J. Phys. Chem. A 2010, 114, 12099–12105
12099
Nature of M-Ga Bonds in Dihalogallyl Complexes (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I): A DFT Study Krishna K. Pandey,*,† Pankaj Patidar,† and Simon Aldridge*,‡ School of Chemical Sciences, DeVi Ahilya UniVersity Indore, Indore 452001, India, and Inorganic Chemistry Laboratory, Department of Chemistry, UniVersity of Oxford, South Parks Road, Oxford, United Kingdom OX1 3QR ReceiVed: August 4, 2010; ReVised Manuscript ReceiVed: October 4, 2010
Density functional theory (DFT) calculations have been performed on the terminal dihalogallyl complexes of iron, ruthenium, and osmium (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os; X ) Cl, Br, I) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) at the BP86/TZ2P/ZORA level of theory. On the basis of analyses suggested by Pauling, the M-Ga bonds in all of the dihalogallyl complexes are shorter than M-Ga single bonds; moreover, on going from X ) Cl to X ) I, the optimized M-Ga bond distances are found to increase. From the perspective of covalent bonding, however, π-symmetry contributions are, in all complexes, significantly smaller than the corresponding σ-bonding contribution, representing only 4-10% of the total orbital interaction. Thus, in these GaX2 complexes, the gallyl ligand behaves predominantly as a σ donor, and the short M-Ga bond lengths can be attributed to high gallium s-orbital character in the M-Ga σ-bonding orbitals. The natural population analysis (NPA) charge distributions indicate that the group 8 metal atom carries a negative charge (from -1.38 to -1.62) and the gallium atom carries a significant positive charge in all cases (from +0.76 to +1.18). Moreover, the contributions of the electrostatic interaction terms (∆Eelstat) are significantly larger in all gallyl complexes than the covalent bonding term (∆Eorb); thus, the M-Ga bonds have predominantly ionic character (60-72%). The magnitude of the charge separation is greatest for dichlorogallyl complexes (compared to the corresponding GaBr2 and GaI2 systems), leading to a larger attractive ∆Eelstat term and to M-Ga bonds that are stronger and marginally shorter than in the dibromo and diiodo analogues. Introduction Since the first reports of structurally characterized transitionmetal boryl complexes, LnM(BX2)m, in 1990,1,2 the chemistry of such systems has blossomed, leading to an in-depth understanding of both electronic structure and reactivity.1-15 In sharp contrast, the coordination chemistry of gallyl ligands -GaX2 (e.g., 1 and 2; see Chart 1) has been developed only over a more recent time frame. A significant fraction of such transitionmetal complexes contain heterocyclic gallyl ligands (1), primarily resulting from the work of Jones and co-workers; M-Ga bond distances for structurally characterized gallyl complexes are given in Table 1.17-37 Within this sphere, very few structurally characterized mononuclear dihalogallyl complexes have been reported (i.e., of type 2). Fischer et al. reported the iron dibromogallyl complex (η5C5Me5)[Ga(η5-C5Me5)](PPh3)Fe(GaBr2),18 and Aldridge and coworkers reported the first example of an organometallic diiodogallyl complex, (η5-C5Me5)(dppe)Fe(GaI2) [dppe )1,2-bis(diphenylphosphino)ethane].19,20 In addition, Braunschweig et al. recently isolated dibromogallyl and diiodogallyl complexes of platinum, trans-X(PCy3)2Pt(GaX2) (X ) Br, I), using an oxidative addition methodology.21 To the best of our knowledge, optimized structures and M-Ga bonding analyses for half-sandwich dihalogallyl complexes of group 8 metals have not been reported. Thus, in the current article, we report geometric and electronic structure
calculations on the iron, ruthenium, and osmium complexes (η5C5H5)(Me3P)2M(GaX2) (for M ) Fe, I corresponds to X ) Cl, II to X ) Br, and III to X ) I; for M ) Ru, IV corresponds to X ) Cl, V to X ) Br, and VI to X ) I; and for M ) Os, VII corresponds to X ) Cl, VIII to X ) Br, and IX to X ) I) and (η5-C5H5)(OC)2Fe(GaX2) (X corresponds to X ) Cl, XI to X ) Br, and XII to X ) I). An alternative conformer of (η5C5H5)(Me3P)2Fe(GaI2) (XIII) with both iodine atoms constrained to lie in the Cs plane was also optimized, although this system is 3.3 kcal mol-1 less stable than the global minimum (see Supporting Information). In carrying out these analyses, we intended to answer two questions; the first addresses the degree of ionic and covalent character of the M-Ga bonds, whereas the second addresses the extent of the M r Ga σ-bonding and M f Ga π-back-bonding contributions to these bonds. These factors are addressed not only as a function of the halogen substituent X, but also as a function of the metal, M, and the ancillary ligand framework. CHART 1
* To whom correspondence should be addressed. E-mail: kkpandey.
