Is a transition metal with 7 d electons ever diamagnetic? A transition metal with 7 d electrons forms a complex ion with cyanide. I know that cyanide is a high field ligand, which means the electrons will pair before moving to the next energy level, but wouldn't there still be one unpaired electron? And wouldn't that make it paramagnetic? I was told that my prediction was wrong and that it was in fact diamagnetic, but I don't understand why.
As I noted in my comment above, I think the question is whether there are 7d electrons in the atom or in the ion. 

For example, if you take 3 electrons from 7, you get 4d electrons in the $\ce{M^{3+}}$ ion which obviously could (and should) be diamagnetic.

This is all, of course, a guess given what little you posted.

Let's say I had a similar question in my class. Students might then assume this $\ce{M^{3+}}$ is octahedral. (Personally, I'd make it a bit more obvious in the question.)

Given that the ligand is high-field, you get an initial MO diagram like this:



But... we're not done. This electron configuration is subject to a weak Jahn-Teller distortion. So you'd end up with an MO diagram like this:



That, of course, is diamagnetic.