Understand the concept of spin-only magnetic moment.

The spin-only magnetic moment ($\mu$) in coordination chemistry is a measure of the magnetic strength of a complex, primarily arising from the spin of its unpaired electrons.

The formula for the spin-only magnetic moment is derived from the number of unpaired electrons ($n$) in the metal ion. It is given by:

where $n$ represents the number of unpaired electrons and the magnetic moment is expressed in Bohr magnetons (BM).

• A. $\sqrt{n(n+1)}$
• B. $2n+1$
• C. $n^2$
• D. $\sqrt{n(n+2)}$

The correct formula matches option D.

The final answer is $\boxed{\text{D. }\sqrt{n(n+2)}}$