Magnetic moment of first series of transition metal complexes is determined by..........

spin angular momentum only

The correct answer is Option (3) → spin angular momentum only

In most first-series (3d) transition metal complexes, the orbital angular momentum is largely quenched due to the strong ligand field, which removes the degeneracy of d-orbitals. As a result, the contribution from orbital motion to the total magnetic moment is negligible or zero.

The observed magnetic moment is therefore primarily due to the unpaired electron spins.

The formula used is the spin-only formula:

$\mu_{s.o.} = \sqrt{n(n+2)} \, \text{B.M.}$

where:

• n = number of unpaired electrons
• B.M. = Bohr Magnetons

This approximation matches experimental values very well for most 3d metal complexes.

Thus, the correct option is spin angular momentum only.