[email protected] (K.K.P.),
[email protected] (S.A.). † Devi Ahilya University Indore. ‡ University of Oxford.
10.1021/jp1073297 2010 American Chemical Society Published on Web 10/26/2010
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TABLE 1: Selected Structurally Characterized Transition-Metal Gallyl Complexes complexa
M-Ga bond distance (Å)
ref(s)
Cp*Fe(GaCp*)(GaBr2)(PPh3) Cp*Fe(dppe)(GaI2) Cp*Fe(dppe)Ga(Mes)I Cp*Fe(CO)2Ga(Mes)I trans-(Cy3P)2Pt(Br)(GaBr2) trans-(Cy3P)2Pt(I)(GaI2) CpFe(CO)2Ga(Mes*)Cl Cp*Fe(CO)2Ga(Mes*)I [CpV(CO)3[Ga{[N(Dipp)C(H)]2}]][Cp′Mn(CO)2[Ga{[N(Dipp)C(H)]2}]][CpCo(CO)[Ga{[N(Dipp)C(H)]2}]]Cp′2V[Ga{[N(Dipp)C(H)]2}] Cp′2V[Ga{[N(Dipp)C(H)]2}]2 Mn{CH(SiMe3)2}2[Ga{[N(Dipp)C(H)]2}] Fe(CO)4[Ga{[N(Dipp)C(H)]2}] CpNi[Ga{[N(Dipp)C(H)]2}]2 Ni{C[N(Me)C(Me)]2}2[Ga{[N(Dipp)C(H)]2}2 Ir(COD)(IMes)[Ga{[N(Dipp)C(H)]2}] Rh(COD)(IMes)[Ga{[N(Dipp)C(H)]2}] (IMes)Cu[Ga{[N(Dipp)C(H)]2}] (IMes)Ag[Ga{[N(Dipp)C(H)]2}] (IMes)Au[Ga{[N(Dipp)C(H)]2}] (IPr)Cu[Ga{[N(Dipp)C(H)]2}] (IPr)Ag[Ga{[N(Dipp)C(H)]2}] trans-Pt{Ga{[N(Dipp)C(H)]2}}2(PEt3)2 cis-Pt{Ga{[N(Dipp)C(H)]2}}2(PEt3)2 trans-Ni{Ga{[N(Dipp)C(H)]2}}2(PEt3)2 trans-Pd{Ga{[N(Dipp)C(H)]2}}2(PEt3)2 trans-PdCl{Ga{[N(Dipp)C(H)]2}}(PEt3)2 trans-NiCl{Ga{[N(Dipp)C(H)]2}}(PEt3)2 PtCl{Ga{[N(Dipp)C(H)]2}}(dcpe) PtCl{Ga{[N(Dipp)C(H)]2}}(dppe) Pt{Ga{[N(Dipp)C(H)]2}}2(dppe) Ni{Ga{[N(Dipp)C(H)]2}}2(tmeda) Pd{Ga{[N(Dipp)C(H)]2}}2(tmeda) Pd{Ga{[N(Dipp)C(H)]2}}2(dppm) Pt{Ga{[N(Dipp)C(H)]2}}2(dppm) Pt{Ga{[N(Dipp)C(H)]2}}2(COD)
2.281(1) 2.3236(14) 2.3550(1) 2.3113(12) 2.3403(4) 2.3383(4) 2.346(1) 2.372(2) 2.4618(13) 2.3105(9) 2.2347(7) 2.5303(9) 2.5093(12) 2.6658(10) 2.3068(8) 2.2196(11), 2.2154(11) 2.3242(6) 2.4689(5) 2.4259(6) 2.3066(6) 2.4161(5) 2.3782(6) 2.2807(5) 2.4108(8) 2.4308(6) 2.4495(6), 2.4313(7) 2.3614(7) 2.4514(8) 2.3551(6) 2.2878(5) 2.4151(7) 2.3929(7) 2.4157(6), 2.4167(7) 2.3051(8), 2.3503(8) 2.3959(9) 2.4032(8) 2.4170(8), 2.4218(7) 2.3838(7)
18 19, 20 19, 20 19, 20 21 21 22 22 23 23 23 23 23 23 24 25 25 27 27 27 27 27 27 27 37 37 37 37 37 37 37 37 37 37 37 37 37 37
a Cy ) cyclohexyl; Cp ) C5H5, Cp* ) C5Me5, Cp′ ) C5H4Me, Mes ) C6H2Me3-2,4,6, Mes* ) C6H2But3-2,4,6, Dipp ) C6H3Pri2-2,6, COD ) 1,5-cyclooctadiene, IMes ) :C{N(Mes)C(H)}2, IPr ) :C{N(Dipp)C(H)}2. dppe )1,2-bis(diphenylphosphino)ethane, dcpe ) 1,2-bis(dicyclohexylphosphino)ethane, dppm ) bis(diphenylphosphino)methane, tmeda ) N,N,N′,N′-tetramethylethylenediamine.
