Why is gadolinium not suitable for use as a contrast agent in its free ionic form?

Gadolinium is not suitable for use as a contrast agent in its free ionic form primarily because it is toxic and can form insoluble salts. In its free form, gadolinium is a heavy metal that poses significant health risks, including nephrotoxicity and other adverse effects. The body cannot easily excrete free gadolinium, which can lead to accumulation and increase the risk of serious complications, especially in patients with compromised kidney function.

To mitigate these risks, gadolinium is used in a chelated form, where it is bound to a chemical compound that stabilizes the ion and prevents toxicity. This method reduces the potential for free gadolinium ions to cause harm while allowing the compound to be effective as a contrast agent in MRI scans.

In contrast to this, the other options do not accurately capture the reasons for gadolinium's unsuitability in its free form. While it has a low molecular weight and may form soluble salts, these aspects do not address the primary concern of its toxicity. Additionally, describing gadolinium as non-toxic contradicts the reasons why its free form is unsafe for medical use.