Oxyanions coordinated

ESR spectra, 2, 511 Oxovanadium(V) complexes aqueous solution, 3,1026 Oxyanions coordinated... 

Fignie 23 Trigonal oxyanion coordination to the copper(II) cation... 

Complex Chromo- phore Coordination no. Stereo- chemistry0 Oxyanion coordination no. Type of lattice Ref. 

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. 

The deoxygenation step was both unprecedented at the time and unique to titanium, thus its mechanism was more challenging to probe. McMurry postulated three general mechanisms (a) titanacycle formation, (b) an acyclic variant with two coordinated titaniums, and (c) a hybrid mechanism where both oxyanions coordinate to the surface of a finely... 

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). 

As for all elements, the distribution of Mo in the environment depends critically on chemical speciation, including oxidation state (Bertine and Turekian 1973 Morford and Emerson 1999). However, Mo is somewhat unusual in both respects. In terms of ligand coordination. Mo is one of a small number of transition metals that commonly form oxy anions and coordinate only weakly with other environmentally common ligands such as Cl" or OH". Other such metals include Cr and W, which sit above and below Mo, respectively, in Group VI of the Periodic Table, as well as Tc, Re, Os and U. Hence, Mo chemistry has some analogies with these metals, as well as with nonmetals such as S, Se, P and As which also form oxyanions. 

Fig. 16A,B Proposed mechanism of the pre-mRNA splicesomal splicing. A catalytic metal ion is essential for both steps A the first step B the second step. A catalytic magnesium ion is inferred to directly coordinate the 3 oxyanion leaving group in the transition state. Catalysts which deprotonate from the nucleophiles have still remained unclear [152]...

Oxyanions also affect the coordination chemistry of the metal center (84). Molybdate and tungstate are tightly bound noncompetitive inhibitors (Ki s of ca. 4 (iM) (85). These anions bind to the reduced form of the enzyme, changing the rhombic EPR spectrum of the native enzyme to axial (Figure 1) and affecting the NMR shifts observed (84,85). Comparisons of the ENDOR spectra of reduced uterofenin and its molybdate complex show that molybdate binding causes the loss of iH features which are also lost when the reduced enzyme is placed in deuterated solvent (86). These observations suggest that molybdate displaces a bound water upon complexation. 

Adsorption of simple inorganic anions, oxyanions and organic ions on iron oxides has been widely investigated (see Tables 11.1 and 11.2). Anions are ligands, i.e. they possess one or more atoms with a lone pair of electrons and so can function as the donor in a coordinate bond. Ligands may adsorb on Fe oxides either specifically or non specifically. 

The affinity between the tetrahedrally arranged orthophosphate oxyanion, P04, and hexava-lently coordinated metal cations lends itself to a classification of phosphate minerals in a scheme similar to silicates (SiO -) framework, insular, chain, and layer phosphates. Examples of this scheme, advanced by Povarennykh (1972) and further elaborated by Lindsay Vlek (1977), include berlinite (AIPO4 framework) hydroxyapatite (insular) monetite (CaHP04, chain) and vivianite (Fe4(P04)2-2H2O, layer). 

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 ligands of a lone-pair cation lie on the surface of a sphere. When TU is surrounded by weakly bonding oxyanions, it lies at the centre of the sphere, forming nine bonds of 297 pm each (O.llvu, Fig. 8.2(a)). When it bonds to strongly bonding anions, as in TI3BO3, TU moves about 70 pm away from its centre to form three primary bonds of 266 pm (0.33 vu), and six secondary bonds of 324-372 pm (Fig. 8.2(b)). In the process the radius of the coordination sphere increases from 297 to 322 pm in accordance with the distortion theorem (Rule 3.6). 

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