Computational Method 5
Calculations on the halogallyl complexes (η -C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) were performed at the nonlocal DFT level of theory using the exchange functional of Becke and the correlation functional of Perdew (BP86).38,39 Scalar relativistic effects were considered using the ZORA formalism.40 Uncontracted Slatertype orbitals (STOs) using triple-ζ basis sets augmented by two sets of polarization functions were employed for the SCF calculations.41 The (1s)2 core electrons of carbon and oxygen, (1s2s2p)10 core electrons of phosphorus and chlorine, (1s2s2p3s3p)18 core electrons of gallium and bromine, (1s2s2p3s3p3d)28 core electrons of ruthenium, (1s2s2p3s3p3d4s4p)36 core electrons of iodine, and (1s2s2p3s3p3d4s4p4d)46 core electrons of osmium were treated by the frozen-core approximation.42 An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to present the Coulomb and exchange potentials accurately in each SCF cycle.43 The calculations were performed utilizing the ADF-2008.01 package.44 The bonding between the metal {[(η5-C5H5)(Me3P)2M]+ or [(η5-C5H5)(OC)2Fe]+} and singlet gallyl [GaX2]- fragments in all complexes studied (I-XII) was analyzed in Cs symmetry using the energy decomposition scheme of the ADF package, based on the methods of Morokuma45 and Ziegler and Rauk.46
Based on these studies, the bond energy ∆E between the fragments can be decomposed as
∆E ) ∆Eint + ∆Eprep
(1)
where ∆Eprep is the energy required to promote the free fragments from their equilibrium structure in the electronic ground state to that which they take up in the molecule
∆Eprep ) Etotal(distorted fragments) Etotal(fragments in the equilibrium structure) (2) In eq 1 ∆Eint is the instantaneous interaction energy between the two fragments of the molecule. It can be decomposed into three main components
∆Eint ) ∆Eelstat + ∆EPauli + ∆Eorb
(3)
where ∆Eelstat describes the classical Coulombic interaction between the fragments; ∆EPauli (the exchange repulsion or Pauli repulsion) takes into account the destabilizing two-orbital, threeor four-electron interactions between the occupied orbitals of
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TABLE 2: Selected Optimized Geometric Parameters of the Metal Gallyl Complexes (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os; X ) Cl, Br, I) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) (η5-C5H5)(Me3P)2Fe(GaX2)
(η5-C5H5)(Me3P)2Ru(GaX2)
(η5-C5H5)(Me3P)2Os(GaX2)
(η5-C5H5)(OC)2Fe(GaX2)
Cl
Br
I
Cl
Br
I
Cl
Br
I
Cl
Br
I
(I)
(II)
(III)
(IV)
(V)
(VI)
(VII)
(VIII)
(IX)
(X)
(XI)
(XII)
M-Ga
2.274
2.286
Ga-X
2.251
2.419
M-P/C C-O
2.205
2.206
M-Ga-X
128.2
127.9
P-M-Ga X-Ga-X
91.3 102.9
92.2 103.0
a
2.284 [2.3240(12)]a 2.638 [2.6287(9)]a 2.208
127.9 [131.67(4)]a 92.6 102.7
2.373
Bond Distances (Å) 2.383 2.385
2.396
2.404
2.406
2.309
2.323
2.325
2.250
2.415
2.634
2.246
2.412
2.630
2.191
2.350
2.558
2.300
2.302
2.302
2.306
2.308
2.309
1.750 1.159
1.750 1.159
1.751 1.159
128.3
Bond Angles (deg) 128.1 127.9 128.2
128.0
127.8
125.1
124.7
124.3
89.6 102.8
90.5 102.9
90.3 102.9
90.8 103.0
89.9 109.2
90.1 110.1
89.9 110.9
91.1 103.0
89.4 102.8
X-ray structure data for (η5-C5Me5)(dppe)Fe(GaI2).19,20
Figure 1. Optimized geometries of diiodogallyl complexes (η5-C5H5)(Me3P)2M(GaI2) (M ) Fe, Ru, Os) and (η5-C5H5)(OC)2Fe(GaI2). Important calculated bond distances and angles for I-XII are given in Table 2.
