Journal of Applied Chemical Research Journal of Applied Chemical Research, 20, 1, 7-13 (2012) w w w. j a c r. k i a u . a c . i r Natural Bond Orbital (NBO) Population Analysis of Iridabenzene (C5H5Ir)(PH3)3 R. Ghiasi*, E. Ebrahimi Mokaram Department 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) Abstract The 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 knowledge concerning iridabenzenes For the past decade the synthesis of compounds is still relatively limited due to the metallabenzenes have been examining [1- subtle nature of such compounds. 7] and their valence isomers starting from The objective of the present work is to (Z)-3-(2-iodoethenyl)cyclopropenes [8- investigate the nature of bonding in an 12]. Recently, the direct synthesis of a iridabenzene (Figure 1), by using natural bond series of iridabenzenes from nucleophilic orbital (NBO) analysis. We have shown that 3-vinylcyclopropenes reported. From the results from NBO calculations can provide experimental and theoretical examinations, the detailed insight into the electronic structure it is obersrved that the actual experimental 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 Natural bond orbital analysis stresses the role All calculations were carried out with the of intermolecular orbital interaction in the Gaussian 03 suite of program [13]. The complex, particularly charge transfer. This calculations of systems contain C, H, and F is is carried out by considering all possible described by the standard 6-31G(d) basis set interactions between illed donor and empty [14, 15]. For Ir element standard LANL2DZ acceptor NBOs and estimating their energetic basis set [16-18] are used and Ir is described by importance by second-order perturbation effective core potential (ECP) of Wadt and Hay theory. For each donor NBO (i) and acceptor pseudopotential [19] with a double-ζ valance NBO (j), the stabilization energy E(2) associated using the LANL2DZ. Geometry optimization with electron delocalization between donor was performed utilizing Becke’s hybrid and acceptor is estimated as: three-parameter exchange functional and the nonlocal correlation functional of Lee, Yang, and Parr (B3LYP) [20]. Vibrational analysis Where q is the orbital occupancy, ε , ε are i i j was performed at each stationary point found, diagonal elements and F is the off-diagonal i,j that conirm its identity as an energy minimum. NBO Fock matrix element. The population analysis has also been performed by the natural bond orbital method Result and discussion [21] at B3LYP/6-31G(d) level of theory using Structure NBO program [22] under Gaussian 2003 The optimized molecular structure of (C5H5Ir) program package. (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 C3, form a square pyramidal environment angles are listed in Table 1. According to the around iridium. The M-C bond length are on theoretical results, three phosphine phosphorous, the border between experimentally determined P11, P15, P19, and two carbon atoms, C2 and M-C (2.0-2.1) and M=C (1.8-2.0) bond lengths. 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 Bond distance(Å) Ir-P15H3 2.378 Ir-P19H3 2.378 Ir-P11H3 2.290 Ir-C2 2.001 Ir-C3 2.000 NBO analysis (0.48) 5d (8.47)6p (0.01)6d ( 0.02)7p (0.01). The Natural Bond Orbital (NBO) analysis of Thus, 68 core electrons, 8.95 valence electrons iridabenzene has provided the detailed insight (on 5d and 6s atomic orbitals) and 0.03 Rydberg into the nature of electronic conjugation between electrons (mainly on 6d and 7p orbitals) give the bonds in this molecule. Table 2 collects the the total of 75.98 electrons. This is consistent natural charges on atoms. The largest negative with the calculated natural charge on Ir atom in charges (-0.415 e) are located on two carbon iridabenzene +0.02 e, which corresponds to the atoms, C1 and C5. According to the NBO results, difference between 75.98e and the total number the electron coniguration of Ir is: [core]6s of electrons in the isolated Ir atom (77e). 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 Natural charge Ir 0.026 C1 -0.319 C2 -0.416 C3 -0.146 C4 -0.319 C5 -0.169 P11 0.175 P15 0.160 P19 0.160 Of the two carbon atoms in ring, the C2 natural orbitals. Three classes of NBOs are and C3 atoms coordinated to iridium have included, the Lewis-type (σ and π bonding larger negative charge (-0.415e). It should be or lone pair) orbitals, the valence non-Lewis emphasized that the calculated natural charge (acceptors, formally unilled) orbitals and the on the Hi atom of the phosphine group is more Rydberg NBOs, which originate from orbitals positive (0.15e) than the charge, on the other H outside the atomic valence shell. The calculated atoms. natural hybrids on atoms are also given in this Table 3 lists the calculated occupancies of table. C4 R. Ghiasi et al., J. Appl. Chem. Res., 20, 1, 7-13 (2012) 10 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) bond s(Ir-C) s(Ir-C) p(Ir-C) LP ( 1) P15 LP ( 1) P11 LP ( 1) P19 sP11- H13 sP11- H14 sP11- H12 sP15- H16 sP15- H17 occupancy Hybridb AO (%)c 1.83003 1.86769 1.63678 1.65218 1.62366 1.65237 1.98496 sd 1.18 sd 1.76 sd 5.21 sp 0.79 sp 0.82 sp 0.79 sp 5.27 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%) s( 55.81%)p( 44.17%)d( 0.02%) s(54.85%)p( 45.11%)d( 0.04%) s( 55.80%)p( 44.18%)d ( 0.02%) s( 15.80%)p( 83.19%)d( 1.01%) 1.98493 sp 5.26 s( 15.81%)p( 83.18%)d( 1.01%) 1.98137 sp 6.31 s( 13.54%)p( 85.44%)d ( 1.02%) 1.98802 1.98778 sp 5.52 sp 5.57 s( 15.20%)p( 83.83%)d( 0.98%) s( 15.06%)p( 83.95%)d ( 0.99%) 1.98621 sp 6.14 s( 13.87%)p( 85.14%)d( sP15- H18 5.57 1.98778 sp s( 15.07%)p( 83.94%)d( sP19- H20 1.98804 sp 5.51 s( 15.20%)p( 83.82%)d( sP19-H21 1.98618 sp 6.14 s( 13.86%)p( 85.15%)d( sP19- H22 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. 