Tricoordinated cations

Cationic tricoordinate boron intermediates, including N-heterocychc carbenes as stabilizing Hgands for borenium salts 12CRV4246. 

The monomer coordination equilibrium constant was estimated to be roughly 1.3 N from the polymer NMR as a function of propylene and with the assumption that the m placements were strictly attributable to coordinatively unsaturated species. This is in the normal range reported from kinetic measurements with heterogeneous catalysts but is surprisingly small for a cationic tricoordinate metallocene. 

In this compound, synthesized in the low temperature reaction between diborane and excess ammonia, the cationic boron is coordinatively saturated in a tetrahedral environment. More recendy, cations having boron in tricoordinate or dicoordinate environments have been observed. These cationic species, called borenium and borinum ions, respectively, have been reviewed (19,20). 

TIricooRlinate caibocations are fiequendy called carbonium ions. The terms methyl cation, butyl cation, etc., are used to describe the c >rTesixiiulir.ji tricoordinate cations. Chemical Abstracts uses as specific names methylium, ethyUum, propylium. We will use carbocation as a generic term for trivalent carbon cations. 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

The converse of polymerization is heterolytic bond scission leading either to R3Sn+ or R3Sn species. Tricoordinate organotin(IV) cations can readily be synthesized at room temperature by hydride or halide abstraction reactions in benzene or other solvents.For example, with R = Me, Bu or Ph ... 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Since the central carbon of tricoordinated carbocations has only three bonds and no other valence electrons, the bonds are sp and should be planar. Raman, IR, and NMR spectroscopic data on simple alkyl cations show this to be so. In methylcycohexyl cations there are two chair conformations where the carbon bearing the positive charge is planar (9 and 10), and there is evidence that difference is hyperconjugation make 10 more stable. Other evidence is that carbocations are difficult to form at bridgehead atoms in [2.2.1] systems, where they cannot be planar (see p. 397). ° Bridgehead carbocations are known, however, as in [2.1.1]... 

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. 

In the field of stannylium ions, n-BusSn B (C6F5)3H , synthesized by the oxidation of stannyl hydride with B(CgF5)3 (Section 2.1.2.2, Scheme 2.2, D), was characterized by Lambert et al. as a tricoordinate stannyl cation becanse of its low-field Sn NMR resonance of 360ppm. The same n-BusSn cation, having a TPFPB connterion, exhibited a less deshielded valne of +263 ppm (at room temperature), which, however, was corrected in snbseqnent stndies to +434ppm (at -Bn3Sn+ TFPB, ... 

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... 

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)]. 

In the cationic systems, the positive charges are delocalized over almost all atoms, even if the individual structures may be described by the Zintl concept that assigns localized positive charges to tricoordinate E atoms. It appears that the Zintl concept is better suited, yet not sufficient, to describe the structures of the heavier chalcogen elements. 

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

The reported Si NMR data for silyl cation 3 demonstrate its homoaromatic character and characterize 3 as a free silyl cation in solution. The four-membered ring in 3 is identified by three Si NMR signals 8 Si = 77.3 (Si Si ), 315.7 (Si ) and 34.3 (Si ). Remarkably, the most deshielded silicon atom in 3 is the central tricoordinated silicon. This is in agreement with some homoaromatic nature of the cation 3 with charge localization at Si (see Fig. 2a) and it discards the possibility of a classical... 

Historically, the first solid state structure of a truly tricoordinated cation of the heavier group 14 elements (Si-Pb) was that of the cyclotrigermenium cation [((t-Bu3Si)Ge)3], 2 published in 1997. Cation 2 was prepared by one-electron oxidation of the cyclotrigermene 20 (E = Si) with trityl cation (see Scheme 8). " ... 

J. Lambert, Preparation of the First Tricoordinate Silyl Cation J. Phys. Org. Chem. 2001, 14, 370. 

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