CHAPTER 3: Question 12

3D explanations

Consider first the stereocentre at the molybdenum atom on the left as drawn. The six ligands around this stereocentre are first ranked into an order of priority (1-6) using the CIP rules. Rule 1 is sufficient to allow the chlorine atom to be ranked 1, sulfur 2, phosphorus 3, and the nitrogen 4. The other two ligands both start with a carbon atom but are distinguished by CIP rule 2, allowing the C=O group to be ranked 5 and the butyne 6. Next, a principal axis is defined, as the axis containing the ligand ranked 1 and the molybdenum atom. This is shown in red in the diagram below. Next, the molecule is viewed along the principal axis with the atom ranked 1 towards the viewer. The rotatable 3D structure may aid in visualizing this rotation. The three highest ranked ligands in the plane perpendicular to the principal axis are seen to decrease in priority in an anti-clockwise direction, so this stereocentre has the (A)-configuration.

The process is then repeated for the other molybdenum atom. Since the same six ligands are present, the ranking order and principal axis will be as above. In this case however, when the molecule is viewed along the principal axis with the ligand of highest priority at the front, then the three highest ranked ligands in the plane perpendicular to the principal axis are seen to decrease in priority in a clockwise direction. The rotatable 3D structure may aid in visualizing this rotation (note the phenyl groups have been replaced by hydrogen for clafity, and the butyne ligands have been replaced by fluorines). Hence, this stereocentre has the (C)-configuration.

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