Potassium chloride coordination number

Solution X-ray diffraction measurements for saturated aqueous solutions of the KCl-MgCl2-6H20 and CsCl-MgCl2-6H20 double salts at 25°C reveal that magnesium(II) ions in the solutions are fully hydrated as [Mg(H20)6]2+ with a Mg-0 bond length of 208-209 pm. This is essentially the same bond length as in the double salt crystals, and the K+ and Cs+ ions have both water molecules and chloride ions in their first coordination sphere. The coordination numbers for water molecules and chloride ions around a K+ ion are 4.7 and 2.4, respectively, and those around a Cs+ ion are 4.7 and 2.0, respectively. The K+-OH2 and K+-C1 interatomic distances are found to be 227 and 320 pm, respectively, and the Cs+-OH2 and Cs + -Cl distances are 315 and 339 pm, respectively (58). The interatomic distances determined are essentially the same as those that have been reported in the literature for aqueous solutions of potassium and cesium salts. 

The significance of these critical ratios was first pointed out by Magnus for ionic complexes and later applied by Goldschmidt to crystals. In the case of GsCl, CsBr and Csl, the value of the ratio rA is greater than 0 73. It is therefore understandable that these salts form crystals with a coordination number of eight. There are, however, many exceptions to the conditions formulated above. For example, the halides of rubidium, potassium chloride and potassium fluoride, which crystallize with a NaCl lattice, have a value of rA greater than 0 732. 

The halogen hydracids are all weaker than perchloric acid. Actually they are not ideal strong electrolytes, although they a pproach this behavior when water is used as the solvent. Certainly, these compounds differ distinctly from typical strong electrolytes such as potassium chloride and other neutral salts. The difference probably originates in the structure of the solid form. Neutral salts in the solid crystalline state possess a coordination lattice. Simple molecules do not exist in this type of lattice since the constituents of the salt are present. solely in the ionic form. Each ion is surrounded in a uniform manner by a definite number of other ions of opposite charge. Indeed it is no longer correct to speak of undissociated molecules in the solid state. 

In the complex [Co(NH3)6]Cl3, the cation is [Co(NH3)6]3+, and it is named first. The coordinated ammonia molecules are named as ammine, with the number of them being indicated by the prefix hexa. Therefore, the name for the compound is hexaamminecobalt(III) chloride. There are no spaces in the name of the cation. [Co(NH3)5C1]C12 has five NH3 molecules and one CN coordinated to Co3+. Following the rules just listed leads to the name pentaamminechlorocobalt(III) chloride. Potassium hexacyanoferrate(III) is K3[Fe(CN)6j. Reinecke s salt, NH4[Cr(NCS)4(NH3)2], would be named as ammonium diamminetetrathiocyanatochro mate (III). In Magnus s green salt, [Pt(NH3)4][PtCl4], both cation and anion are complexes. The name of the complex is tetraammineplatinum(II) tetrachloroplatinate(II). The compound [Co(en)3](N03)3 is named as tris(ethylenediamine)cobalt(III) nitrate. 

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