Structures and Bonding in the Solid State

While the 20 Group 1 metal halides all adopt either a face-centered or a body-centered crystal structure, crystals of the 32 dihalides of the Group 2 and 12 metals form a bewildering array of at least 15 different structure types. Both the crystal and the gas-phase structures have recently been described and correlated in comprehensive reviews by Hoffman and coworkers [16, 17]. In this section we shall be particularly concerned with the crystal stractures of the eight metal difluorides and the four mercury dihalides. 

The difluorides of strontium, barium, cadmium and mercury all adopt the fluorite crystal stmcture. The difluorides of magnesium and zinc adopt a mtile -type stmcture where each metal cation is surrounded by six F anions at the comers of an octahedron, and each anion is surrounded by three metal caritnis in a planar trigonal arrangement [19]. 

BeF2 is tetramorphic Crystal stmctures similar to those of a- and p- quartz and a- and p- cristobalite have all been characterized. Only very recently has one 

3 The crystal structinc of ilurailc (CaF2). Reproduced frran Proceedings of the Royal Society A (1914) 

Crystals of Mgl2, Cal2, and Cdl2 are isomorphic. Lattice energy calculations yield atomization energies that are 17% below the experimental for Ca, 43% lower for Mg, and 80% lower for Cd [21]. It is clear that the tendency for fluorine to form 

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