Silver difluoride, structure

A gas, the major product of fluorination of cyanogen with silver difluoride was assigned this, mechanistically improbable, structure. Usually decomposing above 150°C, it twice detonated spontaneously at room temperature (initial pressure not stated). 

Transition metal difluorides are known mainly for first transition series elements, with palladium and silver difluorides from the second series, and no examples from the third. All these compounds have either the rutile structure, or, for chromium, copper, and silver, a distorted variant, which can be correlated with a Jahn-Teller distortion of the octahedral coordination of the ions. This rutile structure type is associated with smaller cations and, for comparison, although zinc difluoride has the same rutile structure, cadmium and mercury difluorides have the cubic fluorite structure with eight coordination of the cations (12). 

Fig. 1. The puckered layers of [MF4J units in the structures of (a) silver and (b) copper difluorides. Single circles show atoms in one puckered layer and dashed circles those in the adjacent layer below. Those atoms from the adjacent layer above, which complete the distorted octahedral coordination, are shown hatched. (In silver difluoride they are directly above the dashed circles.)...

The change in symmetry from copper to silver is related to rather subtle structural differences. If only the four short M—F bonds are considered, then both structures consist of puckered layers formed by joining the square, [MF4] units through corners. These puckered layers lie directly over one another, related by a translation in copper difluoride, but are displaced alternately, related by a glide plane, in silver difluoride (27), (see Fig. 1). In both structures, the fluorine atoms are arranged in considerably distorted hexagonal close-packed arrays, the distortion being more marked for silver. 

When subjected to high temperatures and pressures, polymorphs have been prepared for some of the difluorides. These all have distorted variants of the fluorite structure, with cubic, or pseudocubic, tetragonal cells. Manganese difluoride has been most studied (28-30) and similar polymorphism reported for cobalt and zinc difluorides (30). Recently, palladium and silver difluorides have been shown to behave in a similar way forming cubic metastable phases (31). In all cases there is a decrease in volume for the structure change. 

Salts of the Agp4 ion were first described by Hoppe [3] and Hoppe and Homann [4]. These were alkali-cation salts and were typically prepared by higher temperature (300-400°C) fluorination of alkali and silver nitrates, mixed halides, or oxides. The products of these syntheses were sufficiently free of silver difluoride and other contaminants to provide X-ray powder diffraction data [4] to establish the unit cell character for the salts of the heavier alkali metals, and to show the diamagnetism (low spin tf ) of AgF4 in these salts. The LiAgp4, however, was not structurally defined. 

Selective reaction of the bromochloro compound 83 with silver fluoride in acetonitrile gave a good yield of the product of nucleophilic displacement of bromide with inversion of configuration, namely, 151, and treatment with an excess of the reagent afforded25 the g m-difluoride (152). With alcohols in the presence of silver triflate, compound 83 affords access to orthoesters (for example, the dimethyl compound 153), and with diols sp/ro-analogs (for example, 154, which has the orthoester structural feature of the orthosomy-cins), are obtained.88... 

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