Zigzag chain

The entropy value of gaseous HCl is a sum of contributions from the various transitions summarized in Table 4. Independent calculations based on the spectroscopic data of H Cl and H Cl separately, show the entropy of HCl at 298 K to be 186.686 and 187.372 J/(mol K) (44.619 and 44.783 cal/(mol K), respectively. The low temperature (rhombic) phase is ferroelectric (6). SoHd hydrogen chloride consists of hydrogen-bonded molecular crystals consisting of zigzag chains having an angle of 93.5° (6). Proton nmr studies at low temperatures have also shown the existence of a dimer (HC1)2 (7). 

Zirconium tetrachloride is a tetrahedral monomer in the gas phase, but the soHd is a polymer of ZrCl octahedra arranged in zigzag chains in such a way that each zirconium has two pairs of bridging chlorine anions and two terminal or t-chlorine anions. The octahedra are distorted with unequal Zr—Cl bridge bonds of 0.2498 and 0.2655 nm. The physical properties of zirconium tetrachloride are given in Table 7. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

H-bonded zigzag chains similar to those in solid HF. At higher temperatures substantial disorder sets in. 

The principal compounds in this category are the monochalacogenides, which are formed by all three metals. It is a notable indication of the stability of tetrahedral coordination for the elements of Group 12 that, of the 12 compounds of this type, only CdO, HgO and HgS adopt a structure other than wurtzite or zinc blende (both of which involve tetrahedral coordination of the cation — see below). CdO adopts the 6-coordinate rock-salt structure HgO features zigzag chains of almost linear O-Hg-0 units and HgS exists in both a zinc-blende form and in a rock-salt form. 

HgO exists in a red and a yellow variety. The former is obtained by pyrolysis of Hg(N03)2 or by heating the metal in O2 at about 350°C the latter by cold methods such as precipitation from aqueous solutions of Hg" by addition of alkali (Hg(OH)2 is not known). The difference in colour is entirely due to particle size, both forms having the same structure which consists of zigzag chains of virtually linear O-Hg-O units with Hg-O 205pm and angle Hg-O-Hg 107°. The shortest Hg - O distance between chains is 282 pm. 

The oxide (p. 1209), chalcogenides (p. 1210) and halides (p. 1211) have already been described. Of them, the only ionic compound is HgF2 but other compounds in which there is appreciable charge separation are the hydrated salts of strong oxoacids, e.g. the nitrate, perchlorate, and sulfate. In aqueous solution such salts are extensively hydrolysed (HgO is only very weakly basic) and they require acidification to prevent the formation of polynuclear hydroxo-bridged species or the precipitation of basic salts such as Hg(OH)(N03) which contains infinite zigzag chains ... 

Reaction of [Rh(/z-Cl)(CO)2]2 with sodium pyrazolate leads to 206 (85CJC699). The Rh2N2Cl ring has the envelope conformation. The rhodium atom has distorted square-planar coordination. The molecules in the crystalline lattice form onedimensional stacking units with alternating rhodium atoms in the binuclear units, intermolecularly interacting in a zigzag chain. 

M(0H)2SOi, H2O where M=Zr (8), Hf (12) also have been determined and reveal the presence of almost planar zigzag chains of metal atoms joined by double hydroxide bridges. The single exception to this trend toward formation of double hydroxy-bridged metal dimers or chains is the compound which is best described as CeOSOif,H20 (17). However, even in this structure the cerium ions form chains which are linked by bridging oxide ions. 

The bottom views show how the zigzag chains are linked by bridging sulfate groups. 

IrB lrBo.7 No Trigonal prism Defect zigzag chains... 

Boron atoms in infinite boron zigzag chains form covalent B—B bonds at 165-190 pm with bond angles of 115°. The boron coordination is trigonal prismatic, giving rise to coordinated rows of face-connected trigonal metal prisms (Table 1). Most typical are the binary transition-metal monoborides such as the FeB, CrB and MoB structure types. Transformations between pairs by simple geometric shifts ... 

NbCoB2 type trigonal prism Zigzag chains Ta3[Pg.193]

F6j/m o-Ni4B.,., B-chain atoms in octahedron and isolated boron atoms in trigonal prisms Zigzag chains o-Ni4B,./ ... 

Boron chains linked to form open nets trigonal prism Zigzag chains with side chains trigonal prism... 

Zigzag chains at 200-pm distance, and boron pairs trigonal prisms Boron net fragments. 

With decreasing My B ratio, two or three B zigzag chains are connected at bonding distances, thus forming double or triple chains (Fig. 1). This tendency toward two-dimensional boron net formation (i.e., AlBj type) is recognized from the structural series within the V-B system VnB +, = (n - 1) VB -I- VBj (n = 1,2,3,5 not known for n = 4). ... 

The first two examples of boron nitrides with N atoms branehing off a typical B—B zigzag chain are UBN (UBC type) and NG2BN. 

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