The synthesis, molecular photophysics, redox characteristics, and electronic interactions, as well as an X-ray photoelectron spectroscopic (XPS) study of a series of Ru(II) and Os(II) complexes with a polyphosphine/cumulene spacer, namely, 1,1',4,4'-tetrakis(diphenylphosphino)cumulene (C4P4), are studied and compared with the corresponding systems containing spacers with shorter sp carbon chain (C-n) lengths. Characterizations of all mono-, homo-, and heterobimetallic complexes with PF6- counteranions are accomplished using H-1, C-13, and P-31{H-1} NMR, fast atom bombardment (FAB/MS), and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF/MS) mass spectroscopy and elemental analysis. From the electrochemical study it is observed that the length of the C-n bridges has a profound influence on redox potentials and the electronic interaction between the two metal-based termini. XPS studies reveal that a simple change in carbon chain length affects the electron donation of the phosphine spacer to the metal-based termini. As a result, the redox potential of the Ru(II) or Os(II) center is shifted significantly. The comproportionation constant, K-c, is calculated as 1.3 x 10(7) (M = Ru-II) or 4.5 x 10(10) (M = Os-II) for homobimetallic [(bpy)(2)M(C4P4)M(bpy)(2)](4+), suggesting a strong electronic communication across the C4P4 spacer. However, the K-c value is estimated to be ca. 4 for the corresponding complexes [(bpy)(2)M(C3P4)M(bpy)(2)](4+) (M = Ru, Os; C3P4 = 1,1',3,3'-tetrakis(diphenylphosphino)allene), indicative of a system with electronic isolation between the two termini. In heterobimetallic [(bpy)(2)Ru(CnP4)Os(bpy)(2)](4+) (n = 3, 4), the energy transfer from Ru(LI) to Os(II) is found to be very efficient, with rate constants k(cn) of ca. 3 x 10(9) s(-1) (n = 3) and 1 x 10(11) s(-1) (n = 4). The increased value of k(cn) upon the change from C-3 to C-4 can be explained by the increase in the electronic communication across spacers. Detailed studies and calculations have revealed a Dexter-type of mechanism for the triplet energy transfer in the system.