Coordination number 3

   3-Coordinate complexes are not common. Usually, trigonal planar structures are observed, and examples involving d10 metal centres include:

• Cu(I) in [Cu(CN)₃]^2-  (19.2), [CuCN)₂]^- (see above), [CuSPMe₃)₃]⁺ ;
• Ag(I) in [AgTe₇]^3- (Figure 19.4b),[AgPPh₃)₃]⁺;
• Au(I) in [Au{PPh(C₆H₁₁)₂}₃] ⁺;
• Hg(II) in [HgI₃]^-, [HgSPh₃)₃]^- ;
• Pt(0) in [PtPPh₃)₃], [PtP ^tBu₂H)₃].

   Sterically demanding amido ligands have been used to stabilize complexes containing 3-coordinate metal ions, e.g. [Fe{N(SiMe₃)₂}₃] (Figure 19.4c). In the solid state, [Y{N(SiMe₃)₂}₃] and [Sc{N(SiMe₃)₂}₃] possess trigonal pyramidal metal centres (∠N–Y–N = 115⁰ and N–Sc–N = 115.5⁰), but it is likely that crystal packing effects cause the deviation from planarity. The fact that in the gas phase [Sc{N(SiMe₃)₂}₃] contains a trigonal planar Sc(III) centre tends to support this proposal. p-Block chemistry has a number of examples of T-shaped molecules (e.g. ClF₃) in which stereochemically active lone pairs play a crucial role. d-Block metal complexes do not mimic this behaviour, although ligand constraints (e.g. the bite angle of a chelate) may distort a 3-coordinate structure away from the expected trigonal planar structure.