both fragments; and ∆Eorb represents orbital interactions between the occupied and virtual orbitals of the two fragments. It has been suggested that the covalent and electrostatic character of the bond can be given by the ratio ∆Eelstat/∆Eorb.47-50 The bond order and natural population analysis (NPA) charges of the complexes (I-XII) were examined by NBO analysis.51 Atomic charges were also computed using the recently developed Voronoi deformation density (VDD).52 All molecular orbital pictures were constructed using MOLDEN.53 Results and Discussion Geometries. The important bond distances and angles of the dihalogallyl complexes I-XII calculated at the BP86/TZ2P level of theory are presented in Table 2. The structures of (η5C5H5)(Me3P)2M(GaI2) (M ) Fe, Ru, Os) and (η5-C5H5)(OC)2Fe(GaI2) (i.e., complexes III, VI, IX, and XII) are shown in Figure 1. The structures of the related chloro- and bromogallyl
complexes are very similar to those presented in this figure and are therefore not included. The optimized Cartesian coordinates of all complexes are given in the Supporting Information. Only single representative examples of monomeric group 8 dibromo- and diiodogallyl complexes have been reported in the literature;18-20 related dichloro complexes have not yet been structurally characterized. Thus, we report the calculated structures of a range of dihalogallyl complexes of iron, ruthenium, and osmium.54 As seen in Table 2, the optimized Fe-Ga and Ga-I bond lengths for III are in good agreement with the experimental values for (η5-C5H5)(dppe)Fe(GaI2).20 The M-Ga bond distances in the complexes (η5-C5H5)(Me3P)2M(GaX2) (where M ) Fe, Ru, and Os; I-IX) and (η5C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, and I; X-XII) are shorter than expected for M-Ga single bonds estimated on the basis of covalent radius predictions (Fe-Ga ) 2.37 Å, Ru-Ga ) 2.45 Å, Os-Ga ) 2.47 Å).55 Using the relationship between
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Figure 2. Natural population analysis (NPA) charge distributions in dihalogallyl complexes (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os) (I-IX) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) (X-XII). The values in square brackets are the Wiberg bond indices (WBIs).
the bond order and bond distance suggested by Pauling,56 the bond orders for the optimized M-Ga bond distances in these complexes are in the range of 1.2-1.4. Interestingly, on going from X ) Cl to X ) I, the calculated M-Ga bond distance increases, for example, in the orders 2.274 Å (I) < 2.286 Å (II) ≈ 2.284 Å (III) for the Fe/PMe3 systems, 2.373 Å (IV) < 2.383 Å (V) < 2.385 Å (VI) for the Ru/PMe3 systems, 2.396 Å (VII) < 2.404 Å (VIII) < 2.406 Å (IX) for the Os/PMe3 systems, and 2.309 Å (X) < 2.323 Å (XI) < 2.325 Å (XII) for the Fe/CO systems. The gallium-bound halides thus exert a (relatively minor) influence on the length of the M-Ga bonds. This effect we relate to the increased charge separation s and consequently larger (attractive) electrostatic interaction term s for GaCl2 complexes compared to the analogous GaBr2- and GaI2containing systems (vide infra). Bonding Analysis of the M-GaX2 Bonds in Complexes I-XII. We begin the analysis of the M-Ga bonding in metal gallyl complexes I-XII with a discussion of bond orders (Wiberg bond indices, WBIs),57 and atomic charges. The WBIs and natural population analysis (NPA) charges of complexes I-XII are presented in Figure 2. The results of the NBO analyses of iron gallyl complexes (η5-C5H5)(Me3P)2M(GaX2) (I-III) and (η5-C5H5)(OC)2Fe(GaX2) (X-XII) are reported in Table 3. As shown in Figure 2, the Wiberg bond indices of the M-Ga bonds in I-XII are in the range 0.61-0.79, thus suggesting
