Journal of Applied Chemical Research, 20, 1, 7-13 (2012)

Journal of App l ied Chemical Research

www.jacr.k iau.ac. i r

Natural Bond Orbital (NBO) Population Analysis of Iridabenzene (C5H5Ir)(PH3)3

R. Ghiasi*, E. Ebrahimi MokaramDepartment of Chemistry, Basic science faculty, East Tehran Branch, Islamic Azad University,

Qiam Dasht, Tehran, IRAN (Received 07 June 2011; Final version received 10 November 2011)

AbstractThe molecular structure of iridabenzene (C5H5Ir)(PH3)3 was calculated by the B3LYP density functional model using LANL2DZ basis set for Ir and 6-31G(d) for other atoms. The results from natural bond orbital (NBO) analysis have provided new insights into Ir–ligand bonding, the hybridization of atoms and the electronic structure of the title molecule. The NBO calculations show that σ(Ir-C2) bonds are formed from an sd1.18 hybrid on iridium atom π(Ir–C3) bond is formed from an sd5.21. Also, these calculations determined that strongest electron donation occurs from a lone pair orbital on the phosphorous atoms, LP(1)P to the antibonding acceptor σ*(Ir–C) orbitals.Keywords: Density functional theory (DFT), Metallabenzenes, Iridabenzene, Natural bond orbital (NBO).

Introduction

For the past decade the synthesis of

metallabenzenes have been examining [1-

7] and their valence isomers starting from

(Z)-3-(2-iodoethenyl)cyclopropenes [8-

12]. Recently, the direct synthesis of a

series of iridabenzenes from nucleophilic

3-vinylcyclopropenes reported. From

experimental and theoretical examinations,

it is obersrved that the actual experimental

knowledge concerning iridabenzenes

compounds is still relatively limited due to the

subtle nature of such compounds.

The objective of the present work is to

investigate the nature of bonding in an

iridabenzene (Figure 1), by using natural bond

orbital (NBO) analysis. We have shown that

the results from NBO calculations can provide

the detailed insight into the electronic structure

of molecule.

* Corresponding author. Reza Ghiasi, Tel/fax:+98-21-33584011.E-mail: rezaghiasi1353@yahoo.com

R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012)8

Figure 1. The optimized equilibrium structure of iridabenzene, and the numbering of atoms.

Computational Methods

All calculations were carried out with the

Gaussian 03 suite of program [13]. The

calculations of systems contain C, H, and F is

described by the standard 6-31G(d) basis set

[14, 15]. For Ir element standard LANL2DZ

basis set [16-18] are used and Ir is described by

effective core potential (ECP) of Wadt and Hay

pseudopotential [19] with a double-ζ valance

using the LANL2DZ. Geometry optimization

was performed utilizing Becke’s hybrid

three-parameter exchange functional and the

nonlocal correlation functional of Lee, Yang,

and Parr (B3LYP) [20]. Vibrational analysis

was performed at each stationary point found,

that confirm its identity as an energy minimum.

The population analysis has also been

performed by the natural bond orbital method

[21] at B3LYP/6-31G(d) level of theory using

NBO program [22] under Gaussian 2003

program package.

Natural bond orbital analysis stresses the role

of intermolecular orbital interaction in the

complex, particularly charge transfer. This

is carried out by considering all possible

interactions between filled donor and empty

acceptor NBOs and estimating their energetic

importance by second-order perturbation

theory. For each donor NBO (i) and acceptor

NBO (j), the stabilization energy E(2) associated

with electron delocalization between donor

and acceptor is estimated as:

Where qi is the orbital occupancy, εi, εj are

diagonal elements and Fi,j is the off-diagonal

NBO Fock matrix element.

Result and discussion

Structure

The optimized molecular structure of (C5H5Ir)

(PH3)3 and numbering of atoms are shown in

R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012) 9

Figure 1. The calculated bond lengths and bond

angles are listed in Table 1. According to the

theoretical results, three phosphine phosphorous,

P11, P15, P19, and two carbon atoms, C2 and

C3, form a square pyramidal environment

around iridium. The M-C bond length are on

the border between experimentally determined

M-C (2.0-2.1) and M=C (1.8-2.0) bond lengths.

