`NH_(3)` is a better complexing agent than `PH_(3)` - why?

Explanation

The atomic size of N is much smaller than that of P. Therefore, the electron density of N in `NH_(3)` is much higher than that of P in `PH_(3)` . Thus, `NH_(3)` is a stronger Lewis base than `PH_(3)` . Consequently, the tendency of `NH_(3)` to form a complex by donating an electron pair to the metal ion is much higher than that of `PH_(3)` . Hence, `NH_(3)` is a better complexing agent than `PH_(3)` .