6 mL of distilled water, 2.2 mL of isopropanol and 1.2 mL of 1,5-cyclooctadiene were measured into 50 mL two neck round bottomed flask. The solvent mixture was first purged with argon after which a condenser was connected to the flask. Then, the mixture was refluxed for 1hour prior to the addition of 1 g of IrCl₃ and further refluxed for 8 hours after the addition under strong stirring. The suspended liquid was removed via cannula and the cold methanol was added to red oil left to effect formation of red crystalline solid. The solid was later filtered, washed with cold methanol and dried under vacuum.

1 g of RhCl₃ was weighed in 50 mL two neck round bottomed flask fitted with condenser, then vacuumed and filled with argon. Then, 10 mL of ethanol, 2 mL of distilled water and 1.4 mL of 1,5-cyclooctadiene were added into the flask. Thereafter, the reaction mixture was refluxed for 8 hours under strong stirring. The resulting yellow precipitate was filtered, washed with distilled water and ethanol and dried under high vacuum.

The NMR analysis of the deuterated chloroform solution of the two synthesized complexes was carried. The infrared spectrum of 1,5-cyclooctadiene was recorded through smart orbit technique while two complexes were sampled in KBr pellets. Raman spectra for the cyclooctadiene and the two complexes were recorded for sample in a 1 mm glass tube using a laser source of 1064 nm with 0.3 W power.

Ab initio density functional theory (DFT) calculations at B3LYP level were performed on the molecules using Gaussian 09 program [9]. Optimization was carried out using basis set of 6 - 311 ++ G(d, p) for the nonmetals and SDD for the transition metals. The same basis sets were used for vibrational frequency calculations with the population analysis being undertaken through full natural bond orbital analysis calculation while the NMR was computed using Gauge-Independent Atomic Orbital (GIAO) method. The Polarizable Continuum Model (PCM) using the integral equation formalism variant (IEF) was implemented to predict the solvent effect on the theoretical spectral data for both complexes.

Two symmetries D₂ and D_2h were fully optimized and their relative energies are shown in Table 1. The D₂ configuration is predicted to be more stable in the gas phase compared to D_2h by 5.88 and 5.04 kcal/mol, respectively for [Rh(µ-Cl)(COD)]₂ and [Ir(µ-Cl)(COD)]₂. The D₂ symmetry is obtained from the C_i COD, while the D_2h complex is associated with C₂ COD [7] [8].

The ¹H NMR calculations were carried out for the two configurations of both complexes in gas phase and in CHCl₃ solvent using the PCM model and compared to the experimental ones. Three sets of 8H peaks for D_2h symmetry but six sets of 4H peaks for D₂ symmetry were predicted (Table 2). These sets have different chemical environments. The D_2h ¹H NMR result is in agreement with the theoretical ¹H NMR spectra for both complexes confirming the higher level symmetry even in solution phase. It is also interesting to notice that the average shielding values for the D₂ configuration matches with shielding value of each set in that of D_2h as depicted in Table 2.

Selected modes of vibrations associated with C=C and C-H bonds of COD were followed theoretically and experimentally for the free configuration and coordinated ones in the complex as indicated in Table 3. There is a quite agreement between the two methods for the % shift in frequency depicted in Table 4. All the corresponding frequencies of mode of vibrations in the free diolefin shift to lower values in the complex with the exception of out-of-plane wagging in both complexes. The sum of shift in C=C stretch (in-phase) and C-H in-plane wag (in- plane) frequencies between the free olefin and coordinated one reflects the total (σ + π) metal-olefin interaction [8]. There is agreement between the sums obtained from theoretical calculation and experiment with [Ir(µ-Cl)(COD)]₂ (13%) having slightly higher value than [Rh(µ-Cl)(COD)]₂ (12%) while its % shift in out of plane wag almost twice that of Rh complex. These results indicate a stronger metal-olefin interaction for Ir complex. This conclusion is ascertained by results obtained from natural bond orbital (NBO) calculation with hybridization values of carbons in C=C and C-H of the Ir complex being higher than those in the Rh complex as seen in Table 5. Iridium

Mode of Vibrations: ^a: asymmetric stretch; ^b: symmetric stretch; ^c: in-plane wag; ^d: in-plane wag; ^e: out-of-plane wag.

is more electropositive than Rh and as such it populates carbon with more electron than Rh, thus decreasing the bond order which in return increases the bond length. This consequently resulted in shift to lower wavenumbers for such vibrational modes for coordinated olefinic C=C with Ir than Rh.