Search

Hard and soft acid-base interactions are investigated from the point of view of various factors contributing to the total bond energy. A method of estimating the total energy as the sum of three contributing energies is proposed. The three energies are the Madelung energy, derived from a coulombic model; the covalent energy, related to Pauling's geometric mean but allowing for loss of overlap with increasing ionicity; the electronegativity energy, the energy resulting from transfer of electron density from a less electronegative element to a more electronegative element. The latter is estimated from Mulliken-Jaffe electronegativies. The results correlate quite well (r = 0·98) with experimental enthalpies of atomization. It is concluded that hard-hard interactions contain considerable covalent bonding as well as Madelung energies. Fajans' rules and covalency, the “anomalous” bond energies of sodium and magnesium compounds, and the validity of the principle of electronegativity equalization are discussed.

The thermodynamic parameters $$\Delta \bar H, \Delta \bar G, \Delta \bar S_{298} $$ and lg βn resp. of the reactions indicated in the title have been computed from polarographic data. The numerical values obtained are nearly independent from the xanthate used. The overall formation constants increase as follows: Zn(II)

Symmetrical disilthianes of the type M(C6H5)2SiSSi(C6H502M(M  CH3; CH2  CH3, C6H4X; X  H, F, Cl, Br and CH3) has been prepared by reaction of the appropriate chlorosilane in the presence of triethylamine with hydrogen sulfide. Selected alcoholysis attempts indicate that these symmetrical aromatic disilthianes are resistant to electrophylic cleavage of the SiSSi linkage. Physical properties of the new disithianes as well as their IR and PMR spectra are discussed.

The reactions of SiX4(X = F, Br, Cl) with trimethylphosphine and of silicon tetrabromide with trimethylamine are examined and the approximate stability relationships found to be: SiF4 SiCl4, SiBr4 towards trimethylphosphine SiF4 > SiCl4 > SiBr4 towards trimethylamine, and Me3P Me3N towards silicon tetrachloride and silicon tetrabromide. The compounds SiCl4,2Me3P, SiBr4,2Me3P, SiF4,Me3P, SiF4,2Me3P, and SiClO4,2Me3P are characterised.

Complexes containing the soft acid metals Pt, Pd, Ag, and Au, with the ligand 1,3-dimethylimidazolium-2-dithiocarboxylate (L) have been synthesized and characterized by analytical data and infrared and electronic spectroscopy. The data suggest that the ligand acts as a bidentate ligand with Pt and Pd, a monodentate ligand with Ag and a bridging ligand between two Au atoms.

Dethioacetalization with thallium trinitrate (TTN) was tried on thioacetals 3-13, and good results were obtained. Then, the utility of co-solvents to be used with methanol was checked, and a wide range of co-solvents were shown to be available. The mechanism was also discussed. Subsequently, dethioacetalization with mercuric perchlorate was found to be another excellent method. This procedure provided a great success for the dethioacetalization of compounds 4 and 11 which was not successful by the TTN procedure.

Although the terms acid and base have been in use since the early days of chemistry, the meaning of these terms has undergone considerable modification through the centuries and continues even today with the recent introduction of new terms such as hard acid, soft acid and superacid. It is the purpose of this chapter to consider some of the classic definitions of the terms acid and base and also some of the more recent developments of our ideas concerning acids and bases [1].

One point that will be repeated over and over again in later chapters is that the speciation of a metal, rather than its total concentration, is the key to understanding its effects on the biota. The realization of this was due as much to a breakthrough in analytical techniques as it was to advances in theoretical knowledge of the subject; the technology for distinguishing different species of heavy metals at low concentrations has only become widely available relatively recently.

Today I am very honored to be given this opportunity to talk about our recent studies of some comparative aspects of cochlear mechanism at this symposium.

Several qualitative microprobe studies have been focused on the distribution of essential and non-essential metals between the various types of metal-rich invertebrate spherites (Brown, 1982; Morgan, 1984; Morgan and Winters, 1987). The conclusion drawn from these observations is that, in general and despite the competitive tendancy extant in multimetal and multi-ligand situations in vivo, the metals are preferentially bound to cellular ligands according to their equilibrium constants for metal-ligand complexes (Nieboer and Richardson, 1980): Class A (‘hard acid’) metals such as Ca, Mg, Mn, Sr and Pb show a preference for oxygen donating ligands according to the sequence O>N>S; Class B (’soft acid’) metals such as Cd, Cu, Hg conform to the ligand sequence S>N>O (Simkiss, 1981; Simkiss and Taylor, 1981; Simkiss et al., 1982; Morgan and Winters, 1987).

The use of cellulose nanofibers (CNF) as an additive in papermaking is an attractive alternative to improve paper’s strength. However, the costs of CNF production need to be competitive compared to other approaches aimed at reducing mechanical beating. Five different types of CNFs were prepared following different pretreatments: TEMPO-mediated oxidation at basic and neutral conditions, soft acid hydrolysis, enzymatic treatment, and mechanical beating. All of the pretreated fibers were later passed through a high-pressure homogenizer. The resulting CNFs were each applied to a papermaking pulp to investigate their reinforcing ability. Results indicated that the TEMPO-oxidized CNFs offered the highest increase at the same nanofiber content compared to the other types of CNFs. However, an analysis of the cost of increasing paper’s breaking length by 75% indicated that TEMPO-oxidized CNFs were more expensive than the other CNF grades, whereas CNFs from mechanical and acid pre-treatment offered similar increases at lower prices. The results indicated that CNFs of high fibrillation degree were not necessary to induce dramatic increases in paper strength. This finding offers a new possibility for the escalation of CNF production to industrial levels with competitive prices The authors whis to aknowledge the finalcial support of the Spanish Economy and Competitiveness Ministry to the project CTM2011-28506-C02-01

The thiophosphorylcyclopentadienyl-thiolate complexes [(5-C5H4R)(5-C 5H4P(S)Ph2)Ti(SPh)2] react with cis-[Pt(C6F5)2(THF)2] initially give the adducts [(5-C5H4R)(SPh)Ti(, 5-S-C5H4P(S)Ph 2)(-SPh)Pt(C6F5)2] [R=P(S)Ph2 (1); SiMe3 (4)], which finally rearrange in solution to form the double thiolate-bridged derivatives [(5-C5H4R)(5-C 5H4P(S)Ph2)Ti(-SPh)2Pt(C 6F5)2] [R=P(S)Ph2 (2) (syn); SiMe3 (5) (syn/anti)]. This paper presents the crystal structure of 4, a complex displaying an unusual mixed thiolate-thiophosphorylcyclopentadienyl bridging system. In contrast, similar reactions of cis-[Pt(C6F5)2(THF)2] with [(5-C5H4R)(5-C 5H4P(O)Ph2)Ti(SPh)2] (R=P(O)Ph2; SiMe3) only gives [(5-C5H4R)(5-C 5H4P(O)Ph2)Ti(-SPh)2Pt(C 6F5)2] [R=P(O)Ph2 (3) (syn); SiMe3 (6) (syn/anti mixtures)] as expected due to the soft acid nature of the platinum centre. © 1999 Elsevier Science S.A.

Generally the reactive loci of sulfonyl chlorides reside at sulfur. Since the formal oxidation state of the sulfur atom in these compounds is +6, most of their reactions are characterized by hard-hard interactions (charge-controlled), as exemplified by the sulfonylation of alcohols and amines.

The hard / soft acids / bases principle has been justifiably criticized because of the lack of a precise definition of hardness and the inability to quantify this property; recent developments have overcome these objections, however.

Using the principles of hard and soft acids and bases to estimate the strength and softness of an acid or base.