Tetrahedral anions

—Tetrahedral Anions. The stepwise hydrol3 is of the [BHJ anion has been investigated in some detail in 88% methanol solution, 0.1—I.IM add. The first step, which results in the production of [BH3(OH2)], is rapid, even at —78 °C. The rate law for the subsequent hydrolysis of this spedes to [BH2(0H2)2]+ is 

The BXa may then react with the replacing halide Y to give [BXaY], and 

The intermediate species (1) is reminiscent of the known [B2F7] anion. Hydrolyses of pyrrolyl-1-borate anions [BPhn(C4H4N)4 nl are second-order reactions, and are catalysed by acids. The activation enthalpy for hydrolysis of the [B(C4H4N)4] anion, which is 14.8 0.9 kcal mol, is intermediate between those for the hydrolyses of the [BHa] and [BPhi]- anions.  

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Manganates V f), [MnOJ", permanganates. Dark purple tetrahedral anion (electrolyte oxidation of [Mn04]. Powerful oxidizing agent... 

All the following compounds are charactenzed by ionic bonding between a group I metal cation and a tetrahedral anion Wnte an appropriate Lewis structure for each anion remembenng to specify formal charges where they exist... 

Figure 3. Schematic representation of the lithium-ion conductor LiAICl4. The A1C14 may be considered as tetrahedral anions, as indicated by green. The lithium ions are located between them.

Several ternary phases occur, others exhibiting 3D structures (like BaVS3, BaTaS3) and others containing discrete tetrahedral anions, such as the [VS4] unit in M3VS4 (M = Na, K, Tl, NH4). 

The long-known K2Hg(CN)4 and related compounds are still the object of structural studies. Thus, a neutron-diffraction study confirmed the spinel structure of K2Hg(CN)4 at room temperature (298 K) with exactly tetrahedral anions, Hg(CN)42 (r(Hg—C) 215.2, r(C—N) 114.9 pm).112... 

The free selenite ion has a pyramidal shape (C3v symmetry) owing to the lone electron pair at the selenium atom. Thus, the Se032- ion can be treated as a pseudo-tetrahedral anion and the lone electron pair often acts as an invisible ligand within the crystal structures of selenites. This observation is called the stereochemical activity of the lone electron pair and it will turn out as one of the... 

A direct evidence of the way of tetrahedral anion adsorption at three-fold sites and the degree of hydratation is not available at present. However, a strong indication of such adsorption of sulphates is found in voltammetry on gold (14) and in our data for platinum surfaces (12). A pronounced difference between the sulphate and perchlorate adsorption effects is... 

Figure 3. Models for the stepped surfaces, with tetrahedral anions adsorbed on the (111) trigonal sites for a) n(lll)-(100), b) n(100)- (111), o) n(100)-(100) and d) n(lll)-(lll) terrace-step orientations.

Transition states of this tetrahedral nature have now been mimicked effectively by a range of stable analogues, including phosphonic acids, phosphonate esters, a-difluoroketones, and hydroxymethylene functional groups (Jacobs, 1991). Lerner s group elicited antibodies to a tetrahedral anionic phosphonate hapten [3] (Appendix entry 2.9)2 whilst Schultz s group isolated a protein with high affinity for p-nitrophenyl cholyl phosphate [4] (Fig. 4) (Appendix entry 3.2). 

Amide hydrolysis at alkaline pH involves a tetrahedral anionic intermediate, which was mimicked by the transition state analogue [49], an /V-aryl arylphosphonamidate, appropriately related to substrate anilide [50] (Fig. 18) (Appendix entry 2.8). 

As noted earlier, cobalt(II) in strong electrolyte solutions will readily form tetrahedral anionic complexes that can be extracted into an immiscible... 

Base hydrolysis of amides also requires quite vigorous conditions, but mechanistically it is exactly equivalent to base hydrolysis of esters. After nucleophilic attack of hydroxide on to the carbonyl, the tetrahedral anionic intermediate is able to lose either an amide anion (care with nomenclature here, the amide anion is quite different from the amide molecule) or hydroxide. Although loss of hydroxide is preferred, since the amide anion is a stronger base than hydroxide, this would merely reverse the reaction. 

Esters also react smoothly with amines, which is a useful reaction if the corresponding acyl halides or anhydrides are not easily available. The reaction proceeds through the anticipated tetrahedral anionic intermediate. There are two possible leaving groups... 

The initial reaction is effectively the same as with an aldehyde or ketone, in that hydride is transferred from the reducing agent, and that the tetrahedral anionic intermediate then complexes with the Lewis acid aluminium hydride. However, the typical reactivity of the carboxylic acid derivatives arises because of the presence of a leaving group. 

The X-ray crystal structure of K[RuO ] shows the salt to have the scheelite (CaWO ) structure with a slightly flattened tetrahedral anion (mean Ru=(0) 1.79 A) [451], A single crystal X-ray structure of TRAP shows this to contain the expected tetrahedral anion [472],... 

The balance of evidence (electrochemical and Raman data, cf. 1.2.2 Figs. 1.2, 1.3 Tables 1.1 and 1.2) suggests that ruthenate is likely to be the tetrahedral anion [RuO ] in solution, so this formulation is used in the text, hi the solid state the anion can be either [RuO or, more commonly, rranx-[Ru(OH) (O) ] . 

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