Soft Lewis base

Once acids and bases have been classified as hard or soft, a simple rule can be given hard acids prefer to bond to hard bases, and soft acids prefer to bond to soft bases (the HSAB principle)P The rule has nothing to do with acid or base strength but merely says that the product A—B will have extra stability if both A and B are hard or if both are soft. Another rule is that a soft Lewis acid and a soft Lewis base... 

A soft Lewis base has a large donor atom of high polarizability and low electronegativity. Iodide ion has its valence electrons in large a = 5 orbitals, making this anion highly polarizable and a very soft base. Other molecules and polyatomic anions with donor atoms from rows 3 to 6 are also soft bases. To summarize, the donor atom becomes softer from top to bottom of a column of the periodic table. 

Soft Lewis acids tend to combine with soft Lewis bases. 

Aluminum is unique among the main group metals. All other p block metals have filled valence d orbitals. As a consequence, these metals have much in common with their transition metal neighbors. They tend to be soft Lewis bases. Aluminum, on the other hand, lacks a filled d orbital set and is a hard Lewis acid that has more in common with its nearest neighbor, magnesium. Highly reactive, aluminum is found naturally in the +3 oxidation state and is difficult to reduce to the pure metal. Thus, although tin and lead have been known since antiquity, aluminum was not discovered until 1825 and did not become a common commodity until more than 60 years later. 

Metals that are soft Lewis acids, for example cadmium, mercury, and lead, are extremely hazardous to living organisms. Tin, in contrast, is not. One reason is that tin oxide is highly insoluble, so tin seldom is found at measurable levels in aqueous solution. Perhaps more important, the toxic metals generally act by binding to sulfur in essential enz Tnes. Tin is a harder Lewis acid than the other heavy metals, so it has a lower affinity for sulfur, a relatively soft Lewis base. 

The ditrigonal cavity formed by six corner sharing silica tetrahedra (Fig. 3.10) has a diameter of 0.26 nm and is bordered by six sets of lone-pair electron orbitals emanating from the surrounding ring of oxygen atoms. These structural features - as is pointed out by Sposito (1984) - qualifies the ditrigonal cavity as a soft Lewis base capable to complex water molecules (and possibly other neutral dipolar molecules). 

Because of the small ionic radius of lithium ion, most simple salts of lithium fail to meet the minimum solubility requirement in low dielectric media. Examples are halides, LiX (where X = Cl and F), or the oxides Li20. Although solubility in nonaqueous solvents would increase if the anion is replaced by a so-called soft Lewis base such as Br , I , S , or carboxylates (R—C02 ), the improvement is usually realized at the expense of the anodic stability of the salt because these anions are readily oxidized on the charged surfaces of cathode materials at <4.0 V vs Li. 

In the phosphonium iodide and chloride salt catalyzed TMSCN addition on aldehydes and ketones, a double activation should exist. Not only the activation of the ketones or aldehydes with the phosphonium cation is necessary, but also the activation of the TMSCN by the soft Lewis base [I] or the harder Lewis base [Cl], which can form a pentavalent silicon intermediate [121]. 

In a general way, it can be predicted that favorable bonding between a metal ion and ligands will occur when they have similar sizes and polarizabilities. Metal ions such as Co3+ and Cr3+ are hard Lewis acids and NH3 is a hard Lewis base, so it is expected that species such as [Co(NH3)6]3+ or [Cr(NH3)6]3+ would be stable. Uncharged metals are soft Lewis acids and CO is a soft Lewis base. Consequently, matching the hard-soft properties of the metal and ligands allows us to predict that Fe(CO)s will be a stable complex. Conversely, complexes such as [Co(CO)6]3+ (CO is a soft ligand) or [Fe(NH3)6] (Fe° is a soft Lewis acid) would not be expected to be stable. Both sets of predictions are in accord with experimental observations. 

With this type of shift reagent, the silver ion binds to soft Lewis base donors, such as olefins, aromatics, phosphenes, and halogenated compounds. At the same time, silver remains as an ion pair with Yb(fod)4 in a specific orientation such that a paramagnetic shift is induced in the resonances of the organic soft base. This method has proven to be useful for the separation of NMR resonance peaks of cis and trans geometric isomer mixtures of long-chain alkenes and complex mixtures of aromatic molecules [5]. 

The palladiumCn) ion forms a 1 1 complex with 5-Br-PADAP, leaving one site for the coordination of another ligand. The Pd(II)-5-Br-PADAP (PdL) complex cation has specific properties such as an extremely high molar absorptivity ( 554 = 4.33 x 10 M - cm in toluene), a high adsorptivity to the liquid/liquid interface and a soft Lewis acid easy to be bound to a soft Lewis base. Therefore, PdL is expected to function as an interfacial molecular recognition reagent of Lewis bases. 

Mercury(II), Hg , is a soft Lewis acid, and so is found in nature only combined with soft Lewis bases, the most common of which is S ". Sulfide can readily and permanently abstract Hg from its complexes with harder bases in ore-forming geological reaction mixtures. Zinc(II), which exhibits borderline behaviour, is harder and forms... 

In the nineteen-eighties many researchers developed a variety of Lewis acid catalysts of the Mukaiyama aldol reaction. In particular, TrClO4 [23] and TMSOTf [24] effectively promote reaction of silyl enolates with aldehydes or acetals. These studies suggested that introduction of a soft Lewis base such as the CIO4 or OTf anion into the Lewis acidic center should lead to effective catalysts. Based on this concept, further studies have been continued to develop novel Lewis acid catalysts with higher catalytic activity or higher chemo- and stereoselectivity. 

Mercury(II) is a very soft Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as sulfur ligands. You should remember here that the major natural form of mercury is sulfides. Increasing the pH of the aqueous solution due to... 

Hard Lewis acids and bases have inflexible electron orbitals that form ionic bonds. The electron orbitals of soft Lewis acids and bases are more polarizable and more likely to form covalent bonds. Soft Lewis acids and bases are also called covalent-bonding ions and are siderophile (sulfur-loving) ions in the geology literature. Organic ligands and soil organic matter range from hard to soft Lewis bases. ... 

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