Why Do Complex Ions with the Same Number of Ligands Form Different Shapes?

[joeyjo100]

I'm currently trying to cram knowledge of the transition metals into my head for a chemistry final exam, although i have come across a stumbling block when reading about the 3 dimensional shapes that complex ions form.

My textbooks say that complex ion with 4 coordination bonds will form a tetrahedral shape, such as [Cu(Cl)4]2+.

But then it says that some complex ions with coordination numbers of four can form square planar shapes, such as [NiCl2(NH3)2].

What causes these two complex ions with the same number of ligands to have different shapes? Has it got something to do with the shape or charge of the ligands?

 

[SpectraCat]

I'm currently trying to cram knowledge of the transition metals into my head for a chemistry final exam, although i have come across a stumbling block when reading about the 3 dimensional shapes that complex ions form.

My textbooks say that complex ion with 4 coordination bonds will form a tetrahedral shape, such as [Cu(Cl)4]2+.

But then it says that some complex ions with coordination numbers of four can form square planar shapes, such as [NiCl2(NH3)2].

What causes these two complex ions with the same number of ligands to have different shapes? Has it got something to do with the shape or charge of the ligands?

Most transition metal complexes with a coordination number of 4 favor tetrahedral geometries. The exception is for ions with a d⁸ configuration of their valence electrons. This is because the complex can lower its energy in this geometry (obviously). IIRC, you can explain this in terms of valence bond theory, by hybridizing the empty s, p_x, p_y, and d_x²-y² orbitals to make 4 empty sp²d hybrid orbitals in a square planar configuration to accept the ligand electron pairs. The occupied d-orbitals are then the d_z², d_xy,d_xz, and d_yz orbitals. If you are good at visualizing orbitals, you will see that the lobes of those orbitals holding the 8 valence electrons of the metal ion are more or less optimally oriented to point between the lobes of the empty sp²d hybrids that are acting as lone pair acceptors, which minimizes the overall electron-electron repulsion energy in the complex.