It does indeed alter the O-H stretch frequency, but you are mistaking the OH stretch of newly backboned elements with those that were already present in the hydrated metallic reagent. Those particular OH stretches we can eliminate along with the other water present. No new O-H stretches are formed in the complex. Yes, the C-O stretches will give you indications the degree to which the metal has complexed with the compound, absolutely no argument, but with respect, that is not what the question asked, nor will the MCAT expect anyone to know.