0.99%) 0.99%) 0.98%) 0.99%) Table 4. Second-order interaction energy (E , kcal/mol) between donor and acceptor orbitals in In the NBO method, delocalization of electron phosphorous atom to iridium molecular orbitals. density (ED) between occupied Lewis-type E2 As seen from E(j)-E(i) Table 3, the σ(Ir–C2) bond is a F(i,j) ® 273.76 0.129 on iridium s formally ® sunoccupied (antibonding orbitals and formed from an0.03 sd1.18 hybrid ®s 116.48 0.53 of 45.83%s, 0.228 0.09%p and ®s NBOs corresponds to or Rydberg) non-Lewis (which is the mixture 120.51 ®p 0.56 0.236 a stabilizing interaction. The47.3854.08%d atomic 0.15 orbitals). On0.081 the other hand, p donor–acceptor ®p p 55.66 ®s 0.38 0.140 strength ofs this interaction can be estimated by32.65σ(Ir–C3) bond is0.52 formed from an sd1.76 hybrid 0.125 ®s 35.83 0.30 0.096 0.34 0.091 ®s the secondp order perturbation theory. Thus, the30.00on iridium (which is the mixture of 36.16%s, results ®s s ®p p obtained from ®p p NBO analysis 26.41 provide24.770.07%p 16.41 and 0.56 0.28 63.77%d 0.27 0.117 atomic0.076 orbitals). 0.061 The convenient basis for investigating conjugative π(Ir–C3) bond is formed from an sd5.21 hybrid interactions in molecular systems. on iridium (which is the mixture of 16.09%s, According to calculations, the iridium atom 0.11%p and 83.81%d atomic orbitals). forms a single bond (sigma bond) and a double Table 4 lists the selected values of the calculated bond (σ and π bonds) with two carbon atoms second order interaction energy (E2) between C2 and C3 atoms, respectively. While the two donor–acceptor orbitals in iridabenzene. The bonds between iridium and the phosphine strongest interactions are the electron donations groups can be described as donation of electron from a lone pair orbital on the phosphorous density from a lone pair (LP) orbital on each 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) of electron density from the phosphine P19 σ*( Ir–C2). As shown in Table 3, the ligands to the Ir molecular orbitals has a clear LP(1)P orbital has 44.2% p-character and is correspondence to a chemical picture of the occupied by 1.652 electrons (this is consistent coordination bonds (H3P Ir). As follows from with a delocalization of electron density from the calculated (E2) values, the P Ir bonds are the idealized occupancy of 2.0e). The donation stronger than σ(Ir–C) bonds in iridabenzene. Table 4. Second-order interaction energy (E 2, kcal/mol) between donor and acceptor orbitals in iridabenzene. E2 E(j)-E(i) Donor®acceptora 273.76 0.03 s* C 3-Ir23 ® s*C 2-Ir23 120.51 0.56 LP ( 1) P15® s*C 2-Ir23 116.48 0.53 LP ( 1) P19 ®s*C 3-Ir23 55.66 0.38 LP ( 1) P11® p* C 3-Ir23 47.38 0.15 pC 4- C 5 ® p*C 3-Ir23 35.83 0.30 pC 3-Ir23 ® s*C 3-Ir23 32.65 0.52 sC 2-Ir23 ® s*C 3-Ir23 30.00 0.34 pC 3-Ir23 ® s*C 2-Ir23 26.41 0.56 sC 3-Ir23 ® s*C 2-Ir23 24.77 0.28 pC 1- C 2® p*C 4- C 5 16.41 0.27 pC 4- C 5® p* C 1- C 2 a Starred label (*) denotes antibonding, and Ry corresponds to the Rydberg NBO orbital. F(i,j) 0.129 0.236 0.228 0.140 0.081 0.096 0.125 0.091 0.117 0.076 0.061 As follows from Table 4, the two carbon atoms comparison to 0.05e on σ*(P–Hapical) orbital. in each ring are strongly conjugated by the On the other hand, the P-H bonds length of following electron density donations: LP (1) P15 basal phosphine group indicate that P–Ha > P– σ*C2-Ir and LP (1) P19 σ*C3-Ir (where Hb > P–Hc. Again, the NBO calculations show LP(1)P15 and LP(1)P19 are s orbitals of atoms that the occupancy on σ*(P–Ha) orbital, 0.07e, P15 and P19, respectively). This mechanism can is larger, in comparison to 0.05e on σ*(P–Hb) explain a relatively high occupancy of σ*C2-Ir and 0.04e on σ*(P–Hc) orbital. antibonding orbital, as shown in Table 3. A gain of occupancy in antibonding acceptor orbital Conclusion can be directly correlated with a weakening of According to our results, following conclusions the bond associated with this orbital. are derived for the iridabenzene: The results from NBO calculations may also 1.The molecular structure of iridabenzene explain the fact that the calculated basal P–H calculated by the B3LYP density functional bonds are slightly longer than the remaining method shows the square pyramidal P–H bonds of the apical phosphine groups. The environment around iridium. NBO calculations show that the occupancy 2. 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