Anions tricoordinated

Experimental studies of tricoordinated anions of heavier group 14 elements (M = Ge, Sn and Pb) were reviewed by Riviere and coworkers484. 

The electron affinity (EA) of the MH3 radicals increases significantly from CH3 to SiH3 while SiH3 and GeII3 have similar EAs i.e. the calculated electron affinities 

FIGURE 37. A molecular orbital diagram which explains the operation of a second-order Jahn-Teller effect in the pyramidalization of MH3. Reprinted with permission from Reference 57. Copyright (1996) American Chemical Society 

Hoz and coworkers have recently studied the intrinsic barriers in identity Sn2 reactions (equation 44)490. 

The interesting aromatic metalolyl anions c-(C4II4)MII- (90) and c-(C4H4)M2 (91) are discussed in Section VI.F.3. 

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Anionic ligands play a very significant role in oxidative and transmetallation addition reactions [91, 92]. Thus, for example, Amatore and Jutand [93, 94] concluded that in the presence of acetate, the tricoordinated anionic species [PdL2(OAc)] are the effective complexes in oxidative addition [94], instead of the usually postulated neutral [PdLj] complex. In the presence of halide anions, anionic complexes are also formed [95-97]. In general, the following order of stabilization of the anionic Pd(0) species is observed 1 > Br > Cl [98]. 

The addition of anions to the reaction mixture may affect the rate of oxidative addition, creating a more polar medium or bind directly to the palladium and thereby changing the mechanism [38, 39]. The neutral bisphosphine palladium(O) complex is intrinsically more reactive than the anionic analogous due to the more naked central metal, but the concentration of the anionic species is significantly higher in polar media [2, 37]. Hence, the reaction will predominantly proceed through the tricoordinated anionic species, when polar solvents are used generating an intermediary pentacoordinated anionic palladium(ll) complex. 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

Tn this contribution we will deal only with the low-coordinated (tricoordinated) cations, radicals, and anions. The vast number of hypercoordinated (penta-, hexacoordinated) species is outside the scope of the present review and will not be considered. 

A family of tricoordinated persilyl substituted anions of the heavier group 14 elements (RaSOaE (R = alkyl E = Si, Ge, Sn) was synthesized in recent years. Sekiguchi and co-workers employed the straightforward one-electron rednction of the corresponding radicals (r-Bu2MeSi)aE 43-45 with metallic Li or K to form anionic derivatives (r-Bu2MeSi)aE ]VI (E = Si, Ge, Sn M = Li, K) 58 M (Scheme 2.55). ... 

Such overlap requires a planarity between the phenyl ring and the trigonal, tricoordinate boron plane. Where that planarity is significantly disrupted by ortho substituents, as is evident in the XRD structure of lithium(12-crown-4)2 cation-trimesitylboron anion,41 only 65% of the free spin is delocalized onto the rings.42... 

The mental substitution of an olefinic carbon with tricoordinate boron would give rise to boraethenes or methyleneboranes (51) a similar substitution with a tetracoordinate borate anion would produce borataethenes or methyleneborates (52a), which are but a resonance representation of borylcarbanions (52b). 

Sometimes more complicated ion exchange reactions are observed, especially when the cation and the anion are able to interact with one another. Then, further transformations of tricoordinated phosphorus derivatives into ammonium 1,3,2,5-dioxaborataphosphorinanes are observed [Eq. (124)]. 

Chalcogenic acids, R E(0)0H, are also tricoordinate and considered to have pyramidal structures. However, no studies on their optical activity have been reported since facile racemization of chalcogenic acids may occur via achiral chalcogenate anions, which are formed by the extrusion of a proton and/or by an intra- or intermolecular proton transfer reaction. 

In phosphonium ylides, as we shall see in Section II. A, the phosphorus atom is tetracoordi-nate and the unique (anionic) carbon is tricoordinate. For exactly the same reasons as in the oxides2, all previous discussions3-14 of the P—C bond have been in terms of a resonance hybrid between a dipolar form 1A and a double bond form IB ... 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

Cyano Anions of Dicoordinated, Tricoordinated, Tetracoordinated, Pentacoordinated, and Hexacoordinated Phosphorus... 

Phosphoranide anions 1 - i.e. species based on pentacoordina-ted phosphorus having a lone pair as one of its five substituents -were proposed by Wittig and Maercker as early as 1967, to act as intermediates or transition states in nucleophilic substitutions at tricoordinated phosphorus ( p. Shortly afterwards in 1969, Hellwinkel brought indirect evidence for the formation of phosphoranide 3 in equilibrium with the carbanion 4, since the action of a base on phosphorane 2 gave, after acidic treatment, a mixture of 5 and 6 in proportions that depend on the experimental conditions (i). 

The 1-oxide function appears to play a major role in the cleavage of the tetrahydrothiopyran ring in the 2-(3-pyridinyl)tetrahydrothiopyran 1-oxide derivative 397, promoting nucleophilic attack by the methanethiol anion at the tricoordinate S atom (Equation 104) <1996JOC8701>. 

The tendency of the finite clusters toward close packing of metals is clearly evident in the structure of the anions [Rhu (CO) 2)H5 n]n, (n = 2, 3), reported in Fig. 11 (IS). The central metal atom is dodecacoordi-nated, whereas the 12 rhodium atoms on the surface are pentacoordinated toward the other metal atoms and tricoordinated toward carbon monoxide. The average Rh—Rh distance is 2.81 A without significant differences between internal and surface bonds. The high symmetry of the metal skeleton, Dsh, decreases to apparent C in the molecule it has not been possible so far to determine the positions of the hydrogen atoms. 

In this section we will review mainly the theoretical studies of tricoordinated MR3 cations, anions and radicals of group 14 elements. Relevant experimental data will be mentioned briefly to supplement and complete the theoretical discussion. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

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