for the transition metals. 

Coordination to metals follows the usual trends. The transition metals try to achieve octahedral coordination (with a few exceptions), but the cations of the electropositive group 1-3 elements exhibit a rich variety. The coordination polyhedra are determined by radius ratios more than by topological preferences. For polyphosphides in general, all P atoms are involved in M-P interactions according to the number of lone pairs present. The anionic (lb)P and (2b)P as well as the neutral (3b)P° species adopt quasi-tetrahedral coordination, especially if main-group cations are involved. Only a few exceptions are known, for example Li3P7. With more covalent M-P bonds, the number (m + n) of available lone pairs of a polyanion P " is strongly related to the metal coordination number that is, CN(M) < m + n). If CN(M) > m + n), ion-ion and ion-dipole interactions dominate. The relation <7[M-(2b)P] > <7[M-(3b)P] is true in most cases. 

Some important properties of the first five alkali metals are shown in Table 12.9. The data in Table 12.9 show that when we move down the group, the first ionization energy decreases and the atomic radius increases. This agrees with the general trends discussed in Section 12.15. 

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... 

The size of an atom is not an exact radius due to of the probabilistic nature of electron density, but we may compare radii among different atoms using a standard. As seen below, the sizes of neutral atoms increase with period number and decrease with group number. This trend is similar to the trend described above for metallic character. The smallest atom is helium. 

There are a number of trends to note. In the well-behaved alkali metals and alkaline earth metals, the radius of an atom increases smoothly as the atomic number increases. The transition metals all have rather similar radii as one passes along the period, and these increase slightly with atomic number going down a group. The same is true for the lanthanides and actinides. 

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