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 type it’s reference code (refcode) in the Entry box above the structure list on the left hand side of the WebCSD page:

 

To manipulate structures in the 3D viewer:

 

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 both the shift key and the left-hand mouse button pressed down.

 

3. To translate structures 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 r button on the keyboard.

 

•	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 geometric parameters in the 3D viewer:

 

1. Select the type of measurment you wish to make by right-clicking within the 3D viewer and selecting Measure, followed by either Distances, Angles or Torsions

 

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 right-click within the 3D viewer and select Measure, followed by either Clear distances, Clear angles or Clear torsions.

 

•	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).