DFT calculations on IrH(PH3)3(OH)+

Geometrical Studies

We started the study of the IrH(PH3)3(OH)+ complex by performing a geometry optimisation on the isolated cation. The results are shown on the first column of Table 2. In particular, the angle H-O-Ir stays far away from the observed one, 108.6 and 91.1° respectively.

In a second attempt at reproducing the experimental structure, we performed a geometry optimisation on the pair [IrH(PH3)3(OH)+][PF6-]. The results are shown on the second column of Table 2. The third column lists the correspondent experimental values.

Table 2: Structural Parameters for IrH(PH3)3(OH)+. A-Without PF6-, B-With PF6-, C-Experimental.
A (Å)B (Å) C (Å) A (°)B (°)
B (°)
Ir - H21.647 1.6621.711 H12 - O - Ir108.6 106.4
91.1
Ir - O2.101 2.1192.119 O - Ir - H293.9 91.0
92.6
Ir - P32.396 2.3742.257 H - Ir - P384.5 82.5
77.6
Ir - P72.513 2.5322.370 P3 - Ir - P7102.0 107.3
100.1
Ir - P132.413 2.3952.337 P13 - Ir - P17177.0 176.0
158.7

Relevant atoms for structural analysis

Albeit the introduction of the PF6- anion does not improve significantly the H-O-Ir bond, the overall geometry becomes closer to the observed one.

Electronic Structure Analysis.

To investigate further the nature of the interaction of the Ir-H····H-O groups, we performed a Mulliken population and density study.

The results of the Mulliken population and charge analysis are shown in Table 3.

Table 3: Charges and Overlap Populations for: A-[IrH(PH3)3(OH)+][PF6-] optimized, B-[IrH(PH3)3(OH)+][PF6-] experimental.
A B
Ir0.1951 0.6703
H20.0312 -0.0226
O-0.7352 -0.7479
H120.3370 0.3182
OP(H····H)0.0029 0.0146

Examination of the electronic distribution shows that the geometry of O-H and Ir-H groups are adequate for an electrostatic interaction.

Charge distribution

This interaction is very similar to the ones observed in typical hydrogen bonds, except for the directionality.

The overlap population between Hδ+ and Hδ- have positive, although small, values. Due to the uncertainties of these partition schemes the results should be looked at with care. Despite that fact, there is an increase in overlap population when the angle H-O-Ir diminishes.

Bond Energy Analysis [5] between IrH(PH3)3(OH)+ and PF6- for the experimental geometry.

Bond energies between IrH(PH3)3(OH)+ and PF6 were decomposed in several terms. All have been explained in detail in several occasions. In Table 4 we present such a decomposition for the formation of [IrH(PH3)3(OH)+][PF6-].

Table 4: Energy decomposition for the formation of [IrH(PH3)3(OH)+][PF6-] in the experimental geometry.
A
δEPauli-0.0063
δEElect-2.2889
δESteric-2.2952
δE(A')-0.1677
δE(A")-0.0399
δEoi-0.2076
δETotal-2.5028

As expected there is a stabilising interaction between both fragments, the electrostatic contribution being the most significant one.

Total Energy Change as Function of the H-O-Ir angle.

In Figure 2 the total energy, calculated with the PW91 Non-Local corrections to the LSDA, is plotted for several H-O-Ir angles. The energy for the experimental geometry is only 0.113 eV higher than the minimum.

Figure 2: Energy changes as function of the H-O-Ir angle.

Energy changes as function of the H-O-Ir angle

As a consequence, we conclude that all weak interactions are important in determining the geometry of the IrH(PH3)3(OH)+ species.

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