Nonclassical coordination

One of the key indicators for the so-called nonclassical coordination in the solid state has been the observation of rotational transitions by INS [57,58]. These studies have led to a value for the rotational barrier of the dihydrogen ligand under the assumption of a model for this rotation. The barrier in turn was then tentatively analysed in terms of distinctive interactions by theoretical calculations or a series of experiments where one or more of these factors were changed. 

Penta- (or higher) coordinate ( nonclassical carbonium ions contain five or (higher) coordinate carbon atoms. They cannot be described by two-electron two-center single bonds alone but also neces-... 

Penta (or higher) coordinate "nonclassical" ions (carbonium ions) CH5 is parent... 

In the past several years, it has become apparent that with proper tuning of ligation, it is possible to prepare organome-tallic compounds of actinide elements with very high coordinative unsaturation and very high chemical reactivity (JJ./Jj2, J 3) In regard to exploring "nonclassical" modes of carbon monoxide acti-... 

In addition, the C3-H coupling constant (from a gated decoupling NMR experiment) of 161.8 Hz in 106 compared with 162 Hz in allenyllithium vs 167.5 Hz in methoxyallene and 168 Hz in allene is also in agreement with an allenic structure. However, neither the C-H coupling constant nor the NMR chemical shifts distinguish between the alternatives that 106 has a nonclassical 1,3-bridged structure 108 (M = Li) or an O-coordinated allenic structure (109). Hence the 6Li, -HOESY NMR technique which can be used to detect close proximities (ca < 3.5 A) between XH and 6Li nuclei was applied. The HOESY spectrum of a-lithiomethoxyallene in THF solution (in which 106 is dimeric) is shown... 

The prototypical pentacoordinated carbocation, the parent nonclassical carbocation, CH, 1, has been extensively studied in the gas phase as well as by theoretical calculations.Among the higher coordinate carbocations, the hexacoordinated methylium dication, 2, and heptacoordinated trication, 3, were also shown to be energy minima by ab initio calculations. High-level ab initio calculations show that they have C2V and Csv structures, respectively.Their isoelectronic boron analogs were also studied. 

Carbocations are a class of reactive intermediates that have been studied for 100 years, since the colored solution formed when triphenylmethanol was dissolved in sulfuric acid was characterized as containing the triphenylmethyl cation. In the early literature, cations such as Ph3C and the tert-butyl cation were referred to as carbonium ions. Following suggestions of Olah, such cations where the positive carbon has a coordination number of 3 are now termed carbenium ions with carbonium ions reserved for cases such as nonclassical ions where the coordination number is 5 or greater. Carbocation is the generic name for an ion with a positive charge on carbon. 

In this section we shall discuss carbocations in which at least one carbon-through a three-center bond (see Section 5.3) is coordinated to four orJive-atoxns. By Olah s terminology these are carbonium ions as opposed to tricoordinated carbenium ions. 61 By older terminology the more highly coordinated carbocations are called nonclassical carbonium ions to differentiate them from the tricoordinated classical carbonium ions. ... 

This chapter begins with a short historic retrospect about the development of the carbocation concepts and covers the techniques used for their generation, observation, and characterization under superacidic long-lived conditions. This is followed by an extensive coverage of the multitude of trivalent (classical) and equilibrating (degenerate) and higher (five or six) coordinate (nonclassical) carbocations. 

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