The physics of the two-center shell model for the a-a potential and covalent valence neutrons and protons with up to 4 valence nucleons is discussed. The structure of nuclear dimers and their rotational bands is illustrated using published transfer reaction data for Be and Boron isotopes. Based on the state in 12C at an excitation energy of 7.65 MeV which is assumed to be an 3\alpha particle chain and adding the covalent nucleons, chain states (trimers) in the system ¹²C*+ x neutrons can be constructed.

PACS numbers: 21.10.--k, 21.60.Gx