Orbital Analysis

The shapes of selected frontier orbitals for the RuIIarene complexes1−6and 8−16in the ground state (S0) were analysed, in particular the Highest Occupied Molecular Orbitals (HOMOs) and the first four Lowest Unoccupied Molecular Orbitals (LUMOs). The Single Occupied Molecular Orbitals (SOMOs) of the lowest-lying triplet state are also described along with their Spin Density Surfaces (total electron density of electrons of one spin (α), minus the total electron density of the electrons of the other spin (β)).11

Visualisation of the SOMOs and the spin density surface of a metal complex is useful to understand where unpaired electrons of a triplet state are localised.

4.3.4.1 Molecular Orbitals in the Ground State

Complexes [(6-p-cym)Ru(bpm)(Py)]2+ (1) and [(6-p-cym)Ru(bpm)(4-MePy)]2+(2) are found to have similar frontier orbitals. The HOMO is delocalised mainly on the Ru−p-cym moiety, with some contribution from the bpm chelating ligand. The LUMO is bpm-centred while the LUMO+1 is delocalised around the Ru−p-cym bond. The LUMO+1 for complexes 1 (Figure 4.10) and2 has an important σ*-antibonding

character towards the pyridine-derivative ligand (Py and 4-MePy, respectively). The LUMO+2 is mainly bpm-centred, whereas the LUMO+3 resembles the LUMO+1 in character. In the case of [(6-p-cym)Ru(bpm)(4-MeOPy)]2+ (3), all the orbitals share similar features to those found for complex 1 and its analogue complex 2, with the

exception of the HOMO, where a significant delocalisation over the 4-MeOPy was observed. The 4,4'-bpy ligand in complex 4 (Figure 4.10) contributes to the overall character of the LUMO+2 and LUMO+3, evidence which, however, is not observed for the corresponding complexes 1 and 3. The HOMOs of [(6-p-cym)Ru(bpm)(4- PhPy)]2+ (5) and [(6-p-cym)Ru(bpm)(4-BzPy)]2+ (6) are fully centred on the corresponding pyridine-derivative, while the empty orbitals are consistently similar to those found in 1, as well as for the other derivatives complexes2−4. A direct orbital comparison between complex 1 and complexes 14, 15 (Figure 4.10) and 16 reveals that while complexes 15 and 16 maintain the similar features found for complex 1, complex 14 displays a different character. Its high-energy LUMOs (i.e. LUMO+1, LUMO+2, and LUMO+3) display not only a significant σ*-antibonding character towards the 3-AcPy ligand, but also an important contribution coming from its π orbitals. Additionally, LUMO+3 in complex14has a reduced arene(p-cym)character, as well as an increased bpm contribution. The character of both the low- and the high- energy orbitals for complexes8and9resemble more to those observed for1, with the only exception of the LUMO+1 which is observed to have lost the σ*-antibonding character towards the Py ligand. Only small differences were found between complexes 9 and 13 when compared to1. The RuII complex 13 has HOMO, LUMO and LUMO+1 very similar to complex 9. However, the LUMO+2 has a σ*-

antibonding character toward the Py ligand, while the LUMO+3 shares the bpm- centred nature of LUMO+2 found in the other derivatives, along with a considerable contribution from its π orbitals. Finally, complexes 10 and 12have HOMOs with an increased contribution from the corresponding N,N'-chelating ligand, particularly in the case of 12 (bathophen). Compared to complex 1, the LUMO+1 and LUMO+2 have reversed characters, so that LUMO+2 and LUMO+3 are more σ*-antibonding in

character. Complex 11 is found to have orbital character similar to complex 1. A complete representation of the frontier orbitals for complexes 1−6 and 8−16 are reported in the Appendix section.

Figure 4.10. Selected frontier orbitals for complexes 1, 4, 14, and 15 in the ground state (S0) geometry.

4.3.4.2 Molecular Orbitals in the Lowest-Lying Triplet State

In the case of complex 1, both the highest-SOMO and the lowest-SOMO (h-SOMO and l-SOMO, respectively) are delocalised over the Ru−arene(p-cym) and Ru−N,N'(bpm) bonds, Figure 4.11. In the case of complex 2, these two orbitals (h-SOMO and l- SOMO) are in general, similar to those found in complex 1, only differing in the contribution from the 4-MePy to thel-SOMO. For complexes3and4this same effect

is also observed but rather increased, and the contribution of the 4,4'-bpy in the latter complex comes mainly from the uncoordinated Py ring which contributes to the l- SOMO also through the non-coordinated-N-centred σ orbital.

Figure 4.11. Calculated lowest- and highest- Single Occupied Molecular Orbitals (h- SOMO and l-SOMO, respectively), and spin density surface for [(6-p- cym)Ru(bpm)(Py)]2+(1).

Complex [(6-p-cym)Ru(bpm)(4-PhPy)]2+ (5) is very similar to1, where both the h- SOMO and the l-SOMO are delocalised over the Ru−arene(p-cym) and Ru−N,N'(bpm) bonds. As observed for complex 4, thel-SOMO in5is also delocalised on the phenyl ring of the 4-PhPy ligand. Similar features are found for complex [(6-p- cym)Ru(bpm)(4-BzPy)]2+ (6), where both the h-SOMO and the l-SOMO are centred in the benzyl ring of the 4-BzPy ligand. The spin density maps for complexes 1−6

show that the nature of these lowest-lying triplet states are mainly Ru−arene(p-cym) centred. Comparison of complex 1 with complexes 14, 15 and 16, show that the l- SOMO on the two first complexes has the same character as that in complex 1, whereas the h-SOMO is N,N'(bpm) localised and similar to the LUMO in the ground state (S0). In the case of 16, the l-SOMO and the h-SOMO lie at inverted energy levels compared to those of complexes 14 and 15. The spin density maps for complexes 14 and 15 show that the unpaired electrons are mainly localised on the

pyridine-derivative ligand, while for 16 they are centred on the Ru−arene(p-cym) moiety. The l-SOMO and theh-SOMO of complex 11 are centred on thep-cym and the phendio N,N'-chelating ligand, respectively. In the case of complexes 10 (phen) and 12 (bathophen), both the l-SOMO and the h-SOMO display a σ*-antibonding character towards the Py ligand (particularly theh-SOMO). Thel-SOMO is also more localised on the corresponding N,N' chelating ligand (phen in 10, and bathophen in

12), whereas theh-SOMO is centred on the Ru-arene(p-cym)fragment. The spin density map of complex 11 is mainly N,N'(phendio) centred, while for 10 and 12 they are delocalised over all the RuII complex. Complexes [(6-hmb)Ru(bpm)(Py)]2+(8) and [(6-ind)Ru(bpm)(Py)]2+(9) display high similarity when compared to 1. However, their h-SOMOs have some σ*-antibonding character towards the monodentate pyridine ligand. Their spin density maps are mainly Ru−arene centred (only the aromatic ring of the ind arene is involved in the case of 9). Complex 13 shows a N,N'(bpy)-l-SOMO localisation as that observed for complex1. The spin density maps of complexes9and13 are Ru−arene(ind)centred in both cases.