Subject tetragonal

A similar transformation to a metallic, tetragonal -form can be effected in Si and Ge by subjecting them to pressures of 200 and I20kbar respectively along the c-axis, and again the density increases by 25% from the value at atmospheric pressure. Lead is familiar as a blue-grey, malleable metal with a fairly high density (nearly 5 times that of Si and twice those of Ge and Sn, but only half that of Os and Ir). 

The structure of white tin had been subjected to investigation first by Bijl and Kolkmeijer in 1918, who reported an incorrect structure. In 1923 Mark and Polanyi carried out a second investigation, and found the correct structure for this tetragonal crystal, a structure involving no variable parameters. In 1924 Mark and Hassel reported the results of their reinvestigation of the structure of bismuth, whose structure had been determined in 1921 by R. W. James, in Manchester, who assigned the value 0.232 + 0.004 to the variable parameter. Hassel and Mark verified the James structure, with the parameter equal to 0.236 + 0.003. The presently accepted value for the parameter is 0.2339. 

The tetrakis-dithiophosphinate complex [PPI14][Pr(S2PMe2)4], whose crystal structure has also been determined, has a distorted tetragonal antiprismatic geometry with Pr—S = 2.888-3.0150.400 The complex ions [M S2P(OEt)2 4]", have been the subject of an NMR paramagnetic shift study (see Section 39.2.9.4). 

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. 

The 3(f ions Cr2+ and Mn3+ and the 3d9 ion Cu2+ are subject to Jahn-Teller distortions. For example, CuO does not have the rock salt structure, but one with four close Cu 0 neighbours and two at longer distance similar tetragonally distorted coordination is found in most other simple compounds of Cr2+ and Cu2+. (Note that CrO is unknown). 

Chromium in its dichloride has a ct electronic configuration and in its crystals is subject to the Jahn-Teller effect. Indeed, chromium has a tetragonally distorted octahedral coordination in its crystals [89], CrCl2 is a fascinating molecule in that it displays both the Jahn-Teller and the Renner-Teller effects. 

Interestingly, aU cyanide SMMs reported thus far are characterized by the presence of ions with appreciable spin-orbit coupling. In the prototypical Mnj2 SMMs, the orbital angular momenta of Mn(III) centers are quenched due to the tetragonal Jahn-Teller distortion. This is not the case for the compounds listed in Table XI. In the molecules based solely on 3d transition metals, important orbital contribution results from the presence of low-spin Fe(III) centers. The only exception is the pentanuclear TBP cluster [Mn (tmphen)2l3[Mn (CN)6]2 (177). This compound was subjected to a detailed theoretical treatment that... 

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