The coordination number and structure for the complex [UO2 C2O4 H2O 2]2- can be determined by examining the ligands and their coordination to the central metal ion, in this case, uranium U .In this complex, the uranium ion is coordinated to two oxygen atoms from the uranyl ion UO2 , two oxygen atoms from the oxalate ion C2O4 2-, and two oxygen atoms from the two water molecules H2O . This gives a total of six ligands coordinated to the uranium ion, which means the coordination number is 6.The structure of this complex is likely to be octahedral, as this is the most common geometry for complexes with a coordination number of 6. In an octahedral structure, the ligands are arranged around the central metal ion with 90 angles between them.The size of the ligands can affect the coordination number and structure of complexes for lanthanide and actinide ions. Larger ligands may lead to lower coordination numbers, as they occupy more space around the central metal ion and may not allow for as many ligands to coordinate. Conversely, smaller ligands may lead to higher coordination numbers, as they can fit more easily around the central metal ion.Additionally, the size of the ligands can also affect the geometry of the complex. For example, larger ligands may cause a distortion in the geometry due to steric hindrance, while smaller ligands may allow for more symmetrical geometries.In the case of lanthanide and actinide ions, their ionic radii generally decrease with increasing atomic number known as the lanthanide and actinide contraction . This means that the size of the central metal ion can also play a role in determining the coordination number and structure of the complex. Smaller metal ions may preferentially form complexes with lower coordination numbers and more compact geometries, while larger metal ions may form complexes with higher coordination numbers and more expanded geometries.