The complex [Co(H2O)6]I3 consists of a cobalt ion coordinated by six water molecules. This type of complex is known as a hexaqua complex. The geometry of [Co(H2O)6] typically features an octahedral arrangement. In an octahedral configuration, the six coordinating water molecules are symmetrically arranged around the central cobalt ion, forming an octahedron.
The presence of three iodide ions (I^-) in the complex, indicated by I3, suggests that these iodide ions are counterions. They do not affect the coordination number or the geometry of the cobalt complex itself, which remains octahedral.
In summary, the geometry for [Co(H2O)6]I3 is octahedral due to the six water molecules coordinating around the cobalt ion.