Oxyanions coordination number

For a discussion of oxyanion coordination numbers see Chapter 15.5, structures I-IV. b tsglyo — JV-tosylglycinate. cCTd = compressed tetrahedral SBP - square-based pyramidal ERO = elongated rhombic octahedral TB = trigonal bipyramidal LB = long bipyramidal SP - square planar. 

An increase in the coordination number of molybdenum takes place in the second protonation step, which has a dramatic effect on the value of K2. Instead of the typical decrease of 3 to 5 log units from the first to the second protonation constant, K2 has in this case about the same value as Kx. In fact, these unusual values for the protonation constants compared to those of other oxyanions, along with the thermodynamic parameters AH° and AS0, were the basis on which the change in coordination number in the second protonation step was first proposed (54). Previously the small difference between the first and second pK value was interpreted in terms of an anomalously high first protonation constant, assumed to be caused by an increase in the coordination number in the first step (2, 3, 54-57). 

Cations with still larger charges or lower coordination numbers lose progressively more and more H ions until with typically only one or two are left, (P02(0H)2) or (P030H) , and with none are left (SO, Fig. 4.1(b)). At this stage these are no longer thought of as deprotonated hydrated cations but as oxyanions or protonated oxyanions. 

The largest nonmetals show coordination numbers as high as eight in the oc-tafiuoroanions, ]Fjj" and XeFg- (see Chapter 17). The corresponding oxyacids and oxyanions show a maximum coordination number of six [Sb(OH)fi]-, Te(OH)6, OI(OH)s, and [XeO, ]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 01(0H)S or HI04. 

The oxyanions as ligands may be classified according to (a) the structural type of the oxyanion (X02, X03, X04 or X06) (b) the coordination number of the oxyanion (1-18), i.e. the number of metal atoms to which a single oxyanion may be coordinated (c) the mode of coordination of the oxyanion, i.e. monodentate, bidentate, tridentate, etc. and (d) the number of oxyanions per metal atom, the stoichiometry p, from one to six, i.e. [M(XO ) ]. Table 1 lists the oxyanions that will be considered in this section according to their structural types, with their approximate stereochemistry and point group symmetry. The carbon-containing oxyanions will be described in Chapter 15.6, and the cyanates in Chapter 13.5, For reasons of space this review will be primarily restricted to mononuclear oxyanions. Figures 2-5 illustrate the mode of coordination of the oxyanions as a function of their coordination number 1-18. 

Within each coordination number the oxyanion may function as a monodentate, a bidentate or, very occasionally, a tridentate ligand to an individual metal cation. With the higher anion coordination numbers the M O distances to different metal atoms may not be identical. In practice, coordination numbers of 1 and 2 predominate and are independent of the three main structural types X02, X03 and X04. The coordination number 3 is essentially confined to tetrahedral X04 anions, and the much less numerous coordination numbers of 4-12 also involve mainly tetrahedral X04 type anions, particularly in their anhydrous oxyacid salts M(XO )9 or double salts M M(XO )(f+,. The stoichiometry number, p, is very often a funciton of the oxyanion/metal ratio of the preparative conditions. The higher p, the lower the coordination number of the oxyanion and the more the bonding is likely to involve a monodentate rather than a bidentate function. Nevertheless, the latter is very little influenced by the stoichiometry p. This is illustrated for the bidentate nitrate ion in the six structures (l)-(6),29,31-34 in which p increases from one to six and the bonding role of the nitrato group is essentially unchanged. 

This coordination number (Figure 4) is an order of magnitude less common than for one- and two-coordinate oxyanions (Figures 2 and 3). The coordination number does not occur for X02-type... 

The above analysis demonstrates that the coordination of oxyanions varies by the anion species. And it accords well with the stoichiometry of oxyanion / Fe at the adsorption saturations. The coordination structure of en is still unclear and the data analysis is in progress in order to clarify the origin of the small coordination numbers after the adsorption of anions. 

The next two series of nonmetals, silicon through chlorine and germanium through krypton, show a maximum coordination number of. six in hexafluoro anions., SF . and TcF. Even here the oxyacids and oxyanions typically. show a coordination... 

The radius of oxygen (0 ) (1.4 nm) roughly equals that of hydroxyl ion and the water molecule, so that similar maximum coordination numbers are found for all three. Of course, when the ligand is an oxyanion and contains multiple oxygens and/or larger atoms such as Cl or S " (r = 1.8 A for both), fewer ligands can associate with a given cation, so its maximum possible coordination number will be less. 

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