*One more RfC comment:  I can see both sides.  I don't see the issue with adding a cite, nor do I think it should be difficult to find one.  I would also agree that "Ac" for "Acetyl" is common knowledge, and as an abbreviation doesn't need to cited so much as defined.  What has been done for Methyl?  Is there a mention of the commonly used (in organic chem) "Me" abbreviation?  As to the comment above that Ac is non-standard informal shorthand - it really is more than that.  The prevalence of AcAc ligands, and the practical concerns about writing out the entire formula make it more than that.  I wold say its usage in organic / organometallic chem is fairly standard.