Navya, your questions always interests me because they are very practical! My few impressions are below.
Whether you can claim that reagent or not depends upon how your provisional specification was written. If you have drafted your provisional application in such a manner that this reagent can reasonably be foreseen to be a part of it, then yes. If your provisional has any kinds of limitations with respect to this category of reagent, then the chances are less. Your complete specification must fairly be based upon your provisional specification. Another important aspect is conception of invention. If you have conceived your invention to encompass this reagent as on your provisional filing, even if you have not explicitly mentioned this reagent in your prov spec, then you can claim it in your complete spec. For example, if this reagent is a functional equivalent of your other reagents.
If your provisional is in any way limiting to include this reagent, and this reagent form part of single inventive concept, then what I suggest is file another provisional with this reagent. Request post dating of your earlier provisional application as per the provisions of Sec 9. You can post date your application for a period up to 6 months, subject to the satisfaction of the Controller. But there is a caveat, you will lose 6 months of priority.
Study your provisional closely and take a decision accordingly. This does not constitute any legal advice.