A new series of heteronuclear metal strings, MRhRh(dpa)Cl and MRhRhM(dpa)X, from the reactions of Rh(dpa) with metal ions of group 7 to group 12.

We report the syntheses, structures, magnetic and electrochemical properties of MRhRh metal cores helically wrapped by four dpa- (2,2'-dipyridylamide) ligands. We successfully synthesized the precursor Rh2(dpa)4 (1) in high yield and characterized its structure including its oxidized form (1+) which facilitated the syntheses of this series of metal springs. By the reactions of (1) and the metal ions of group 7 to group 12 (M = Mn(2), Fe(3), Co(4), Ni(5), Cu(6), Pd(8), Pt(9), Ru(10), Ir(11) and Rh(12)), ten MRh2(dpa)4Cl2 complexes were successfully isolated. Note that Cd(7) can only be obtained by the one-pot method. The yield of Rh3(dpa)4Cl2 (12) is also improved by this stepwise method. The oxidized complexes [MRh2(dpa)4Cl2](PF6) (M: Ni(5+), Ru(10+), Ir(11+)) are also synthesized for the studies of electronic structures and magnetic properties. The X-ray diffraction technique is applied to characterize all of their structures. The results of these structural, magnetic, and electrochemical studies provide us with in-depth knowledge and comprehensive insight into metal-metal bonds and interactions for this new series of metal strings. In particular, four metal-metal bonds with short distances are found: Pd-Rh (2.372(13) Å), Pt-Rh (2.385(7) Å), Ru-Rh (2.33(3) Å), and Ir-Rh (2.373(5) Å). The remaining ones show no evidence of covalent metal bonds judging from their metal-metal distances, magnetic behaviour, and redox couples in electrochemical analysis. Besides, two unique tetranuclear MRhRhM(dpa)4X2 (M: Cu+(13) and Ag+(14)) complexes with a Rh2(dpa)4 framework are developed. Four metals are aligned linearly. This coordination mode of metal strings provides a unique synthetic route for constructing longer metal chains from a smaller number of dentate ligands.