Factors governing the metal coordination number in isolated group IA and IIA metal hydrates

Many of the group IA and IIA metal ions, such as Na+, K+, Mg2+, and Ca2+, play crucial roles in biological functions. Previous theoretical studies generally focus on the number of water molecules bound to a particular (as opposed to all) alkali or alkaline earth cations and could not establish a single preferred CN for the heavier alkali and alkaline earth ion-water complexes. Crystal structures of hydrated Na+, K+, and Rb+ also cannot establish the preferred number of inner-shell water molecules bound to these cations. Consequently, it is unclear if the gas-phase CNs of group IA metal hydrates increase with increasing ion size, as observed for the group IIA series from the Cambridge Structural Database, and if the same factors govern the gas-phase CNs of both group IA and IIA ion-water complexes. Thus, in this work, we determine the number of water molecules directly bound to the series of alkali (Li+, Na+, K+, and Rb+) and alkaline earth (Be2+, Mg2+, Ca2+, Sr2+, and Ba2+) metal ions in the gas phase by computing the free energy for forming an isolated metal-aqua complex as a function of the number of water molecules at 298 K. The preferred gas-phase CNs of group IA hydrates appear insensitive to the ion size; they are all 4, except for Rb+, where a CN of 6 seems as likely. In contrast, the preferred gas-phase CNs of the group IIA dications increase with increasing ion size; they are 4 for Be2+, 6 for Mg2+ and Ca2+, and 7 for Sr2+ and Ba2+. An entropic penalty disfavors a gas-phase CN greater than 4 for group IA hydrates, but it does not dictate the gas-phase CNs of group IIA hydrates. Instead, interactions between the metal ion and first-shell water molecules and between first-shell and second-shell water molecules govern the preferred gas-phase CNs of the group IIA metal hydrates.     

Protection of primary amino as pyrrole in organometallic reagents

Pyrrole 5 was converted into Grignard and organocuprate reagents. After addition of the latter to alkene 6, the pyrrole ring was converted to a primary amine by ozonolysis, reduction and hydrolysis.     

Novel organometallic reagents: geminal dianionic derivatives of the heavy group 14 elements

This Forum review describes the most recent achievements in the novel prospective field of highly reactive main-group organometallics, namely, geminal dianionic derivatives of the heavy group 14 elements (Si, Ge, Sn). A brief historical introduction to the topic is followed by discussion of the current state of affairs in the field of stable derivatives and prospects for future efforts, highlighting our own synthetic approach and recent results. The most important experimental contributions, including synthesis of 1,1-dilithiosilane, -germane, and -stannane derivatives; dilithio(halo)silanes (lithiosilylenoids); metallole 1,1-dianions; and heavy analogues of the cyclobutadiene dianion derivatives, are presented, along with a discussion of the synthetic applications of the above-mentioned organometallic compounds.     

Use of brucine as an oxidation-reduction indicator in cerimetry

Summary Brucine has been found to work satisfactorily as an inside indicator in the titration of molybdenum(V) and hydroquinone with ceric sulphate. It does not give a good indication of the end point in the titration of arsenic(III) in sulphuric acid medium, using osmium tetroxide as a catalyst or in hydrochloric acid medium using iodine monochloride as a catalyst. Brucine cannot also be used in the titration or uranium(IV) with ceric sulphate. A 0.1 per cent solution of brucine in 2 N–3 N acetic acid has been found to be stable for nine months without showing any discoloration.     

Potentiometric titration of cyanide and chloride, using the silver specific-ion electrode as an indicator

Conditions are given for consecutive potentiometric titration of cyanide and chloride in mmole amounts, a silver-specific electrode being used as indicator electrode.     

Determination of calcium in biological material The use of calcein as an indicator in the edta titration

The EDTA titration of calcium using the change in fluorescence of Calcein indicator under ultraviolet illumination at the end-point was very satisfactory for essentially pure calcium chloride solutions. In order to use this method for biological samples, it was found best to separate the calcium by an oxalate precipitation at a pH of 4.7, convert the calcium oxalate to calcium carbonate, and dissolve the calcium carbonate in hydrochloric acid.     

Ferroin as indicator in the dichrometric titration of uranium(IV)

Ohne Zusammenfassung

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Gravimetric determination of fluoride by precipitation of LiF from homogeneous solution. Comparison with the classical method

     

Argentometric titration of chloride with dichlorofluoroscein as an adsorption indicator: A useful modification

The addition of excess of ethyl alcohol renders more useful the argentometric titration of chloride with dichlorofluoroscein as adsorption indicator by improving the end-point, both when simple counter-ions are present and also when the presence of adsorbable and/or complexing counter-ions renders the normal technique inadequate.     

Potentiometric titration of cyanide using the cation-sensitive glass electrode as indicator

Summary The argentometric determination of cyanide was re-examined in order to compare the cation-sensitive glass electrode and the silver metal electrode as indicator electrodes for this titration. Contrary to earlier reports, the glass electrode is capable of indicating both titration end points and yields stoichiometric information comparable to or better than that obtained with the silver metal electrode. In the presence of redox interferences, the glass electrode is superior to the silver metal electrode for titration purposes.

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Zusammenfassung Die argentometrische Cyanidbestimmung wurde im Hinblick auf einen Vergleich der kation-empfindlichen Glaselektrode und der Silberelektrode als Indicatorelektroden nochmals untersucht. Im Gegensatz zu früheren Berichten ergab sich, daß die Glaselektrode beide Endpunkte der Titration anzeigen kann und Aussagen über die Stöchiometrie gestattet, die vergleichbar oder besser sind als die mit Hilfe der Silberlektrode erhaltenen. In Gegenwart von Oxydations- oder Reduktionsmitteln erweist sich die Glaselektrode als besser geeignet.

     

p-Fluorphenyl group as an indicator of charge on the carbon atom in aromatic compounds

A linear equation is presented for determining, from the chemical shift of¹⁹F in a p-fluorophenyl group, the effective charge of that CH fragment of the original aromatic molecule the carbon atom of which is bonded to the fluorophenyl indicator group. It has been shown that for a cyclic aromatic 6-electron system determination of the effective charges of the CH fragments by quantum-chemical calculations and the evaluation of this charge from the¹⁹F chemical shift of the corresponding p-fluorophenyl substituents lead to coincident results.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2219–2222, October, 1989.The authors are grateful to N. P. Gambaryan for fruitful discussions.     

Use of tosylaminocarbonyl group as an amino protecting group in peptide chemistry]

Several N-tosylaminocarbonyl amino acids and corresponding di- and tripeptides were prepared in order to investigate the suitability of the tosylaminocarbonyl group as an amino protecting group in peptide chemistry. The group is shown to be selectively cleavable under neutral conditions.