bond lengths in ferrocinium
Chip Nataro and I have been having a back-and-forth conversation on this topic for about 18 hours now and neither of us likes where we are, so I am opening up the thread to the larger community. However, I will preface this with, I think the question is poorly written and the answer they want is simple, when in fact, the answer is complex.
Here goes. Spessard and Miessler, 2nd ed, chapter 5, end of question problem 5.5:
The Fe-C distance in ferrocinium+ is 6 pm longer than in Fc, but the Co-C distance in Cobaltocinium+ is 6 pm shorter than that in Colbatocene.