First- and Second-Row Anomalies

Maximum coordination numbers of the nonmetals as shown by the fluorides 

In contrast, for the large polarizable hydride ion which can bond more strongly by a covalent bond the lithium compound is the most stable  

The next two series of nonmetals, silicon through chlorine and germanium through krypton, show a maximum coordination number of six in hexafluoro anions. SFh. and TcFh. Even here ihe oxvacids and oxyanions typically show a coordination 

The largest nonmetals show coordination numbers as high as eight in the oc-tafluoroanions. IF, and XeF (see Chapter 17). The corresponding oxyacids and oxyanions show a maximum coordination number of six [Sb(OH)6], Te(OH)6, OI(OH)5, and [Xe06]4-. Of these, apparently only iodine shows a maximum oxidation state with a coordination number as low as four Periodic acid can exist as either OI(OH)j or HI04. 

It was mentioned previously that a strong resemblance obtains between Li and Mg, Be and Al. C and P. and other diagonal elements, and it was pointed out that this could be related to a size-charge phenomenon. Some examples of these resemblances are as follows  

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Periodicity (Continued) of elements, 27-29 first- and second-row anomalies, 858-861... 

First- and Second-Row Anomalies 858 The Use of p Orbitals in Pi Bonding 861 The Use (or Not) of d Orbitals by Nonmetals 86ft Reactivity and d Orbital Participation 875... 

Generally speaking, the mechanisms of p-block element reactions are not particularly consistent with the rules outlined above. The reason for this boils down to the so-called first-row anomaly, where both first- and second-period elements (H-Ne) are all somewhat unreasonably lumped together as first row. The expression means that the chemical properties of first-row elements are anomalous relative to those of their heavier congeners. Let us go through the above four rules one by one and see how well they hold up in a main-group inorganic context. 

Fig. 109. Schematic illustration of phase relationships, structural transitions and anomalies appearing in the nonstoichiometric range of 123-0,. First row EM investigations of Beyers et al. (1989). Second row XRD and NPD on poly- and single-crystalline samples, after Plakhty et al. (1994, 1995). Third row Hard X-ray single-crystal refinements of superstructures, after von Zimmermann et al. (1999). Fourth and fifth rows under normal and higher pressures, measured in equilibrium samples. Sixth to tenth rows phase relationships deduced from lattice parameters of slowly cooled samples (preparation methods described in sect. 3.1.2.2). Eleventh row EXAFS results after Rohler et al. (1997a,b, 1998).

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