Why orbital angular momentum is not quenched for 4d and 5d series elements?

In order for an electron to contribute to the orbital angular momentum the orbital in which it resides must be able to transform into an exactly identical and degenerate orbital by a simple rotation
Paramagnetism arises from the presence of unpaired electons, each such electron having a magnetic moment associated with its spin angular momentum and orbital angular momentum.For the compounds of the first series of transition metals, the contribution of the orbital angular momentum is effectively quenched and hence is of no significance.

For these, the magnetic moment is determined by the number of unpaired electrons and is calculated by using the ‘spin-only’ formula

U=n(n+2)
wheren is the number of unpaired electrons and u is the magnetic moment in units of Bohr magneton (BM).