Planetary NebulAE(PNe) are known to host an energetic radiation environment that does not avoid the simultaneous presence of different types of molecules and single atoms. For this reason, PNe appear to be a unique laboratory where simple and complex molecules coexist. We will present our recent very sensitive and high-resolution mm observations of PNe IC 418 and NGC 7027, which have provided us a nice mm database of the atomic and molecular content of these objects. In our aim to study the complex organic molecules in mm frequencies, we have found for the first time very weak radio recombination lines (RRLs) of several neutral and ionized atoms never observed before. These new detected RRLs together with the molecular content give useful information about their circumstellar envelopes as well as the complex organic chemistry taking place in this kind of astrophysical environments.