Gaseous trihalides

Struck CW, Baglio JA (1991) Estimates for the enthalpies of formation of rate-earth solid and gaseous trihalides. High Temp Sci 31 209-237... 

The thermochemistry of rare-earth trifluorides was summarized in Gmelin Hand-buch (1976) and the thermochemistry of rare-earth tribromides and triiodides was summarized in Gmelin Handbuch (1978). The thermochemistry of trivalent rare-earth trichlorides was critically assessed by Morss (1976). Enthalpies of formation of most of the lanthanide tribromides were determined by Hurtgen et al. (1980). Thermodynamic properties for europium halides were assessed by Rard (1985). Only enthalpies of formation of Sc, Y, Dy and Tm triiodides have been redetermined since the classical work of Hohmann and Bommer (Morss and Spence 1992). A recent set of literature values of enthalpies of formation of rare-earth solid and gaseous trihalides has been published, accompanied by Born-Haber cycle estimated values for all trihalides (Struck and Baglio 1992). 

It is poisonous because it coordinates to the haemoglobin in blood in a similar fashion to CO and NO (Figure 10.14). The trihalides are all rather volatile even PI3, which forms red crystals, has a melting temperature of 61 °C. All the trihalides are hydrolysed by water (although the hydrolysis of PF3 is slow). The gaseous trihalides are pyramidal in shape. 

The thermodynamic functions of the gaseous lanthanide trihalides have been calculated using standard statistical thermodynamic methods which relate the functions Cp, S, and H to the molecular partition function Q (Lewis et al., 1961) ... 

The selected enthalpies of formation of the gaseous lanthanide trihalides, in kJ-mol 1... 

All the trihalides are rapidly hydrolyzed by water and are rather volatile. The gaseous molecules have pyramidal structures, and some form molecular lattices. Mixed trihalides can be detected in mixtures, but it appears unlikely, despite some older claims, that any can be isolated in pure form since equilibria, such as... 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

The gaseous halogens or interhalogens can add directly to a solid halide to form a corresponding complex salt provided that the temperature of the reaction is below the decomposition temperature of the polyhalide. The reaction usually yields a trihalide, but in the case of the addition... 

Plausible though this mechanism is, it came under criticism (7) because, inter alia, it could not account for the intense hypersensitive transitions of the gaseous rare-earth trihalides (8). However, there is recent evidence that the halides are not planar (9,JL0), as had been previously supposed. If this is in fact the case, the importance of the mechanism based on Y terms in V remains undecided. 

Accepting a bonding pair from an electron-rich atom. In gaseous boron trihalides, B has only six electrons around it (Section 10.1). To attain an octet, it accepts a lone pair from an electron-rich atom and forms a covalent bond ... 

Dihalides of uncertain purity are prepared by the disproportionation of the trihalides. Alternate routes have also been reported. Swaroop and Flengas (549) prepared ZrCU of 95-99% purity by heating the trichloride and metallic zirconium at 675°C for 30-35 hours in an evacuated quartz tube lined with platinum foil. There is also a reference to the production of liquid dihalides by the reaction of the gaseous tetrahalides with loosely packed zirconium at 700°C for the chloride and 400°C for the bromide and iodide (270). The difluoride has been prepared (357) by the reaction of atomic hydrogen on thin layers of zirconium tetrafluoride at 350°C. New data on hafnium are lacking, although Corbett (542) has concluded that hafnium diiodide does not exist. 

A normal amide of trivalent arsenic, As(NH2)3, can be synthesized by the reaction between gaseous ammonia and an arsenic trihalide (except the fluoride) at low temperatures... 

Nanometric boron carbide particles can also be prepared by CVD. The reaction of boron trihalides with carbon or gaseous carbon-containing precursors using radiofrequency (RF) plasma [142, 143] or laser-assisted CVD [144] has been appUed. 

Boron forms two simple series of lower halides, BX3 and B2X4(X2B—BX2). The highly reactive trihalides are volatile covalent compounds whose molecules have the expected trigonal planar geometry. The molecules of the other series are also planar in the solid state however, in the gaseous state, the chloride and the bromide... 

Preparation of the trihalides from the metals has generally been confined to the chlorides, bromides and iodides. Two variations of the procedure are the reaction of the metal with either the gaseous hydrohalic acid or with halogen... 

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