that the covalent contributions to the M-Ga bonds in I-XII might not be the dominant binding interactions. On going from X ) Cl to X ) I, the WBIs of the M-Ga bonds decrease, indicating a progressive weakening of the M-Ga bond. The calculated NPA charge distributions (Figure 2) indicate that the metal atom carries a negative charge (from -1.38 to -1.62) and the gallium atom carries a relatively large positive charge (from 0.76 to 1.18). As a result, a large electrostatic interaction can be predicted between the metal and gallium atoms (see Table 4), which will decrease on going from X ) Cl to X ) I. The PMe3/CO ligands and C5H5 group carry positive charges, whereas the halides are negatively charged in complexes I-XII. A more definitive picture of M-Ga bonding is obtained through NBO analysis of the delocalized Kohn-Sham orbitals. The compositions of the Fe-Ga σ-bonding orbitals are listed in Table 3. In iron gallyl complexes I-III and X-XII, the Fe-Ga σ-bonding orbitals are significantly polarized toward the metal atom, whereas the Ga-X σ-bonding orbitals are significantly polarized toward the halides. The occupations of the Fe-Ga σ-bonding orbitals are in the range of 1.701-1.732. It is significant that the hybridization at the gallium atoms of the M-Ga σ bonds is approximately sp, that is, the gallium atoms in the gallyl complexes are not sp2-hybridized. The relatively short M-Ga bond distances can thus be explained on the basis of the high s character at gallium (ca. 47-53%) in the M-Ga bonds.
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TABLE 3: Results of NBO Analyses of the Iron Gallyl Complexes (η5-C5H5)(L)2Fe(GaX2) (L ) PMe3, CO; X ) Cl, Br, I) at the BP86/TZ2P Level (η5-C5H5)(Me3P)2Fe(GaX2)
Fe-Ga σ-bond occupancy M % %s %p %d %f Ga % %s %p %d Ga-X σ-bond occupancy Ga % %s %p %d X % %s %p %d
(η5-C5H5)(OC)2Fe(GaX2)
Cl
Br
I
Cl
Br
I
(I)
(II)
(III)
(X)
(XI)
(XII)
1.732
1.730
1.720
1.707
1.708
1.701
63.97 15.45 38.63 45.92 0.00
62.65 15.40 39.04 45.56 0.00
62.20 15.32 39.69 44.98 0.00
71.48 18.42 18.13 63.45 0.00
70.06 18.70 18.80 62.50 0.00
69.23 18.96 19.79 61.25 0.00
36.03 52.14 47.72 0.14
37.35 52.77 47.13 0.10
37.80 52.79 47.12 0.09
28.52 47.40 52.42 0.17
29.94 48.10 51.78 0.12
30.77 48.95 50.95 0.10
1.953
1.944
1.931
1.951
1.941
1.927
15.70 24.34 75.16 0.50
18.26 23.86 75.47 0.67
20.96 23.64 75.43 0.94
18.91 26.73 72.85 0.42
22.41 26.29 73.15 0.56
26.41 25.71 73.50 0.78
84.30 26.84 73.02 0.14
81.74 23.71 76.16 0.13
79.04 20.92 78.99 0.10
81.09 23.17 76.64 0.18
77.59 19.83 79.99 0.18
73.59 17.12 82.75 0.14
TABLE 4: Energy Decomposition Analyses of the Metal Gallyl Complexes (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os; X ) Cl, Br, I) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) at the BP86/TZ2P Levela (η5-C5H5)(Me3P)2Fe(GaX2)
∆Eint ∆Epauli ∆Eorb ∆Eσ+π (a′) ∆Eπ(a′′)b ∆Eelstat ICc ∆Eprep ∆E(-BDE)
(η5-C5H5)(Me3P)2Ru(GaX2)
(η5-C5H5)(Me3P)2Os(GaX2)
(η5-C5H5)(OC)2Fe(GaX2)
Cl
Br
I
Cl
Br
I
Cl
Br
I
Cl
Br
I
(I)
(II)
(III)
(IV)
(V)
(VI)
(VII)
(VIII)
(IX)
(X)
(XI)
(XII)
-129.0 171.3 -90.7 82.2 -8.5 (9.4%) -209.6 [69.8%] 8.8 -120.3
-124.0 156.6 -87.4 -78.9 -8.5 (9.7%) -193.3 [68.9%] 7.9 -116.1
-120.1 151.3 -88.4 -79.1 -9.4 (10.5%) -183.0 [67.4%] 7.9 -112.2
-133.0 189.8 -90.9 -82.9 -8.0 (8.8%) -231.9 [71.8%] 10.5 -122.6
-128.0 174.4 -87.7 -79.7 -8.1 (9.2%) -214.7 [71.0%] 9.6 -118.4
-124.0 166.9 -88.5 -79.5 -8.9 (10.1%) -202.4 [69.6%] 9.6 -114.5
-139.2 221.9 -100.4 -92.3 -8.1 (8.1%) -260.7 [72.1%] 11.8 -127.4
-134.2 206.1 -97.6 -89.3 -8.3 (8.5%) -242.7 [71.3%] 11.1 -123.1
-130.1 197.6 98.3 -89.2 -9.1 (9.3%) -229.4 [70.0%] 11.0 -119.0
-172.0 158.5 -121.4 -115.9 -5.5 (4.5%) -209.2 [63.3%] 14.4 -157.7
-166.8 144.1 -118.7 -113.2 -5.4 (4.6%) -192.2 [61.8%] 13.3 -153.4
-163.5 137.4 -120.7 -114.8 -5.9 (4.9%) -180.2 [59.9%] 13.8 -149.6
a Energy contributions in kcal mol-1. b Values in parentheses are the percentage contributions to the total orbital interactions reflecting the π character of the bond. c Percentage ionic character.
