I want to write a commentary (for a journal) about a paper a collaborator has published.
Is there a conflict of interest? Is it research misconduct if I write the commentary?
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If you want to be "safe" just reveal the relationship. Then people can better judge whether what you have to say is just fluffing (if it is too positive) or grumpiness/sabotage (if it is too negative) or that it is a fair assessment of the other work as it should be.
For an explicit answer to the topline question, however, motive matters. It is possibly misconduct either to try to "catapult" the work of a collaborator or to sabotage it. Writing a fair analysis isn't misconduct.
Following up on a comment, if the editor thinks that writing such commentaries is a clear conflict, then you shouldn't bother to submit it. In such a case, publishing it without disclosing the relationship would be misconduct, not because of the writing of the commentary, but from the deceit of withholding information from the editor.