Three Coordination (ML3)
The VSEPR-predicted shape, trigonal planar, is well represented amongst this relatively rare coordination number. ML3 is (like ML2) favoured by transition metal ions with lots of d electrons (d8.d9.d10). Two other shapes are known, however; one is called T-shape (for obvious reasons), and the other called trigonal pyramidal. These latter two can be seen to arise from distortions of the ‘parent’ trigonal planar shape, as depicted in Figure 4.6. It is usual for different shapes for a particular coordination number to be able to interconvert without any bond breaking, simply through rearrangements such as those exemplified in
Figure 4.6.
Trigonal planar, the parent shape of three-coordination, and the way it can transform to other known geometries.
Figure 4.6. There is an obvious outcome of this. These various shapes reported in this chapter should be considered as limiting shapes– they are the extremes or termini of change and as a consequence molecules may adopt a shape that is intermediate or part-way along the process of changing from one basic shape to another. This is something that three-dimensional structural studies have clearly identified. However, it is both convenient and essential if we are to have any pattern to our family of shapes to identify a coordination complex in terms of its nearest basic shape– and in most cases the deviation from a basic shape is small suggesting that they do have inherent stability.
A simple trigonal planar complex ion is [HgI3] −. Two more elaborate examples of trigonal planar geometry both involving bulky ligands, are [Cu (SR2)3] + (where R = CH3) and [Pt (PR3)3] + (where R = C6H5) for which all metal–donor bonds are equivalent (and. Coincidentally both Cu S and Pt P distances are 226 pm) and L M L angles all very close to the expected 120◦ (Figure 4.7). Whereas bond distances are usually identical also for the three-coordinate trigonal pyramidal geometry, the L M L angles are all<120◦ and diminish the further the metal is raised above the plane of the donors.
A rare example of T-shaped geometry is [Rh (PR3)3] + (where R = C6H5), which (unlike its trigonal planar Pt analogue) has one P Rh P angle close to 180◦ and the others near 90◦ with Rh P distances also not identical. Overall trigonal planar is the dominant shape of three-coordination, and this is the shape predicted by Kepert’s amended VSEPR model.
