Werner Complexes - Preparation and Determination of Structural Formula

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Kyle Thomas

March 25th 2009

Werner Complexes - Preparation and Determination of Structural Formula


        In this experiment three Werner complexes, hexaamminecobalt(III) chloride, pentaamminechlorocobalt(III) chloride and hexaamminenickel(II) chloride were synthesized so their structural formula could be examined through magnetic susceptibility, electronic conductance measurements and the analysis of free chloride in the two cobalt complexes through titrations of silver nitrate. Cobalt (III) coordination compounds usually only form in low spin, octahedral complexes where the all t2g are paired leaving all the eg unpaired; whereas nickel(II) complexes are usually formed in the high spin state1. Through these analytical methods, it was found that: hexaamminecobalt(III) chloride has three free chloride ions to form the complex [Co(NH3)6]Cl3, in the low-spin state. Pentaamminechlorocobalt(III) chloride and hexaamminenickel(II) chloride both have two free chlorine ions to form the complexes [Co(NH3)5Cl]Cl2 and [Ni(NH3)6]Cl2 respectively. As expected, pentaamminechlorocobalt(III) chloride has a low spin electron arrangement, and hexaamminenickel(II) chloride is in a high spin state.


        Alfred Werner was the first to discover the structure for coordination compound in 1893, thus they were given the name “Werner Complexes”. For this research he was given the Nobel Prize in 1913. Werner complexes are complex ions that contain a transition metal in the center, which is surrounded by anionic or neutral ligands.2 The metal in this case acts as a Lewis acid (electron pair accepting) and the anionic ligands act as a Lewis base (electron pair donating). Using pentaamminechlorocobalt(III) chloride, Co(NH3)3Cl3, as an example, there are four possible structures depending on how the ligands are arranged.

        To find the actual arrangement and geometry of the three complexes, conductance measurements, magnetic susceptibility and free chloride ions through titrations can be measured and analyzed.

        Electronic conductance measurements helps determine the amount of chloride ions not attached to the metal. This is possible because when dissolving a salt in water, the molecule separates into cations and anions, each which can conduct electricity. The conductance measured can be compared to a literature reference of how many total ions correspond to the value. With this, logically dividing the charges between the cations and anions present will help determine the overall structure of the molecule.

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        The second technique for determining the amount of ions present in a Werner complex is magnetic susceptibility. With finding the magnetic susceptibility using a magnetic susceptibility of a compound, one can calculate whether the compound is paramagnetic or diamagnetic. If the mass magnetic susceptibility, χg, is negative, then the compound is diamagnetic; however, if χg is positive, then the compound is paramagnetic. The apparatus used for this experiment is a Evans/Johnson Mathey Magnetic Susceptibility Balance.

        The last technique used was titrating a sample with silver nitrate. Since the three samples contain chloride ions, titrating it with silver nitrate would precipitate silver ...

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