Describe the structures of tetracarbonylnickel(0), pentacarbonyliron(0), and hexacarbonyl chromium(0) in terms of their coordination geometries.

Tetracarbonylnickel(0) has a tetrahedral structure, where the nickel atom is coordinated to four carbon monoxide ligands. Pentacarbonyliron(0) exhibits a trigonal bipyramidal geometry, with the iron atom coordinated to five carbon monoxide ligands. Hexacarbonyl chromium(0) adopts an octahedral structure, where the chromium atom is coordinated to six carbon monoxide ligands. In all cases, the metal-ligand coordination geometries are determined by the number of available d orbitals on the central metal atom and the nature of the ligands, following the principles of coordination chemistry and the spatial arrangement of ligands around the metal center.