Hexafluorides, structures

Hexadiene complexes with gold, 12 348 with iron, 12 263, 264 with platinum, 12 319 with rhodium, 12 297-299 with silver, 12 340 Hexafluorides, structures, 27 106-108 Hexafluoroacetone, 30 223-312, 44 317-318 metal complexes, 30 279-298 of cobalt, 30 286-287 of gold, 30 295... 

All three elements form gaseous hexafluorides by the direct combination of the elements. They all have octahedral structures... 

An extreme example of hybidization is the structure proposed for sulphur hexafluoride, SFe. The six S-F bonds are dhected to the apices of a regular octahedron. An aiTangement which would satisfy this number of covalent bonds is sp d hybridization. The ground state of the sulphur atom is s p° and... 

Whereas some atoms have only one valency, others have several, e.g. sulphur has valencies of two, four and six and can form compounds as diverse as hydrogen sulphide, H2S (valency two), sulphur dioxide, SO2 (valency four) and sulphur hexafluoride, SF6 (valency six). Clearly some compounds comprise more than two different elements. Thus hydrogen, sulphur and oxygen can combine to produce sulphuric acid, H2SO4. From the structure it can be seen that hydrogen maintains its valency of one, oxygen two and sulphur is in a six valency state. 

The pentafluorides of Rh and Ir may be prepared by the deliberate thermal dissociation of the hexafluorides. They also are highly reactive and are respectively dark-red and yellow solids, with the same tetrameric structure as [Rup5]4 and [Osp5]4 (p. 1083). 

The largest class of molecules to violate the octet rule consists of species in which the central atom is surrounded by more than four pairs of valence electrons. Typical molecules of this type are phosphorus pentachloride, PC15, and sulfur hexafluoride, SF6. The Lewis structures of these molecules are... 

Molybdenum hexafluoride. 3,1412 Molybdenum-iron-sulfur complexes, 4,241 Molybdenum oxide amino acid formation prebiotic systems, 6, 872 Molybdenum storage protein microorganisms, 6, 681 Molybdenum telluride, 3, 1431 Molybdenum tetraalkoxides physical properties, 2, 347 Molybdenum tribromide, 3,1330 Molybdenum trichloride, 3,1330 Molybdenum trifluoride, 3, 1330 Molybdenum trihalides, 3, 1330 bond lengths, 3, 1330 magnetic moments, 3,1330 preparation, 3,1330 properties, 3, 1330 structure, 3,1330 Molybdenum triiodide, 3,1330 Molybdenum trioxide complexes, 3, 1379 Molybdenum triselenide, 3, 143)... 

The starting point for the synthesis of xenon compounds is the preparation of xenon difluoride, XeF2, and xenon tetrafluoride, XeF4, by heating a mixture of the elements to 400°C at 6 atm. At higher pressures, fluorination proceeds as far as xenon hexafluoride, XeFfi. All three fluorides are crystalline solids (Fig. 15.27). In the gas phase, all are molecular compounds. Solid xenon hexafluoride, however, is ionic, with a complex structure consisting of XeF< + cations bridged by F anions. 

More recently, Somfai and coworkers have reported on the efficient coupling of a set of carboxylic acids suitable as potential scaffolds for peptide synthesis to a polymer-bound hydrazide linker [24]. Indole-like scaffolds were selected for this small library synthesis as these structures are found in numerous natural products showing interesting activities. The best results were obtained using 2-(7-aza-l H-benzo-triazol-l-yl)-l,l,3,3-tetramethyluronium hexafluoride (HATU) and N,N-diisopropyl-ethylamine (DIEA) in N,N-dimethylformamide as a solvent. Heating the reaction mixtures at 180 °C for 10 min furnished the desired products in high yields (Scheme 7.4). In this application, no Fmoc protection of the indole nitrogen is required. 

Chemical and physical properties of the hexafluorides recently have been reviewed by Weinstock (321), who also accounted for the structures of these volatile compounds in the solid state. The vibrational properties of hexafluoride molecules have been summarized by Weinstock and Gooiman (325). 

The structural information we have of pentafluorides in the solid state is relatively new. The similar melting points (near 100° C and below) and even more so the almost identical boiling points (close to 230°) of the transition metal fluorides MeFs point to similar structures of these compounds. Their high volatility is clearly less than that of the hexafluorides so that one may assume associated aggregates or polymere molecules in the solid state. New structure analyses showed this assumption to be true. There exist at least three structure types within the 12 pentafluorides of d-transition elements hitherto known. Two crystal... 

As in the hexafluorides there are finite octahedral groups in the structures of the described hexafluorometallates too, one isolated from the other and thus hardly influenced by it. This brings about the character of complex anions, as may be seen from the shrinking of distances Me—F and F—F within the MeFg-group, often below the ionic radii sum. 

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