Coordination Number 5

• Square pyramid Geometry: Oxovanadium salts (Vanadyl, VO²⁺) often show square pyramidal geometry, for example, VO(acac)₂. Note that the Vanadium(IV) can be considered coordinatively unsaturated and addition of pyridine leads to the formation of an octahedral complex.

• Trigonal Bipyramid Geometry: The structure of [Cr(en)₃][Ni(CN)₅] 1.5 H₂O was reported in 1968 to be a remarkable example of a complex exhibiting both types of geometry in the same crystal. The reaction of cyanide ion with Ni²⁺ proceeds via several steps:

with log(β4)=30.1

Figure 1: [Ni(CN)₅]^3-