Fluoro complexes Subject

The structural chemistry of six-coordinated fluoro complexes of the d-transition elements is the subject of an entire review paper by Babel (7). In contrast, six coordination is rare in actinide fluoro-complexes. The relatively large sizes of the actinide ions (Fig. 3) suggest that low coordination numbers should be found only in the fluoride complexes of the higher oxidation states. The radius ratio, r+/r, predicted for the lower hmit of stabihty of octahedral coordination is ]/2 — 1, which corresponds to a positive ion radius of 0.55 A in coordination with fluoride (1.33 A). 

The anomalous iodoacetamide-fluoride reaction violates this rule, in that a less stable -halonium complex (18) must be involved, which then opens to (19) in the Markownikoff sense. This has been rationalized in the following way estimates of nonbonded destabilizing interactions in the possible products suggest that the actual product (16) is more stable than the alternative 6)5-fluoro-5a-iodo compound, so the reaction may be subject to a measure of thermodynamic control in the final attack of fluoride ion on the iodonium intermediate. To permit this, the a- and -iodonium complexes would have to exist in equilibrium with the original olefin, product formation being determined by a relatively high rate of attack upon the minor proportion of the less stable )9-iodonium ion. 

These complex anions are very weakly basic, i.e. of very low nucleophilicity, and have been used deliberately as counter-ions in the isolation of solid compounds where the cation is highly electrophilic and subject to disproportionation or other reaction with species of reasonable base strength. Relative basicities of anions can be of great importance in isolating such compounds. An example which will be discussed in detail in Sec. 11.3.4.6 involved attempts to synthesise solid compounds of IJ and BrT. These were unsuccessful when the counter-ion was fluorosulfate, because of disproportionation of the halogen cations, whereas the cations were stable in the presence of less basic fluoro-antimonates. 

Bicyclic aziridines 17 with a l-azabicyclo[n.l, 0]alkane framework can also be subjected to ring opening with either hydrogen fluoride or the hydrogen fluoride-pyridine complex to give 3-fluoro-l-azacycloalkanes 18. ° ° ... 

Photonucleophilic substitution of fluoro- and chloro-anisoles has been the subject of three reports within the year. Cornelisse and co-workers have studied the photocyanation and photohydrolysis of 4-fluoro- and chloro-anisoles by laser spectroscopy and report that the initial step of the reaction involves formation of a triplet state transient complex composed of a ground state and an excited state aromatic molecule. Only in the presence of water does the complex yield radical ions and it is this process which determines the product quantum yield. The radical cation then reacts with the nucleophile to give a neutral radical which yields the substituted arene in a single step. Liu and Weiss report on anomalous effects during photonucleophilic aromatic substitution of 2- and 4-fluoroanisoles and also on the photo-... 

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