Figure 7.2 Mendeléev’s Periodic Table of 1871.
The Uniqueness Principle
As can be seen from Figure 7.2, and of relevance to this chapter, Mendeléev placed an underline beneath the 2nd Period. This underscore was to indicate a degree of difference between these elements and the ones beneath. It was apparent to him that the elements above the line did not wholly resemble the lower members of each Group.
This difference of the topmost member of each Group is known as the Uniqueness Principle. Rogers has identified three factors that cause significantly different behavior among most of the 2nd Period elements. Summarizing his statements, it can be stated that [8]:
•They have exceptionally small atomic radii
•They exhibit a maximum of four bonding directions
•The nonmetallic elements have an enhanced ability to form multiple (π) bonds
The theoretical underpinning of the Uniqueness Principle has been described by Kaupp [9]:
The similarity of the radial extent of 2s- and 2p-shells is a decisive factor that determines the special role of the 2p elements within the p-block . . . The overall small size of the 2p-elements can be appreciated from any tabulation of atomic, ionic, and covalent radii. . . . This of course does lead to overall coordination number preferences of the 2p-element, as is well known. The smaller radii and the high electronegativities of the 2p-elements are behind their strict obedience of the octet rule, in contrast to the apparent behavior of their heavier homologues . . .
The Uniqueness Principle can be illustrated by the formulas of the highest oxidation-state simple oxo-anions for the Group 14 and Group 15 elements. The top member has a preference for the delocalized trigonal-planar π-bonding system (Table 7.1), while the lower tetrahedrally coordinated tetra-oxo-anions are prone to polymerization.
Main Group Organometallic Compounds
Until the latter part of the 20th century, it was assumed that formation of organometallic compounds, particularly those with metal–carbon bonds, was an almost exclusive domain of the transition metals. This belief is no longer held. As Power has described [10]:
The new compounds that were synthesized highlighted the fundamental differences between their [the heavier main-group elements] electronic properties and those of the lighter elements to a degree which was not previously apparent. This has lead to new structural and bonding insights as well as a gradually increasing realization that the chemistry of the heavier main-group elements ever more resembles that of transition metal complexes than that of their lighter main-group congeners. The similarity is underlined by recent work, which has shown that many of the new compounds react with small molecules such as H2, NH3, C2H4 or CO under mild conditions and display potential for applications in catalysis.
Table 7.1 Comparison of the formulas of the simple highest oxidation-state oxo-anions
The d-Block Contraction
Just as the Uniqueness Principle served to differentiate the chemistry of the 2nd Period elements from those in the subsequent periods, so the d-block contraction (sometimes called the scandide contraction) results in a greater similarity between some of the members of the 3rd Period and 4th Period [11].
The contraction is not actually a “contraction,” instead, the expected increase in effective ionic radii upon descending a p-block element is less when passing from the 3rd Period to the 4th Period than would be expected for a systematic trend. Table 7.2 shows that the increase in radius from Al3+ to Ga3+ is only 8.5 pm. For comparison, the Group 3 analogue, Sc3+, which like Al3+ has a noble gas ion configuration has a radius 21 pm greater. Similarly, the ionic radius of In3+ is 10 pm less than that of Y3+.
The accepted explanation for the d-block contraction is that the 3d electrons are poor shielders. Thus the increased effective nuclear charge results in an orbital contraction for gallium and its 3+ ion. The Uniqueness Principle is used to account for the exceptional differences between the 2nd Period and 3rd Period elements of the same Group. Relatedly, the d-block contraction is used to explain exceptional similarities between the 3rd Period and 4th Period elements of the same Group.
Table 7.2 A comparison of the ionic radii of the Group 3 and Group 13 ions
The 4th Period Anomaly
Following from the d-block contraction is the 4th Period Anomaly:
The 4th Period anomaly for the p-block elements is where the properties of the Group member of the 4th Period do not fit the trend for the other members of the Group.
Pyykkö [12] has referred to the phenomenon as secondary periodicity, a pattern first reported by Biron in 1915. According to Biron, descending a Group, many physical and chemical properties exhibit an alternating “sawtooth” pattern. Hildebrand rediscovered this phenomenon in the context of the Group 15 elements in 1941 [13]. He noted that the chemistry of nitrogen, arsenic, and bismuth was more focused toward the +3 oxidation state. By contrast, the chemistry of phosphorus and antimony revolved more around the +5 oxidation state.
Dasent wrote an article on compounds that a chemist would expect should exist, but which had not been synthesized at that time. He categorized the probable reasons for nonexistent compounds; category 3 being that of the 4th Period anomaly [14]:
. . . those whose instability is related to the reluctance of certain atoms of the first long period to assume their highest oxidation state.
It was Sanderson who provided a fuller account of the unique features that he found for 4th Period p-block elements, confirming the existence of this anomaly [15]. As exemplars of the 4th Period anomaly, he cited the difficulty in preparing AsCl5 (yet PCl5 and SbCl5 are well known) [16] and HBrO4 (yet HClO4 and H5IO6 are well known) [17].
In explanation, Pyykkö found that there were two different factors. The 4th Period anomaly resulted from the d-block contraction while the lesser 5th Period factor was a result of the combination of relativistic effects (see Chapter 2) and