Energy Decomposition Analysis of the M-Ga Bonding in Complexes I-XII. To further probe the nature of the M-Ga bonds, we carried out energy decomposition analyses on metal gallyl complexes I-XII in Cs symmetry. The results are presented in Table 4 and Figure 3. It should be emphasized that the calculated energy contribution ∆Eπ gives only the outof-plane (π⊥) component of the [(η5-C5H5)(L)2M]+ f [GaX2]π back-donation. This is because the molecules have Cs symmetry and, thus, the orbitals can only have a′(σ) or a′′(π) symmetry. Thus, the energy contributions of the a′(σ) orbitals come from the [(η5-C5H5)(L)2M]+ r [GaX2]- σ donation but also from the in-plane [(η5-C5H5)(L)2M]+ f [GaX2]- π backdonation. For molecules that have only Cs symmetry, it is not possible to separate the latter two interactions because both orbitals have a′ symmetry. The frontier orbitals of metal fragments [(η5-C5H5)(L)2M]+ are the π highest occupied molecular orbital (HOMO) and the σ lowest unoccupied molecular orbital (LUMO), whereas those of the [GaX2]- fragments are the σ HOMO and π LUMO.
The bond dissociation energies liste in Table 4 reveal the expected periodic trend in bond strengths due to d-orbital extent/ energy: the Os-Ga bonds are stronger than the corresponding Fe-Ga and Ru-Ga linkages. Figure 3 depicts the variation in bond dissociation energies ∆E (-De), interaction energies ∆Eint, orbital interaction energies ∆Eorb, and electrostatic interactions ∆Eelstat for compounds I-XII. The breakdown of the interaction energy ∆Eint into the repulsive term ∆EPauli and the attractive terms ∆Eorb and ∆Eelstat shows that the absolute values of the terms are relatively larger for X ) Cl and decrease on going to X ) Br and X ) I for all four sets of complexes (Table 4). The values of the electrostatic interaction, ∆Eelstat, increase on going from M ) Fe to M ) Os (Figure 3). The nature and properties of the HOMOs and LUMOs of the fragments [(η5C5H5)(Me3P)2M)]+ (M ) Fe, Ru, Os) and [GaX2]- play a role in explaining the orbital interaction differences. The energies of the HOMOs and LUMOs of the metal fragments vary as [(η5-C5H5)(Me3P)2Fe]+ (HOMO, -7.923 eV; LUMO, -7.112 eV), [(η5-C5H5)(Me3P)2Ru]+ (HOMO, -8.186 eV; LUMO,
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Figure 3. Trends of the interaction energy contribution, orbital interaction (covalent contribution), electrostatic interaction (ionic contribution), and bond dissociation energy (-De) to the M-Ga bond in the dihalogallyl complexes (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os) (I-IX) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) (X-XII).
