, L 2 = 2 PPh 3 or 1,1Ј-bis(diphenylphosphanyl)ferrocene (dppf), R = Ph or ferrocenyl] and their spectroscopic and electrochemical characteristics. Three of these compounds possess redox-active, ferrocenebased substituents or ligands − that are oxidized at lower potentials than the ruthenium center itself − attached either to the terminal carbon atom of the allenylidene ligand or to the ruthenium atom. The Fc/Ph-substituted complexes 3a and 3b provide unique examples of hindered rotation of the allenylidene substituent around the M=C bond. For 3a (L 2 = 2 PPh 3 ), two isomers differing in the orientation of the vertically aligned, unsymmetrically substituted allenylidene ligand are discernible even at 388 K. The dppf-substituted congener 3b has the allenylidene ligand in a horizontal orientation and exhibits a rotational barrier, as determined by dynamic 31 P NMR spectroscopy, of ∆G ϶ = 47 kJ/mol at T C = 238 K. The changes in the spectroscopic and electrochemical properties upon replacement of the PPh 3 by a dppf ligand and the Ph by an Fc moiety can be explained in terms of the bonding within these systems. These effects are attenuated when the ferrocene-based redox tags are oxidized, as shown by IR and

cis-[RuCI 2 (depehl (1) (depe = Et 2 PC 2 H 4 PEt 2) is obtained in excellent yield and high isomeric purity by the reaction of [RuCliDMSO)41 with two equivalents of depe in acetone 01" ethyl acetate under reflux conditions. One of the chloride ligands is labile and readily displaced by CH 3 CN to afford the cationic monosubstitution product cis-[RuCl(CH 3 CN)(depe)21 +(2), which was isolated as the PF 6-salt. The X-ray cl"ystal structure of 1 was determined. The redox properties of 1 and 2 as weil as the chloride displacement from 1 by CH 3 CN were studied by cyclic and square wave voltammetry

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