In solution, the pyrazolylborate-zinc-nucleobase complexes show self-association and base pairing with external nucleobases. The self-association was studied quantitatively for Tp(Cum,Me)Zn-hypoxanthinate and Tp(Cum,Me)Zn-thyminate; the dimerization constants K(D) are 63 +/- 8 and 0.2 +/- 0.1 M(-1), respectively. Of the external nucleobases, 9-ethyladenine forms stable base pairs with the thyminate, uracilate, and xanthinate complexes, 9-isobutylguanine only with the cytosinate complex, 1-methylthymine with the adeninate and diaminopurinate complexes, and 1-methyluracil with the diaminopurinate complex. The association constant for the base pair Tp(Cum,Me)Zn-thyminate:9-ethyladenine was determined by NMR methods as K = 66 +/- 10 M(-1). Structure determinations of the crystalline adducts have confirmed the base pairing for Tp(Cum,Me)Zn-thyminate:9-ethyladenine, Tp(Cum,Me)Zn-cytosinate:9-isobutylguanine, and Tp(Cum,Me)Zn-xanthinate:9-ethyladenine. Both Watson-Crick and Hoogsteen base pairs have been observed. In the solid state, extended base pairing leads to quartet and polymer arrangements.