Examine the structures of di-, tri-, and tetrachloro mercury

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Consider the following series of molecules: HgCl2, HgCl3-, and HgCl42- as we move across the series we are successively adding a Cl to the central Hg atom. For each structure how would you expect the Cl atoms to arrange themselves around the Hg atom? Sketch each of the structures to show the shape of the molecule you predict.

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Check you answers by inspecting the corresponding crystal structures. The following structures are provided: HgCl2 (refcode OKAJOZ), HgCl3- (refcode KUSMAM), and HgCl42- (refcode KEYZUK). To display a structure click on the identifier in the Structure Navigator on the right hand side of the main Mercury window.

 

To manipulate structures in Mercury

 

1. Structures can be rotated by moving the cursor in the display area while keeping the left-hand mouse button pressed down.

 

2. To zoom in and out move the cursor up and down in the display area while keeping the right-hand mouse button pressed down.

 

3. To translate structures hold down the middle mouse button while moving the cursor in the display area (three-button mouse only). Alternatively, move the cursor in the display area while keeping both the left-hand mouse button and the keyboard Ctrl key pressed down.

 

4. At any stage the display area can be returned to the default view by hitting the Reset button at the bottom of the window.

 

•	Do the shapes of the experimentally determined structures agree with your predictions? For each of the three structures measure the Cl-Hg-Cl bond angles in the structure. What does this tell you about the observed geometries?

 

To measure angles in Mercury

 

1. Set Picking Mode in the tool bar (near the top of the main Mercury window) to the required parameter type, viz. Measure Distance, Measure Angle or Measure Torsion

 

2. Geometrical measurements (intramolecular or intermolecular) can now be made by clicking on e.g., two atoms for a distance, three atoms for an angle or four atoms for a torsion angle.

 

3. To remove all geometrical measurements from the display click on the Clear Measurements button in the tool bar near the top of the main Mercury window.

 

•	HgCl2 is linear with bonds angles of 180 deg., HgCl3- exists in a trigonal planar arrangement with bond angles of approximately 120 deg., and HgCl42- is tetrahedral with all bond angles approaching 109.5 deg. What is the main factor that determines the geometry that is adopted?

 

Image: showing HgCl2 (linear), HgCl3- (trigonal planar), and HgCl42- (tetrahedral).