High ionic potentials

Beryllium is normally divalent in its compounds and, because of its high ionic potential, has a tendency to form covalent bonds. In free BeX2 molecules, the Be atom is promoted to a state in which the valence electrons occupy two equivalent sp hybrid orbitals and so a linear X—Be—X system is found. However, such a system is coordinatively unsaturated and there is a strong tendency for the Be to attain its maximum coordination of four. This may be done through polymerization, as in solid BeCk, via bridging chloride ligands, or by the Be acting as an acceptor for suitable donor molecules. The concept of coordinative saturation can be applied to the other M"+ cations, and attempts to achieve it have led to attempts to deliberately synthesize new compounds. 

The lack of adequate experimental data on chelates of germanium, boron, titanium, and vanadium prevents a similar comparison for these elements however, their high ionic potentials do indicate that they should form stable chelates. Vanadium does occur naturally in the very stable vanadium porphyrin complex. For similar reasons, molybdenum and tin are not discussed further. 

It is believed that most of the transition metals are complexed to nitrogen donors, such as are found in amino acids or derivatives of chlorophyll, and that the metals with high ionic potentials, such as beryllium, boron, germanium, titanium, gallium, and major elements such as aluminum and silicon, may be bonded to oxygen donors of degraded lignin. 

The actinide ions in 5+ and 6+ oxidation states are prone to severe hydrolysis as compared to lower oxidation states in view of their high ionic potentials. Consequently, these oxidation states exist as the actinyl ions MOt and MO + even under acidic conditions, which can further hydrolyze under high pH conditions. The oxygen atoms of these ions do not possess any basic property and thus do not interact with protons. The tetravalent ions do not exist as the oxy-cations and can be readily hydrolyzed at low to moderate pH solutions. The degree of hydrolysis for actinide ions decreases in the order M4 > MOT > M3 > MOt, which is similar to their complex formation properties (4). In general, the hydrolysis of the actinides ions can be represented as follows ... 

Cr ). Those with a high-ionic potential (>20) form soluble oxyanions e.g. phosphate, nitrate, sulfate). 

The adsorption on insulators is weak and associative due to dipolar forces between the lone pair on the carbon of the CO molecule and exposed cations of high ionic potential but can be more complex surfaces carrying hydroxyl groups or otherwise defective can undergo more complex interactions. 

Figure 7.3 plots the ratio of crystal radius versus charge for selected ions. Oxyanions—sulfate, selenate, phosphate, arsenate, borate, molybdate, carbonate, and silicate—are represented by their central cations S6+, Se6+, P5+, As5+, B3+, Mo4+, C4+, and Si4+. The ions fall into three behavioral groups. Ions of high ionic potential, the alkali and alkaline earth cations and the halide anions, large univalent and divalent ions, are highly water soluble, easily weatherable, and leach readily from soils to the sea over geologic time. 

---