-6.823 eV), [(η5-C5H5)(Me3P)2Os]+ (HOMO, -8.086 eV; LUMO, -6.747 eV), while the energies of the HOMO and LUMO orbitals of the [GaX2]- species are [GaCl2]- (HOMO, -0.942 eV; LUMO, 2.280 eV), [GaBr2]- (HOMO, -1.150 eV; LUMO, 1.932 eV), [GaI2]- (HOMO, -1.337 eV; LUMO, 1.480 eV). As a result, the LUMO of the osmium fragment [(η5C5H5)(Me3P)2Os]+ comes closer in energy to the HOMO of the [GaX2]- fragments, allowing for better [(η5-C5H5)(Me3P)2Os]+ r [GaX2]- electron donation and, thus, relatively greater electrostatic interaction, ∆Eelstat, between the two fragments in osmium complexes than in ruthenium complexes. The contributions of the electrostatic interaction terms ∆Eelstat are significantly larger in all gallyl complexes (I-XII) than the covalent bonding ∆Eorb term; the M-Ga bond in each case has a degree of ionic character of between 60% and 72%. Table 4 also gives the breakdown of the orbital interactions ∆Eorb into M r Ga σ-donation and M f Ga π-back-donation components. It is important to note that the π-bonding contribution is, in all complexes, significantly smaller than the σ-bonding contribution (4-10% of the total orbital contribution; see Table 4). From
Pandey et al. the data presented in Table 4, it can be concluded that (i) gallyl ligands in these systems behave predominantly as σ donors; (ii) irrespective of the halide, the interaction energy increases in the order Fe < Ru < Os; and (iii) the absolute values of the ∆EPauli, ∆Eint, and ∆Eelstat contributions to the M-Ga bonds decrease in the order Cl > Br > I. Upon replacing the ancillary PMe3 ligands (in complexes I-III) by a better π-acceptor CO co-ligand (as in complexes X-XII), the orbital contributions, interaction energies, and dissociation energies of the Fe-Ga bonds increase significantly. Thus, in the complexes (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I; X-XII), the GaX2 ligand acts as a better σ donor and relatively poorer π acceptor than in complexes I-III, consistent with the less electron-rich nature of the iron center. That the ligand binding energies are higher in X-XII than in I-III further reflects the greater importance of σ donation (over π-acceptor behavior) for the GaX2 ligand in these systems. To visualize the significant M-Ga covalent interactions, envelope plots of relevant orbitals for iron complexes (η5C5H5)(Me3P)2Fe(GaI2) (III) and (η5-C5H5)(OC)2Fe(GaI2) (VII) are given in Figures 4 and 5, respectively. Parts A (HOMO) and B (HOMO - 3) of Figure 4 give a pictorial description of the Fe-Ga σ bonding in III, whereas Figure 4C depicts Ga-I σ bonding; Figure 4D (LUMO + 2) is essentially a nonbonding π orbital centered at gallium. Similarly, parts A (HOMO) and B (HOMO - 6) of Figure 5 are Fe-Ga σ bonding, whereas Figure 5C is a Ga-I σ bonding and Figure 5D (LUMO + 1) is a nonbonding π orbital at gallium. Consistent with the idea that dihalogallyl ligands act as poor π-acceptor ligands, the predominant molecular orbital featuring a significant contribution from the gallium pπ atomic orbital is a nonbonding MO in each case (LUMO + 2 for III, LUMO + 1 for VII) occurring at 3.16 and 3.28 eV above the respective HOMOs. Conclusions Bonding analyses carried out on metal halogallyl complexes of the types (η5-C5H5)(Me3P)2M(GaX2) (M ) Fe, Ru, Os) and (η5-C5H5)(OC)2Fe(GaX2) (X ) Cl, Br, I) allow the following conclusions to be drawn: (i) The contributions of the electrostatic interactions ∆Eelstat are significantly larger in all gallyl complexes (I-XII) than the
Figure 4. Plots of (A,B) Fe-Ga σ bonding, (C) Ga-X σ bonding, and (D) nonbonding π molecular orbitals of (η5-C5H5)(Me3P)2Fe(GaI2).
Figure 5. Plots of (A,B) Fe-Ga σ bonding, (C) Ga-X σ bonding, and (D) nonbonding π molecular orbitals of (η5-C5H5)(OC)2Fe(GaI2).
