Pentacarbonyliron(0) is a coordination complex that consists of a central iron atom surrounded by five carbon monoxide (CO) ligands. The geometry of this complex can be determined by considering the number of ligands and the electron pairs around the central metal atom. In the case of pentacarbonyliron(0), the geometry is trigonal bipyramidal.

Explanation:
- Trigonal Bipyramidal Geometry:
In a trigonal bipyramidal geometry, the central metal atom is surrounded by five ligands arranged in a specific manner. This geometry consists of a plane with three ligands arranged around the equator (the equatorial plane) and two other ligands positioned above and below the plane (the axial positions). The angle between the equatorial ligands is 120 degrees, while the angle between the axial ligands and the equatorial ligands is 180 degrees.

- Pentacarbonyliron(0):
The pentacarbonyliron(0) complex contains five carbon monoxide (CO) ligands bonded to a central iron atom. Each CO ligand contributes a total of two electron pairs to the iron atom, resulting in a total of ten electron pairs from the five CO ligands. This count includes both the bonding and non-bonding electron pairs.

- Electron Pair Arrangement:
To determine the geometry, we need to consider the electron pair arrangement around the central iron atom. In the case of pentacarbonyliron(0), the ten electron pairs from the CO ligands are arranged in a trigonal bipyramidal manner. The equatorial plane contains three pairs, while the axial positions each contain one pair.

- Molecular Geometry:
The molecular geometry of pentacarbonyliron(0) is also trigonal bipyramidal, as it mirrors the electron pair arrangement. The five CO ligands are positioned around the iron atom in a trigonal bipyramidal fashion, with three ligands in the equatorial plane and two ligands in the axial positions.

In conclusion, the geometry of pentacarbonyliron(0) is trigonal bipyramidal. It is important to note that the geometry is determined by the number of ligands and the electron pair arrangement around the central metal atom.