Reviewed herein are key aspects of the recent progress in the chemistry of σ-alkynyl compounds of Ru 2 (LL) 4 , where LL is an N,N′-bidentate bridging ligand. Synthetic methodologies for both the mono-alkynyl Ru 2 (II,III) and bis-alkynyl Ru 2 (III,III) compounds have been thoroughly investigated. The basic electronic properties of these Ru 2 (II,III) and Ru 2 (III,III) compounds were elucidated on the basis of solution voltammetry and absorption spectroscopy, which revealed rich redox characteristics and small energy gaps ranging from 1.2 to 1.6 eV. Facile electron transfer across the polyyn-diyl chains was demonstrated on the basis of Ru 2 -(ap) 4 type compounds (ap is 2-anilinopyridinate), and a very small electronic decay constant (β) of 0.063 Å -1 was determined. Also demonstrated was the long-range electronic coupling between Fc-Fc + termini bridged by a -(CtC) m -Ru 2 (DMBA) 4 -(CtC) n -fragment (DMBA is N,N′-dimethylbenzamidinate), and high hole mobility across the Ru 2 (III,III) unit was inferred. Illustrating the potential of metal-alkynyl compounds in molecular electronics, scanning tunneling microscopy (STM) studies of Ru 2 (ap) 4 (σ-OPE) embedded in an alkanethiol matrix revealed significantly improved molecular conductance compared to the OPE of comparable length (OPE is oligo(phenyleneethynylene)). Using the weak-base protocol, several donor-Ru 2 (DMBA) 4 -acceptor type compounds have been obtained, and their voltammetric characteristics are consistent with what are expected for a molecular diode. Ru 2 (ap) 4 (C 2n R) type compounds undergo [2 + 2] cycloaddition/insertion reactions with TCNE, which drastically alter the electronic structures of metal-alkynyl compounds. Also described are the covalent modification chemistry both at the σ-alkynyl ligands and the periphery of N,N′-bridging ligands and supramolecular chemistry based on the latter modification. a m, n ) integers.

Polyyn-diyls capped by Ru(2)(ap)(4) termini (ap = 2-anilinopyridinate), that is, [Ru2(ap)4](mu-C,C'-C2m)[Ru2(ap)4] (compounds 1-5 with m = 1-4 and 6), were synthesized through either a metathesis reaction between Ru2(ap)4Cl and LiC(2m)Li or a Glaser homocoupling reaction of Ru2(ap)4(CmH) under Eglinton/Hay conditions. X-ray diffraction studies of compounds 2 and 4 revealed both the linear rigid rod topology of these compounds and the fine structural details about the Ru2 cores and polyyn-diyl chains. Cyclic and differential pulse voltammetric (CV and DPV) measurements and spectroelectrochemical studies show that reduced and oxidized forms of 1, 2, 4, and 5 are donor-acceptor systems in which the Ru2 termini are coupled to varying degrees depending upon the length of the polyyn-diyl bridge.

Syntheses and characterizations are reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr(-)) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF(3) (6), p-COMe (7), p-CF(3) (8), or Ar is 3,4-Cl(2)C(6)H(3)(-) (9) or 3,5-Cl(2)C(6)H(3)(-) (10). The (quasi)reversible oxidation potentials measured for the Mo(2)(5+)/Mo(2)(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation: DeltaE(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8sigma(X)) mV. Molecular structure determinations of compounds 1, 2, 5, and 10 revealed an invariant core geometry around the Mo(2) center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Å, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of (1)H NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and (1)H NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.