NBO analysis

The Natural Bond Orbital (NBO) analysis of

iridabenzene has provided the detailed insight

into the nature of electronic conjugation between

the bonds in this molecule. Table 2 collects the

natural charges on atoms. The largest negative

charges (-0.415 e) are located on two carbon

atoms, C1 and C5. According to the NBO results,

the electron configuration of Ir is: [core]6s

(0.48) 5d (8.47)6p (0.01)6d ( 0.02)7p (0.01).

Thus, 68 core electrons, 8.95 valence electrons

(on 5d and 6s atomic orbitals) and 0.03 Rydberg

electrons (mainly on 6d and 7p orbitals) give

the total of 75.98 electrons. This is consistent

with the calculated natural charge on Ir atom in

iridabenzene +0.02 e, which corresponds to the

difference between 75.98e and the total number

of electrons in the isolated Ir atom (77e).

Table 1. The theoretical bond lengths (Å) of iridabenzene calculated by the B3LYP method with 6-31G(d) basis set for C, H, P atoms and Lanl2dz for Ir atom.

Bond Ir-P15H3 Ir-P19H3 Ir-P11H3 Ir-C2 Ir-C3 Bond distance(Å) 2.378 2.378 2.290 2.001 2.000

Table 2. The NBO atomic charges of iridabenzene calculated by the B3LYP method with 6-31G(d) basis set for C, H, P atoms and Lanl2dz for Ir atom.

Atom Ir C1 C2 C3 C4 C5 P11 P15 P19 Natural charge 0.026 -0.319 -0.416 -0.146 -0.319 -0.169 0.175 0.160 0.160

Of the two carbon atoms in ring, the C2

and C3 atoms coordinated to iridium have

larger negative charge (-0.415e). It should be

emphasized that the calculated natural charge

on the Hi atom of the phosphine group is more

positive (0.15e) than the charge, on the other H

atoms.

Table 3 lists the calculated occupancies of

natural orbitals. Three classes of NBOs are

included, the Lewis-type (σ and π bonding

or lone pair) orbitals, the valence non-Lewis

(acceptors, formally unfilled) orbitals and the

Rydberg NBOs, which originate from orbitals

outside the atomic valence shell. The calculated

natural hybrids on atoms are also given in this

table.

R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012)10

Table 1. The theoretical bond lengths (Å) of iridabenzene calculated by the B3LYP method with 6-31G(d) basis set for C, H, P atoms and Lanl2dz for Ir atom.

Bond Ir-P15H3 Ir-P19H3 Ir-P11H3 Ir-C2 Ir-C3Bond distance(Å) 2.378 2.378 2.290 2.001 2.000

Table 2. The NBO atomic charges of iridabenzene calculated by the B3LYP method with 6-31G(d) basis set for C, H, P atoms and Lanl2dz for Ir atom.

Atom Ir C1 C2 C3 C4 C5 P11 P15 P19Natural charge

0.026 -0.319 -0.416 -0.146 -0.319 -0.169 0.175 0.160 0.160

Table 3. Occupancy of natural orbitals (NBOs) and hybrids of iridabenzene calculated by the B3LYP method with 6-31G(d) basis set for C, H, P atoms and Lanl2dz for Ir atom.

