# Steric, Activation Method and Solvent Effects on the Structure of Paddlewheel Diruthenium Complexes
## 3 Results and Discussion
### Magnetic Properties  
The magnetic moments at room temperature of compounds **1**-**3** are in the range 3.86-4.17 \(\mu_{\rm B}\), similar to those observed for other tetracarboxylatodiruthenium compounds [1; 4; 33; 42]. These values are close to the theoretical value of 3.87 \(\mu_{\rm B}\) for three unpaired electron considering g = 2. These data are in accordance with the electronic configuration of \(\sigma^{2}\)\(\pi\)\(\delta^{2}\)(\(\pi\)\(\delta\)*)\({}^{3}\) per Ru\({}_{2}\)\({}^{5+}\) unit proposed by [PERSON] et al. [43]. Compounds **1b**, **2** and **3** show similar magnetic behavior. The molar magnetic susceptibility continuously increases with decreasing temperature (300-2 K) while the magnetic moment decreases, especially at low temperature values. This behavior has been attributed to a strong zero-field splitting (D) of the diruthenium unit and a small degree of antiferromagnetic coupling (z)) between the bimetallic units, and it is indicative of molecular or zig-zag polymeric arrangements [1; 4; 32; 33; 38; 42]. The fitting of the experimental magnetic data of compounds **1b**, **2** and **3** has been carried out using the model of [PERSON] et al. [44], which takes into consideration the equations developed by [PERSON] [45] and corrected later by [PERSON] and coworkers [46]. This model takes into account the existence of D and zJ, but also a temperature independent paramagnetism (TIP) and a paramagnetic impurity (P) of a mononuclear complex of Ru(III) with S = 1/2. Good agreements between the experimental data and calculated curves of the molar susceptibility and magnetic moment have been obtained for compounds **1b**, **2** and **3**. Table 4 collects the calculated magnetic parameters (g, D, zJ, TIP and P) obtained in these fits together with the \(\sigma^{2}\) value, which indicates the quality of the fits. As an example, Figure 4 shows the experimental and calculated curves using this model for complex **1b**. Similar fits are obtained for complexes **2** and **3** (Figures S2 and S3, Supplementary Materials). The large D calculated values range between 59.66 and 60.31 cm\({}^{-1}\), which are similar to those found for analogous tetracarboxylatodiruthenium(II,III) compounds [1; 4; 23; 33; 44; 47; 48]. The calculated zJ values are between \(-\)0.09 and \(-\)0.32 cm\({}^{-1}\). These low antiferromagnetic coupling constants agree well with the molecular nature of compound **1b** and with the formation of zig-zag chains in **2** and **3**. \begin{table}
\begin{tabular}{c c c c} \hline
**Compound** & \(\sigma\)**(Axial Ligand)\(\rightarrow\)\(\sigma\)*(Ru\({}_{2}\))** & \(\pi\)**(RuO,Ru\({}_{2}\))\(\rightarrow\)\(\pi\)*(Ru\({}_{2}\))** & \(\delta\)**(Ru\({}_{2}\))\(\rightarrow\)\(\delta\)*(Ru\({}_{2}\))** \\ \hline
**1a** & 332 sh & 450 & 1088 \\
**2** & 328 sh & 490 & 1169 \\
**3** & 352 & 474 & 1162 \\ \hline \end{tabular}
\end{table} 
Table 3: Assignment of the absorption bands observed in the spectra of compounds **1a**, **2** and **3**. Compound **1a** shows a maximum at 49 K in the molar susceptibility vs. temperature curve (Figure 5) and a paramagnetic tail from 13 to 2 K. This behavior indicates a linear chain arrangement in the solid state of the diruthenium units [23; 49; 50; 51; 52; 32]. In this case, there is a strong antiferromagnetic coupling between these units, which are bridged by a chloride ligand giving a 180\({}^{\circ}\) Ru-Cl-Ru angle. In this case, the model of [PERSON] [44] is not valid to fit the experimental magnetic data, since this model does not foresee a maximum in the molar susceptibility curve. A suitable model to fit the magnetic data of linear polymeric diruthenium complexes has also been developed previously [49]. This model considers that every S = 3/2 Ru\({}_{2}\)\({}^{5+}\) unit has a strong antiferromagnetic coupling with the closest neighbors units, but the couplings are negligible with the rest.