Four-coordinate molecules shape

The two shapes associated with four-coordination are tetrahedral and square-planar. In the former only optical activity and in the latter only geometrical isomerism is generally encountered in the appropriate molecules. 

For real molecules or ions, the stereochemical descriptor should be based on the nearest idealized geometry. However, some idealized geometries are closely related [e.g. square planar (SP-A), four-coordinate square pyramidal (SPY-A), see-saw (SS-4), and tetrahedral (T-4) T-shaped (TS-3), trigonal planar (TP-3), and trigonal pyramidal (TPY-3)] and care may therefore be required in making the choice. 

Other transition metal ions also work as glue . When Zn(N03)2 and Ni(N03)2 are reacted with PR44, the ID polyrotaxanes 15 and 16, respectively, are produced (Figure 16 and 17) [39]. In both structures, two pseudorotaxane units are linked by a metal ion which is coordinated by four water molecules and two pyridyl group of the two pseudorotaxanes in an octahedral coordination geometry. The two pyridyl groups occupy cis positions in 15 whereas they occupy trans positions in 16. Therefore, the former ID polyrotaxane has a zigzag shaped polymer chain whereas the latter has a linear structure. 

Fig. 18. The hydrogen-bonded sheet structure of l,l,5,5-tetrakis(hydroxydimethylsi-loxy)-3,3,7,7-tetraphenylcyclotetrasiloxane. Note the crown-shaped eight-membered rings of oxygen atoms at the junctions where four of the molecules meet. All hydrogen and carbon atoms have been omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

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