Four rhodium dimers have been synthesized with a bridging diisocyanide ligand, dmb (2,2-dimethyl-1,3-diisocyanopropane): [Rh-2(dmb)(4)](BPh4)(2), [Rh-2(dmb)(4)Cl-2]Cl-2, [Rh-2(dmb)(4)I-2](PF6)(2), and [Rh-2(dmb)(2)(dppm)(2)](BPh4)(2) (dppm = bis(diphenylphosphino)methane). The complexes have been characterized by elemental analysis and mass spectrometry, as well as UV-visible, IR, and H-1 NMR spectroscopies. X-ray crystal structures of the rhodium(I) complexes, [Rh-2(dmb)(4)](BPh4)(2)center dot 1.5CH(3)CN (3.2330(4), 3.2265(4) angstrom) and [Rh-2(dmb)(2)(dppm)(2)](BPh4)(2)center dot 0.5CH(3)OH center dot 0.2H(2)O (3.0371(5) angstrom), confirm the existence of short Rh center dot center dot center dot Rh interactions. The metal-metal separation for the rhodium(II) adduct, [Rh-2(dmb)(4)Cl-2]Cl-2 center dot 6CHCl(3) (2.8465(6) angstrom), is consistent with a formal Rh-Rh bond. For the two luminescent rhodium(I) dimers and six previously investigated diisocyano-bridged dimers with and without dppm ligands, the intense spin-allowed d sigma*-> p sigma absorption band maximum shifts to longer wavelengths with decreasing Rh center dot center dot center dot Rh separation, and there is an approximate correlation between band energy and the inverse of the metal metal separation cubed. Both [Rh-2(dmb)(4)](2+) and [Rh-2(dmb)(4)(dppm)(2)](2+) undergo oxidative addition in the presence of iodine. In the conversion of [Rh-2(dmb)(4)](2+) to [Rh-2(dmb)(4)I-2](2+), the observed intermediate is tentatively assigned to a tetramer composed of two rhodium dimers. In the case of [Rh-2(dmb)(2)(dppm)(2)](2+), no intermediate was detected.