undesirable coupling mechanisms when the LO inductor is placed off-chip.

I really don't understand what and how undesirable coupling mechanisms in the example. Could you explain it in detail?

Thanks for help.

I am not a chip-fab expert. But in more-general terms, the bond wire is an additional impedance, due to the additional length (inductance L and resistance R). So any current through the bond wire would induce voltage componentss across it, Ri + Ldi/dt, giving conductive coupling into the chip as well as the ground. And since the ground conductor would probably be longer, there might also be more "ground bounce" voltage. But there's also another type of coupling mechanism: The loop enclosed by the inductor current's path could be much larger, since the signal and ground paths are longer, and the signal path and its ground path would probably also have a much greater separation distance. So, according to Faraday's Law, there would be more radiated coupling (either received or transmitted or both), between that loop and all other circuits, and also from all external EM fields, in proportion to the change in the area enclosed by the loop.