Atomic of anions

Fig. 109. Environment of water molecule ( ) attached to oxygen atoms of anions and also to other water molecules.

Neptunium complex with pyridine [Np02(CH3C(0)CHC(0)CH3)2(Py)] (27) is described in the work [84]. The equatorial plane of Np atom is formed by four oxygen atoms of anions and nitrogen atom of pyridine. The Np-0 distances are equal to 2.333-2.372 A, while Np-N bond is longer, 2.564 A. The equatorial surrounding is near planar, the deviation of atoms does not exceed 0.088 A. At... 

We have already remarked that in many hydrates the H2O molecules are not all equivalent, often having very different environments. It is also found that in some hydrates there are two or more kinds of non-equivalent cation. This complication is less frequently encountered an example is Na4P207.10 H20, in which one-half of the Na ions have 6 H2O while the remainder have 4 H2O and 0 atoms of anions as nearest neighbours. We confine our examples in Table 15.5 and the following account to hydrates in which all the cations have similar arrangements of nearest neighbours, as is the case in most hydrates. 

Several recent reviews have projected about the gigantic rise in the number of publications about coordination polymers per year since 1990. In the majority of coordination polymers, either the O-atoms of anions (carboxylates, nitrates, sulfates, phosphates, phenolates) and/or the N-atoms of cyanates, cyanides, amines, pyridines engage in coordination bonds with transition-metal atoms (Tr and Tr-N)- A Cambridge structural database (CSD) search indicates that the coordination polymers propagated by Tr O (7074 hits) are the major portion compared to those of Tr-N (6392 hits). In about 53% of the Tr-O type, the O-atom belongs to carboxylates, whereas in about 40% of the Tr-N kind, the N-atom belongs to pyridine.These statistics indicate the importance of carboxylate and pyridine moieties in generating coordination polymers. 

Biggs S, Mulvaney P, Zukoski C F and Grieser F 1994 Study of anion adsorption at the gold-aqueous solution interface by atomic force microscopy J. Am. Chem. Soc. 116 9150... 

Note that agreement with Pariser and Parr s empirical value is better for Y13 than for Yn ) Use Salem s values to calculate election densities on the three carbon atoms of the allyl anion for one iteration beyond the initial Huckel values, as was done in Exercise 8.9.1. Comment on the results you get, as to the qualitative picture of the anion, the influence of election repulsion on the charge densities, and agreement or lack of agreement with the results already obtained with the Pariser and Parr parameters. 

Removal of an a-hydrogen atom of the ketone as a proton to form n carbauion (acetone anion) ... 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

The nucleophilicity of the nitrogen atom survives in many different functional groups, although its basicity may be lost. Reactions of non-basic, but nucleophilic urea nitrogens provide, for example, an easy entry to sleeping-pills (barbiturates) as well as to stimulants (caffeine). The nitrogen atoms of imidazoles and indole anions are also nucleophilic and the NH protons can be easily substituted. 

A number of novel reactions involving the a carbon atom of aldehydes and ketones involve enol and enolate anion intermediates... 

A hydrogen attached to the a carbon atom of a p keto ester is relatively acidic Typical p keto esters have pA values of about 11 Because the a carbon atom is flanked by two electron withdrawing carbonyl groups a carbanion formed at this site is highly stabi hzed The electron delocalization m the anion of a p keto ester is represented by the res onance structures... 

Stabilized anions exhibit a pronounced tendency to undergo conjugate addition to a p unsaturated carbonyl compounds This reaction called the Michael reaction has been described for anions derived from p diketones m Section 18 13 The enolates of ethyl acetoacetate and diethyl malonate also undergo Michael addition to the p carbon atom of a p unsaturated aldehydes ketones and esters For example... 

Isopolyanions. Isopolyanions are named by indicating with numerical prefixes the number of atoms of the characteristic element. It is not necessary to give the number of oxygen atoms when the charge of the anion or the number of cations is indicated. 

Aldol Addition and Related Reactions. Procedures that involve the formation and subsequent reaction of anions derived from active methylene compounds constitute a very important and synthetically useful class of organic reactions. Perhaps the most common are those reactions in which the anion, usually called an enolate, is formed by removal of a proton from the carbon atom alpha to the carbonyl group. Addition of this enolate to another carbonyl of an aldehyde or ketone, followed by protonation, constitutes aldol addition, for example... 

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Bromine and chlorine convert the 1- and 2-butenes to compounds containing two atoms of halogens attached to adjacent carbons (vicinal dihahdes). Iodine fails to react. In this two-step addition mechanism the first step involves the formation of a cation. The halonium ion formed (a three-membered ring) requires antiaddition by the anion. 

Fig. 10. Pharmacophores for angiotension-converting enzyme. Distances in nm. (a) The stmcture of a semirigid inhibitor and distances between essential atoms from which one pharmacophore was derived (79). (b) In another pharmacophore, atom 1 is a potential zinc ligand (sulfhydryl or carboxylate oxygen), atom 2 is a neutral hydrogen bond acceptor, atom 3 is an anion (deprotonated sulfur or charged oxygen), atom 4 indicates the direction of a hydrogen bond to atom two, and atom 5 is the central atom of a carboxylate, sulfate, or phosphate of which atom 3 is an oxygen, or atom 5 is an unsaturated carbon when atom 3 is a deprotonated sulfur. The angle 1- -2- -3- -4 is —135 to —180° or 135 to 180°, and 1- -2- -3- -5 is —90 to 90°.

The removal of a hydrogen atom from a heterocyclic nitrogen atom of azolones by nucleophiles acting as bases, e.g. (14) (15), gives mesomeric anions, e.g. (15) <-> (16) <->... 

Often electrophilic reagents can attack both nitrogen atoms of the mesomeric pyrazolate and indazolate anions. In this case there is no simple relationship between the tautomeric constant and the product composition. 

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