Pyramidal dication

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41... 

CH)62+-Type Dications. The first known representative of the (CH)62+-type hexacoordinate pyramidal dications is Hogeveen s hexamethyl cation 437.41 789 1108 This dication 437 can be prepared from a variety of precursors in superacidic media (HSO3F, HS03F-SbF5) at low temperature (Scheme 3.28). 

There are several other carbodications that may be represented as 1,3-dicationic systems (or nonclassical, highly delocalized systems), including Schleyer s l,3-dehydro-5,7-adamantanediyl dication (28),11 Hogeveen s pyramidal dication (29),12 diallyl dications (i.e., 30),13 dicationic ethers (i.e., 31),14 diprotonated pyrones (32),15 as well as various aromatic dications. 

Perturbation results by Hogeveen and van Kruchten (1981) conhrm their proposed symmetrical structure for the pyramidal dication [36]. 

Similar arguments applied to the pentagonal pyramidal dication (C,Mc,)" (Fig. 1.17) in which one 2c-2e C-C bond and two 3c-2e C—C—C bonds link the apical carbon atom to the five basal carbon atoms, lead to the following C-C bond orders in the Ce pyramid apical-basal links, 7/15, which is 0.47 and basal-basal links, 17/15, which is 1.13. 

Hogeveen, Heldeweg, and coworkers also prepared the ethyl and isopropyl derivatives of these pyramidal dications (235a-235d) and their bridged structures have been conhrmed by II and NMR spectra and quenching studies ... 

Hogeveen et al. have generated stable pyramidal dications of the general formula (CCH3)jCR and (CCHj)4(CR)2 (R is ethyl, isopropyl), e.g. ... 

The described dications represent nonclassical ions of a special kind — pyramidal mono- and dications the latter can be regarded as a link to the elementoorganic chemistry e.g., boranes, by the peculiarities of their stereochemistry and of their bond nature. Thus, the pyramidal structure of pentaborane 509 is similar to that of monocation (CH)j 510 with a pentacoordinated carbon while the pyramidal dication (CH) 511 is similar to carborane 512. By the peculiarities of their stereochemistry pyramidal cations are similar to metalloorganic sandwich compounds, e. g., ferrocene 513 the dication 511 is regarded as a semi-sandwich . 

Pentacyclononyl and trishomocyclopropenyl cations and pyramidal dications The nmr spectrum of the 9-pentacyclo[4.3.0. 0 ]nonyl cation [118] (Coates and Fretz, 1975) shows only three peaks for the averaged C-6, 7, 9, C-1, 5, 8 and C-2, 3, 4 carbons, independent of temperature. Deuterium at C-9 is scrambled only among the three carbons C-6, 7, 9. This is consistent only with a static trishomocyclopropyl cation type structure. In the spectrum of the 9-deuteriated cation, the peak corresponding to C-6, 7, 9 is shifted less than 0.1 ppm, confirming the static symmetrical structure of the cation (Saunders and Kates, 1980). 

The H and C nmr spectra of the pyramidal dication of Hogeveen shows a five-fold symmetry with no significant line-broadening at temperatures as low as — I50°C. The alternative structures could be either a hypercoordinated five-fold symmetrical pyramidal dication [120] or a set of rapidly equilibrating carbodications [121] with the same time-averaged symmetry, as indicated in (88). 

Figure 127 shows the square pyramidal geometry of the [Cun([15]aneS5)]2+ dication. 

Treatment of 77 with one equivalent of [AuCl(tht)] (tht = tetrahydrothiophene) leads to the type III AuCl bridged pincer complex 79 in high yield (counterion is Cl ) [149]. The unique complex 79 is the only heterodimetallic compound with two metal atoms attached at a carbodiphosphorane carbon(O) atom. The environment at the central carbon atom is planar in 77 and pyramidal in 78 and 79. P-C bond lengths and P-C-P angles in the dication 78 are closely related to those in the neutral Pt complex 36 (Fig. 25). The two metals of 79 attached to the carbon atom are coimected via a very short d -d ° pseudo closed shell interaction with a Pd-Au bond length of 2.8900(3) A a similar bonding situation (aurophilic attraction) is found in the gold complex 29 shown in Fig. 23. 

Vanadyl (acac) 2 is a square pyramidal complex of two molecules of the enolate of acac acetyl acetone, pentan-2,4-dione) and the vanadyl (V=0) dication. It can easily accept another ligand to form an octahedral complex so there is... 

No Au-Au interactions occur in the monocation [RP(AuPPh3)3]+ (R = o-tolyl group) in which the central P atom has an ideal tetrahedral geometry. However, by treatment of this compound with [Ph3Au]+(BF4) (Scheme (3)), the dication [RP(AuPPh3)4] +, which has square-pyramidal geometry with the phosphorus atom at the apex, is formed. 

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