Nature of M-Ga Bonds in Dihalogallyl Complexes covalent bonding ∆Eorb contributions. Thus, the M-Ga bond in these gallyl systems is predominantly ionic in character (60-72%). (ii) The magnitude of the charge separation is greatest for dichlorogallyl complexes (compared to the corresponding GaBr2 and GaI2 systems), leading to a larger attractive ∆Eelstat term and to M-Ga bonds that are stronger and marginally shorter than in dibromo and diiodo analogues. (iii) In all of the complexes studied, the π-bonding component of the total orbital contribution is significantly smaller than the σ-bonding component. Thus, in these complexes, the GaX2 ligand behaves predominantly as a σ donor. Short M-Ga bonds can be attributed to high gallium s-orbital character in the M-Ga σ-bonding orbitals. (iv) The interaction energy increases in the order Fe < Ru < Os, and the absolute values of the ∆EPauli, ∆Eint, and ∆Eelstat contributions to the M-Ga bonds decrease in the order Cl > Br > I. Supporting Information Available: Cartesian coordinates of the optimized geometries of the metal dihalogallyl complexes I-XII. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baker, R. T.; Ovenall, D. W.; Calabrese, J. C.; Westcott, S. A.; Taylor, N. J.; Williams, I. D.; Marder, T. B. J. Am. Chem. Soc. 1990, 112, 9399. (2) Knorr, J. R.; Merola, J. S. Organometallics 1990, 9, 3008. (3) Braunschweig, H. Angew. Chem., Int. Ed. 1998, 37, 1786. (4) Irvine, G. I.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV. 1998, 98, 2685. (5) Smith, M. R., III. Prog. Inorg. Chem. 1999, 48, 505. (6) Braunschweig, H.; Colling, M. Coord. Chem. ReV. 2001, 223, 1. (7) Braunschweig, H. AdV. Organomet. Chem. 2004, 51, 163. (8) Aldridge, S.; Coombs, D. L. Coord. Chem. ReV. 2004, 248, 535. (9) Braunschweig, H.; Rais, D. Heteroat. Chem. 2005, 16, 566. (10) Braunschweig, H.; Kollann, C.; Rais, D. Angew. Chem., Int. Ed. 2006, 45, 5254. (11) Kays, D. L.; Aldridge, S. Struct. Bonding (Berlin) 2008, 130, 29. (12) Braunschweig, H.; Dewhurst, R. D. Angew. Chem., Int. Ed. 2009, 48, 1893. (13) Vidovic, D.; Pierce, G. A.; Aldridge, S. Chem. Commun. 2009, 1157. (14) Dang, L.; Lin, Z.; Marder, T. B. Chem. Commun. 2009, 3987. (15) Braunschweig, H.; Dewhurst, R.; Schneider, A. Chem. ReV. 2010, 110, 3924. (16) For a review of the chemistry of complexes containing transition metal-group 13 element metal bonds in the 1960s and 1970s, see: Hsieh, A. T. T. Inorg. Chim. Acta 1975, 14, 87. (17) Baker, R. J.; Jones, C. Coord. Chem. ReV. 2005, 149, 1857. (18) Buchin, B.; Gemel, C.; Kempter, A.; Cadenbach, T.; Fischer, R. A. Inorg. Chim. Acta 2006, 359, 4833. (19) Coombs, N. D.; Clegg, W.; Thompson, A. L.; Willock, D. J.; Aldridge, S. J. Am. Chem. Soc. 2008, 130, 5449. (20) Coombs, N. D.; Vidovic, D.; Day, J. K.; Thompson, A. L.; Le Pevelen, D. D.; Stasch, A.; Clegg, W.; Russo, L.; Male, L.; Hursthouse, M. B.; Willock, D. J.; Aldridge, S. J. Am. Chem. Soc. 2008, 130, 16111. (21) Braunschweig, H.; Gruss, K.; Radacki, K. Inorg. Chem. 2008, 47, 8595. (22) Bunn, N. R.; Aldridge, S.; Kays, D. L.; Coombs, N. D.; Day, J. K.; Ooi, L. L.; Coles, S. J.; Hursthouse, M. B. Organometallics 2005, 24, 5879. (23) Aldridge, S.; Baker, R. J.; Coombs, N. D.; Jones, C.; Rose, R. P.; Rossin, A.; Willock, D. J. Dalton Trans. 2006, 3313. (24) Baker, R. J.; Jones, C.; Platts, J. A. Dalton Trans. 2003, 3673. (25) Baker, R. J.; Jones, C.; Platts, J. A. J. Am. Chem. Soc. 2003, 125, 10534. (26) Baker, R. J.; Jones, C.; Murphy, D. M. Chem. Commun. 2005, 1339. (27) Green, S. P.; Jones, C.; Mills, D. P.; Stasch, A. Organometallics 2007, 26, 3424. (28) Jones, C.; Rose, R. P.; Stasch, A. Dalton Trans. 2007, 2997.
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