Coordination Number 5

For coordination number 5, there are 2 basic geometries, trigonal bipyramidal and square pyramidal, and distorted versions thereof. Main group compounds mostly prefer the trigonal bipyramidal geometry. It is difficult to predict for the transition metals which structure a compoundwill adopt. Since the two structure types are only slightly different in energy, in many cases, for the same molecule or ion, depending on the conditions (packing in the solid state or counterion), both geometries can be seen. The anion [Ni(CN)₅]^3- can be seen in the same crystal, adopting both geometries. This is due to the stereochemical nonrigidity of the structures.

-Trigonal Bipyramidal

Compounds like iron pentacarbonyl (Fe(CO)₅) and PF₅ adopt the trigonal bipyramidal geometries. If all the ligands are the same, the point group for trigonal bipyramical structures is D3h. In trigonal biyramidal structures, there are 2 positions for the ligands, 2 axial and 3 equatorial positions.

PF5

The positions of the ligands can change, i.e. equatorial ligands can become axial ligands and vice versa. This process is called Pseudorotation or Berry Rotation. The mechanism is illustrated in the figure below.

Watch a movie of the Berry Pseudorotation of phosphorus pentafluoride (about 1MB quicktime format)

View the following animation (by rightclicking into the structure or using the buttons below):

Fe(CO)5 in solution exhibits only one signal for the resonace of the carbonyl-carbon atoms in the 13C-NMR. The explanation for this is that the NMR timescale is too slow for capturing the two different carbon signals, since the activation energy is very low for this process. Instead, an average signal is observed. If the activation energy is increased by changing the ligands, e.g. replacing the carbonyl ligands with more sterically demanding ligands, and/or the temperature is lowered to slow down the reaction, then the signals can bedetected separately, since the NMR timescale is faster than the rate of the pseudorotation. Alternatively, a faster technique, like IR spectroscopy could be employed.
Fe(CO)5	[Cu(CH3COO)(bipy)2]+

-Square Pyramidal

The square pyramidal structure is especually preferred when the metal has one ligand with a multiple bond, like in the vanadium examples shown below. The square pyramidal structure also has two distinct coordination sites, one axial and four basal. The central atom can be on the basal plane or slightly above it.

Also note that the transition state for the Pseudorotation is a square pyramidal structure.