The synthesis and spectroscopic properties of trans-[RuL4(C&3bond; CAr)(2)] (L-4 = two 1,2-bis-(dimethylphosphino)ethane (dmpe)(2); 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, 16-TMC; 1,12-dimethyl-3,4:9,10-dibenzo-1, 12-diaza-5,8-dioxacyclopentadecane, N2O2) are described. Investigations into the effects of varying the [RuL4] core, acetylide ligands, and acetylide chain length for the [C-&3bond; C(C6H4C&3bond; C)(n-1)Ph] and [-C&3bond; C(C6H4)(n-1)Ph] (n = 1-3) series upon the electronic and electrochemical characteristics of trans-[RuL4(C&3bond; CAr)(2)](0/+) are presented. DFT and TD-DFT calculations have been performed on trans-[Ru(L')(4)(C&3bond; CAr)(2)](0/+) (L' = PH3 and NH3) to examine the metal-acetylide pi-interaction and the nature of the associated electronic transition(s). It was observed that (1) the relationship between the transition energy and 1/n for trans-[Ru(dmpe)(2){C&3bond; C(C6H4C&3bond; C)(n-1)Ph}(2)] ( n = 1-3) is linear, and (2) the sum of the d(pi)(Ru-II) -> pi(star)(C&3bond; CAr) MLCT energy for trans-[Ru(16-TMC or N2O2)(C&3bond; CAr)(2)] and the pi-(C&3bond; CAr) -> d(pi)(Ru-III) LMCT energy for trans-[Ru(16-TMC or N2O2)(C&3bond; CAr)(2)](+) corresponds to the intraligand pi pi(star) absorption energy for trans-[Ru(16-TMC or N2O2)(C&3bond; CAr)(2)]. The crystal structure of trans-[Ru(dmpe)(2)-{C&3bond; C(C6H4C&3bond; C)(2)Ph}(2)] shows that the two edges of the molecule are separated by 41.7 (A) over circle. The electrochemical and spectroscopic properties of these complexes can be systematically tuned by modifying L-4 and Ar to give E-1/2 values for oxidation of trans-[RuL4(C&3bond; CAr)(2)] that span over 870 mV and gamma(max) values of trans-[RuL4(C&3bond; CAr)(2)] that range from 19 230 to 31 750 cm(-1). The overall experimental findings suggest that the pi-back-bonding interaction in trans-[RuL4(C&3bond; CAr)(2)] is weak and the [RuL4] moiety in these molecules may be considered to be playing a "dopant" role in a linear rigid pi-conjugated rod.