Evaluate the structure that contains the longest observed Sb-Cl bond length.

•	We have already determined that the shortest observed Sb-Cl bond lengths (~1.85Å) are in error. The hit structure containing these bond lengths is actually a hexafluoro-antimony ion.

•	Return to the Sb-Cl bond length distribution by selecting Searches from the top level menu, followed by Draw1: Sb1 Cl2.

•	The longest Sb-Cl bond length is reported to be 3.1Å. Could this unusually long bond length also be due to error?

•	Examine the CSD structure that contains the long Sb-Cl bond (CSD refcode: CUMMON). Use the Information, Diagram and 3D_Visualiser buttons on the left of the View structures tab to examine the structure in detail.

•	Using the 3D_Visualiser you will see that the SbCl63- anion has a strongly distorted octahedral geometry:

 

•	The structure contains three short (2.41 - 2.52Å) and three long (2.83 -3.11Å) Sb-Cl bonds.

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The angles involving atoms which are mutually cis range from 81.73 (Cl6-Sb1-Cl5) to 97.97 (Cl4-Sb1-Cl5), while the trans angles are 174.47, 173.3 and 170.50, the last involving Cl3 and Cl6. The deviations of these bond angles from the ideal values of 90 and 180 are consistent with the presence of the long bonds, the largest deviations from 90 degrees involve the more weakly bonded chlorine atoms, Cl4-Sb1-Cl5 = 97.97, Cl5-Sb1-Cl6 = 81.7.

•	To investigate the reason for this strongly distorted octahedral geometry open Mercury and read in the file bond_lengths.gcd. Click on the refcode CUMNON in the Structure Navigator on the right hand side of the main Mercury window to view the structure.

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Click on the tick box next to the words H-Bond Default definition in the list box underneath the 3D display. This finds all the hydrogen bonds formed by the two molecules that are currently displayed. Some of the hydrogen bonds are shown as broken red lines. Place the cursor anywhere on one of the broken red lines and left-click. This causes the complete molecule at the other end of the hydrogen bond to be shown. Repeat this process to build up a picture of the hydrogen bonding in this structure:

 

•	It is reasonable to conclude that the presence of multiple hydrogen bonds, mainly involving the three long-bonded chlorine atoms is responsible for the octahedral distortion.

•	Diethylenetriammonium hexachloroantimonate(III) (CSD refcode: CUMNON) is an unusual and interesting structure. However, the reported Sb-Cl bond lengths are not in error.