Pyramidal cations

Numerous crystal structures have been published of compounds containing the pyramidal cations Se "Cl3+, Se "Br3+, Te Cl3, etc. and the anions Se CLj ", Sej Se3Cli3, ... 

In the n = 4 series, there are fixed points in the known structures244,245 of the square pyramidal cations [MnI(Ph3PO)4]+ and [MnC104(Ph2MeAs0)4]+, and there has been a tendency to relate most of the other compounds to these. However, other compounds of the group may turn out to be tetrahedral [Mn04]X2 species, and some, especially the hydrates, may be octahedral. 

A large amount of work has been reported on potentially bis- or trishomocycloprope-nium ions. Many reviews exist including those bv Winstein " , Story and Clark Williams and Kurtz, Koptyug Hogeveen and Kwant, Lenoir and Siehl and a particularly full account by Barkhash The topic also overlaps that of the pyramidal cations and related boranes In this present section we will again not give an exhaustive account but concentrate on a few key systems where there has been some recent definitive work reported. 

Two transition metal stabilized germyl cations, 10 and 11 , have been reported. In the first one the strongly pyramidalized cationic germanium center is bonded to two iron atoms and one t-Bu group, and is stabilized by the coordination to DMAP. The second one has a planar cationic germanium bonded to two tungsten atoms and one methyl group. 

Bishomo-square pyramidal cation, 2-bicyclo[3.2.1]octa-3,6-dienyl cation and related cations 302 Cyclobutenyl cations 308 Cyclopentenyl cations 310 Cyclohexenyl cations 311 Benzenium ions and related cations 313 9,l0-Dihydro-10-phenanthrenium ions and related cations 323 Acenaphthenium ions 332 Vinyl cations 334... 

BISHOMO-SQUARE PYRAMIDAL CATION, 2-BICYCLO[3.2. l]OCTA-3,6-DIENYL CATION AND RELATED CATIONS... 

The square pyramidal structure [242] proposed by Stohrer and Hoffmann (1972) for (CH), is formally a product of addition of a CH+ fragment to cyclobutadiene (155). A bishomo homologue of [242], the octamethyl pyramidal cation [243] was prepared by Hart and Kuzuya (1974) under both solvolytic and stable-ion conditions. When the tetracyclic e/jrfo-alcohol [244] was treated with HCl in ether or CF3COOH-CCI4, nucleophilic substitution occurred exclusively with retention of configuration at the secondary carbon... 

However, the static pyramidal cation [243] could account for the observed spectral properties as shown by a comparison with those reported by Masa-mune et al. (1973) for the related ion [250]. In contrast to ion [243], [250] was captured by nucleophiles exclusively at the basal positions where most of the positive charge is concentrated. Severe steric hindrance by the methyl groups... 

For a study of the stabilizing effect of different substituents on pyramidal cations Hart and Kuzuya (1975a) prepared a diphenyl-substituted analogue of [243] from both the alkene [249] and the alcohol [252 X = OH] precursors in superacid. The diphenylpyramidal cation [253] was less stable than [243]... 

Reaction of tricyclic compound 54a, previously obtained from 37 and PCI3 <1991AGE207> with a stoichiometric amount of AICI3 in CH2CI2 at 20 °C, produced a colorless solution from which a small amount of white crystalline 55 separated (Scheme 20). The square-based pyramidal cation, which is isoelectronic with [C5R5] features four diphosphirane units. The cation may also be considered as a nido-cluster according to Wade s rules <2003AGE2778>. 

As in its phosphorus analogue 55 the arsenic atom is occupying the apical site of the square-based pyramidal cation. From a formal point of view the nido-cluster is constructed from four phosphaarsirane moieties <2006CC1375>. 

Valence group (2,6). Simple examples of this valence group include ions such as Se(CH3)3 and Te(CH3)3 (see later) and SeF. In crystalline Se(CH3)3l there are pyramidal cations but each is associated rather closely with only one I . Although Se- T is only a weak bond (3 78 A) this is less than the estimated van der Waals separation (4T5 A) and in fact shorter than S-T (3-89 A) in the sulphur analogue. Moreover this weak Se -T bond is almost collinear with one of the Se-C bonds, and the ion pair has been described as a charge-transfer complex. Similarly, the Se-I distance in the complex of I2 with 1,4-diselenane is slightly less than the S"T distance in the S analogue. At the same time there is an increase in I—I from the value 2-66 A in the free I2 molecule to 2-79 A in the S compound and to 2-87 A in... 

For example, for the square pyramidal cation C5H5+ and derivatives thereof (Figs. 1.7 and 1.15), there are four ways of assigning the 3c-2e C—C—C bond and two 2c-2e C-C bonds that, in localized bonding terms, link the apical... 

Figure 1.17. Bond orders in pyramidal cations C5H5 and indicated by...

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