Hafnium tetrahalides

Zirconium and hafnium tetrahalides form six-, seven- and eight-coordinate adducts with polydentate phosphine and arsine ligands. The 1 1 adducts [MXt(dppe)] and [MX dpae)] (M = Zr, X = C1 or Br M = Hf, X = C1 dppe = Ph2PCH2CH2PPh2, dpae = Ph2AsCH2CH2AsPh2) have been prepared in benzene media. IR spectra of the [MCLt(dppe)] complexes exhibit four v(M—Cl) modes in the 353-278 cm-1 region, consistent with a six-coordinate octahedral structure.45... 

In the gas phase, the tetrahalides exist as monomeric regular tetrahedral molecules. This has been established by electron diffraction and vibrational spectroscopy. Bond lengths and vibrational frequencies are listed in Table 16. Force constants for M—X bond stretching are slightly larger for the hafnium tetrahalides than for the zirconium analogues the force constants decrease appreciably as X varies in the order Cl > Br > I.506... 

Hafnium carbide is inert to most reagents at room temperature, but is dissolved by hydrofluoric acid solutions which also contain an oxidizing agent. Above 250°C, hafnium carbide reacts exothermically with halogens to form hafnium tetrahalide, and above 500°C, with oxygen to form hafnium dioxide. At higher temperatures in a flow of hydrogen, hafnium carbide slowly loses some of its carbon. 

Zirconium and hafnium tetrahalides react with hydrogen peroxide in alkali solution to yield M(0H)300H. The maximum content of peroxo units per metal atom has been observed in K4Zr(02)4 6H2O, which is a derivative of the unknown orthoperzirconic acid Zr(OOH)4. 

Zirconium and hafnium tetrahalides form a variety of double halides with metal halides. The most widespread composition is Q2MX6 (Q = monovalent cation) but the QMX5 complexes and fluoride complexes Q3MF7 and Q4MF8 are also documented. Q2MX6 chlorides and bromides consist of [MXe] octahedra. The complex chloride anions [MCls]" and [MCle]" have been detected in molten MCI4 (equation 15) ... 

The Lewis acid properties of zirconium and hafnium tetrahalides have been studied with nitrogen bases as well as oxygen bases, although very... 

Direct reaction of zirconium and hafnium tetrahalides and bromides with excess DMSO under strictly anhydrous conditions yielded products of composition Zr(Hf)Cl4 9DMSO and Zr(Hf)Br4 lODMSO (482). Attempts to recover the tetrahalides by heating only resulted in the production of tarry residues. The S-O-stretching frequency in these compounds was found to be 960 cm" for the nine-solvate and 920 cm for the 10-solvate, which indicates that the DMSO molecules are oxygen bound. 

Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides. 

Zirconium(IV) and hafnium(IV) chlorides and bromides form 1 2 adducts of the type [ZrX4(RCN)2] (R = Me, Et, Pr or Ph X = Cl or Br) and [HfX4(MeCN)2] (X = Cl or Br).11SM24 These complexes may be prepared by (i) direct reaction of the metal tetrahalide with an excess of the nitrile120 123 or (ii) electrochemical oxidation of zirconium or hafnium metal in the presence of a solution of chlorine or bromine in acetonitrile.118 The adducts are moisture-sensitive, white solids, insoluble in nonpolar solvents, but soluble in acetonitrile. In the later solvent, [ZrBr4(MeCN)2 behaves as a nonelectrolyte.122... 

Not all the structures have been determined but in the vapour phase all the tetrahalides of titanium and probably all those of zirconium and hafnium have monomeric, tetrahedral structures. In the solid, Tip4 is a polymer consisting of comer-sharing TiF octahedra, but the other tetrahalides of titanium retain the tetrahedral configuration around the metal even in the solids. The larger zirconium exhibits higher coordination numbers. Thus solid MF4 contain... 

The free energies of formation of corresponding hafnium and zirconium compounds are also nearly equal, so that separation by preferential reaction of one species is difficult, too. Table 7.9 compares the free energies of formation of hafnium and zirconium tetrahalides at 1000 K. 

Compounds of zirconium and hafnium metal tetrahalides with the bidentate ligands ( )4As2C2H4 and ( )4P2C2H4 were prepared 447) by the reaction of the tetrahalide and the ligand in a suspension in benzene. These are presumably species with a coordination number of eight. The infrared absorption frequencies in the 460-250 cm region are tabulated in Table VII. 

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. 

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