Donor Lewis-typea

NBOs (Ir–C) bondoccupancy Hybridb AO (%)c

s(Ir-C) s(Ir-C) p(Ir-C)

1.830031.867691.63678

sd 1.18

sd 1.76

sd 5.21

s( 45.83%)p( 0.09%)d( 54.08%)s( 36.16%)p ( 0.07%)d ( 63.77%) s( 16.09%)p( 0.11%)d( 83.81%)

LP ( 1) P15 1.65218 sp 0.79 s( 55.81%)p( 44.17%)d( 0.02%)LP ( 1) P11 1.62366 sp 0.82 s(54.85%)p( 45.11%)d( 0.04%)LP ( 1) P19 1.65237 sp 0.79 s( 55.80%)p( 44.18%)d ( 0.02%)sP11- H13 1.98496 sp 5.27 s( 15.80%)p( 83.19%)d( 1.01%)sP11- H14 1.98493 sp 5.26 s( 15.81%)p( 83.18%)d( 1.01%)sP11- H12 1.98137 sp 6.31 s( 13.54%)p( 85.44%)d ( 1.02%)sP15- H16 1.98802 sp 5.52 s( 15.20%)p( 83.83%)d( 0.98%)sP15- H17 1.98778 sp 5.57 s( 15.06%)p( 83.95%)d ( 0.99%)sP15- H18 1.98621 sp 6.14 s( 13.87%)p( 85.14%)d( 0.99%)sP19- H20 1.98778 sp 5.57 s( 15.07%)p( 83.94%)d( 0.99%)sP19-H21 1.98804 sp 5.51 s( 15.20%)p( 83.82%)d( 0.98%)sP19- H22 1.98618 sp 6.14 s( 13.86%)p( 85.15%)d( 0.99%)

a LP(n)A is a valence lone pair orbital (n) on A atom. b Hybrid on A atom in the A–B bond or otherwise, as indicated. c Percentage contribution of atomic orbitals in NBO hybrid.

Table 4. Second-order interaction energy (E2, kcal/mol) between donor and acceptor orbitals in iridabenzene.

Donor®acceptora E2 E(j)-E(i) F(i,j)s* C 3-Ir23 ® s*C 2-Ir23 273.76 0.03 0.129LP ( 1) P15® s*C 2-Ir23 120.51 0.56 0.236LP ( 1) P19 ®s*C 3-Ir23 116.48 0.53 0.228LP ( 1) P11® p* C 3-Ir23 55.66 0.38 0.140pC 4- C 5 ®p*C 3-Ir23 47.38 0.15 0.081pC 3-Ir23 ® s*C 3-Ir23 35.83 0.30 0.096sC 2-Ir23 ®s*C 3-Ir23 32.65 0.52 0.125pC 3-Ir23 ® s*C 2-Ir23 30.00 0.34 0.091sC 3-Ir23 ® s*C 2-Ir23 26.41 0.56 0.117pC 1- C 2® p*C 4- C 5 24.77 0.28 0.076pC 4- C 5® p* C 1- C 2 16.41 0.27 0.061

a Starred label (*) denotes antibonding, and Ry corresponds to the Rydberg NBO orbital.

In the NBO method, delocalization of electron

density (ED) between occupied Lewis-type

orbitals and formally unoccupied (antibonding

or Rydberg) non-Lewis NBOs corresponds to

a stabilizing donor–acceptor interaction. The

strength of this interaction can be estimated by

the second order perturbation theory. Thus, the

results obtained from NBO analysis provide

convenient basis for investigating conjugative

interactions in molecular systems.

According to calculations, the iridium atom

forms a single bond (sigma bond) and a double

bond (σ and π bonds) with two carbon atoms

C2 and C3 atoms, respectively. While the two

bonds between iridium and the phosphine

groups can be described as donation of electron

density from a lone pair (LP) orbital on each

phosphorous atom to iridium molecular orbitals.

As seen from Table 3, the σ(Ir–C2) bond is

formed from an sd1.18 hybrid on iridium

(which is the mixture of 45.83%s, 0.09%p and

54.08%d atomic orbitals). On the other hand,

σ(Ir–C3) bond is formed from an sd1.76 hybrid

on iridium (which is the mixture of 36.16%s,

0.07%p and 63.77%d atomic orbitals). The

π(Ir–C3) bond is formed from an sd5.21 hybrid

on iridium (which is the mixture of 16.09%s,

0.11%p and 83.81%d atomic orbitals).

