Lewis bases coordination bonds

Lewis bases coordinate to the zinc atom of dialkylzinc. Donation of electrons from the Lewis base weakens the Zn—C bond, and thus enhances the nucleophilicity of the alkyl... 

In addition, the electronic structure, mainly determined by the central metal and Lewis-basic or Lewis-acidic substituents at the ligands, is a considerable aspect for the polymerization activity of metallocene and half-sandwich complexes." Complexes with a sterically accessible oxygen or a nitrogen atom (Lewis bases) coordinated to the central metal are not useful for good catalytic performance. Generally, hafnium metallocene and half-sandwich complexes exhibit lower polymerization activities due to a more stable hafnium—carbon bond," which lowers the insertion rate of the coordinated monomer into the growing polymer chain. 

Most chemists would rationalize the existence of amine borane as due to the fact that the B atom in monomeric BH3 has only six electrons in the valence shell. The formation of the NB bond is described as due to donation of the electron lone pair on N to the electron poor boron atom. A species formed in this manner from two relatively stable chemical entities, is referred to as a complex or coordination compound. The chemical species providing the electron pair is referred to as the electron donor or the Lewis base. The bonding partner is referred to as the electron acceptor or the Lewis acid. The new bond that has been formed between the donor and acceptor atoms, has been referred to as an electron donor-acceptor bond or as a dative bond. 

The dominant decomposition pathway consists of insertion of the carbene into the Ru-C bond formed via C-H activation (Scheme 3.5). This is followed by either P-hydride elimination, which is preferred, or a-hydride elimination if no P-protons are present. As shown in structure 21, this insertion can occur as a result of strong Lewis-base coordination (e.g., CO or small phosphines). The presence of electron-withdrawing carboxylates was also found to be detrimental because they appear to favor insertion (24). Finally, additives that are commonly used to suppress olefin migration reactions, such as benzoquinone, also resulted in catalyst... 

A complex ion is an ion formed from a metal ion with a Lewis base attached to it by a coordinate covalent bond. A complex is a compound containing complex ions. A ligand is a Lewis base that bonds to a metal ion to form a complex ion. For example, Ag(NH3)2 is a complex ion formed from the Ag ion and two NH3 molecnles. The NH3 molecules are the ligands. 

How strong are these solvent interactions with unsaturated fragments such as Cr(CO)s This question has been addressed by photoacoustical calorimetric measurements [29,30]. As expected, traditional Lewis bases coordinate more strongly than do the alkanes but the alkane chromium bond is apparently worth about 10 kcal mol L Time resolved IR spectral experiments have demonstrated an interaction of similar magnitude between W(CO)5 and alkanes in the gas phase [31]. Notably, even xenon has been shown in both solution [32] and gas phase [33] experiments to bind with a dissociation energy of 8 kcal mol . ... 

Thus in the aldol addition, just as in the case of epoxide-opening reactions, the chloride ion, formed as a necessary consequence of the mechanism of Lewis base catalysis with chlorosilanes, is not innocuous. In fact, it is a competent nucleophile that can attack an aldehyde or an epoxide activated by the Lewis base-coordinated silicenium cation in an intermolecular fashion. The desire to understand these two seemingly inconsistent results obtained in our study of the Lewis base-catalyzed reactions of trichlorosilanes presented an opportunity for the development of novel catalytic processes. For example, if a chloride ion can capture these activated electrophiles, could other exogenous nucleophiles be employed to intercept these reactive intermediates If so, a wide variety of bond-forming processes mediated by the phosphoramide-bound chiral Lewis acid [LB SiCls]" would be feasible. At this point it remained unclear if (1) an exogenous nucleophile could compete with the ion-paired chloride and (2) what kinds of nucleophiles could be compatible with the reaction conditions. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

A coordination compound, or complex, is formed when a Lewis base (ligand) is attached to a Lewis acid (acceptor) by means of a lone-pair of electrons. Where the ligand is composed of a number of atoms, the one which is directly attached to the acceptor is called the donor atom . This type of bonding has already been discussed (p. 198) and is exemplified by the addition compounds formed by the trihalides of the elements of Group 13 (p. 237) it is also the basis of much of the chemistry of the... 

In principle, any molecule or anion with an unshared pair of electrons can act as a Lewis base. In other words, it can donate a lone pair to a metal cation to form a coordinate covalent bond. In practice, a ligand usually contains an atom of one of die more electronegative elements (C, N, O, S, F, Cl, Br, I). Several hundred different ligands are known. Those most commonly encountered in general chemistry are NH3 and HzO molecules and CN , Cl-, and OH- ions. 

The diketonates can form Lewis base adducts such as 5-coordinate Pd[P(o-tolyl)3](CF3COCHCOCF3)2 (Figure 3.25), though with acetylacetone square planar adducts of the type M(acac)2(PR3)2 are usually obtained, where the diketone is monodentate O-bonded [63]. 

When a Lewis base donates an electron pair to a Lewis acid, the two species share the pair and become joined by a coordinate covalent bond, a bond in which both electrons come from one of the atoms (see Section 2.11). 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

To remove an ion, we can use the fact that many metal cations are Lewis acids (Section 10.2). When a Lewis acid and a Lewis base react, they form a coordinate covalent bond and the product is called a coordination complex. In this section, we consider complexes in which the Lewis acid is a metal cation, such as Ag+. An example is the formation of Ag(NI 1,)2+ when an aqueous solution of the Lewis base ammonia is added to a solution of silver ions ... 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

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