Table 4 lists the selected values of the calculated

second order interaction energy (E2) between

donor–acceptor orbitals in iridabenzene. The

strongest interactions are the electron donations

from a lone pair orbital on the phosphorous

atoms, LP(1)P to the antibonding acceptor

R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012) 11

σ*(Ir–C) orbitals, in basal position, e.g. LP(1)

P19 σ*( Ir–C2). As shown in Table 3, the

LP(1)P orbital has 44.2% p-character and is

occupied by 1.652 electrons (this is consistent

with a delocalization of electron density from

the idealized occupancy of 2.0e). The donation

of electron density from the phosphine

ligands to the Ir molecular orbitals has a clear

correspondence to a chemical picture of the

coordination bonds (H3P Ir). As follows from

the calculated (E2) values, the P Ir bonds are

stronger than σ(Ir–C) bonds in iridabenzene.

Table 4. Second-order interaction energy (E2, kcal/mol) between donor and acceptor orbitals in iridabenzene.

Donor®acceptora E2 E(j)-E(i) F(i,j)s* C 3-Ir23 ® s*C 2-Ir23 273.76 0.03 0.129 LP ( 1) P15®s*C 2-Ir23 120.51 0.56 0.236 LP ( 1) P19 ®s*C 3-Ir23 116.48 0.53 0.228 LP ( 1) P11®p* C 3-Ir23 55.66 0.38 0.140 pC 4- C 5 ®p*C 3-Ir23 47.38 0.15 0.081 pC 3-Ir23 ®s*C 3-Ir23 35.83 0.30 0.096 sC 2-Ir23 ®s*C 3-Ir23 32.65 0.52 0.125 pC 3-Ir23 ®s*C 2-Ir23 30.00 0.34 0.091 sC 3-Ir23 ®s*C 2-Ir23 26.41 0.56 0.117 pC 1- C 2®p*C 4- C 5 24.77 0.28 0.076 pC 4- C 5®p* C 1- C 2 16.41 0.27 0.061

a Starred label (*) denotes antibonding, and Ry corresponds to the Rydberg NBO orbital.

As follows from Table 4, the two carbon atoms

in each ring are strongly conjugated by the

following electron density donations: LP (1) P15

σ*C2-Ir and LP (1) P19 σ*C3-Ir (where

LP(1)P15 and LP(1)P19 are s orbitals of atoms

P15 and P19, respectively). This mechanism can

explain a relatively high occupancy of σ*C2-Ir

antibonding orbital, as shown in Table 3. A gain

of occupancy in antibonding acceptor orbital

can be directly correlated with a weakening of

the bond associated with this orbital.

The results from NBO calculations may also

explain the fact that the calculated basal P–H

bonds are slightly longer than the remaining

P–H bonds of the apical phosphine groups. The

NBO calculations show that the occupancy

on σ*(P–Hbasal) orbital, 0.06e, is larger, in

comparison to 0.05e on σ*(P–Hapical) orbital.

On the other hand, the P-H bonds length of

basal phosphine group indicate that P–Ha > P–

Hb > P–Hc. Again, the NBO calculations show

that the occupancy on σ*(P–Ha) orbital, 0.07e,

is larger, in comparison to 0.05e on σ*(P–Hb)

and 0.04e on σ*(P–Hc) orbital.

Conclusion

According to our results, following conclusions

are derived for the iridabenzene:

1. The molecular structure of iridabenzene

calculated by the B3LYP density functional

method shows the square pyramidal

environment around iridium.

2. The Natural Bond Orbital (NBO) analysis

has provided the detailed insight into the type

R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012)12

of hybridization and the nature of bonding in

iridabenzene. The σ(Ir-C2) bonds are formed

from an sd1.18 hybrid on iridium atom and

π(Ir–C3) bond is formed from an sd5.21. The

strongest electron donation occurs from a lone

pair orbital on the phosphorous atoms, LP(1)P

to the antibonding acceptor σ*(Ir–C) orbitals.

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