Chemical Bonding in Transition Metal Complexes with Beryllium

Dec 28, 2009 - The equilibrium geometries and bond dissociation energies of the 14 valence electron (VE) complexes. [(PMe3)2M-BeCl2] ...
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J. Phys. Chem. A 2010, 114, 8529–8535

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Chemical Bonding in Transition Metal Complexes with Beryllium Ligands [(PMe3)2M-BeCl2], [(PMe3)2M-BeClMe], and [(PMe3)2M-BeMe2] (M ) Ni, Pd, Pt)† Pattiyil Parameswaran*,‡,§ and Gernot Frenking*,‡ Fachbereich Chemie, Philipps-UniVersita¨t Marburg, Hans-Meerwein-Strasse, D-35043 Marburg, Germany, and Fukui Institute for Fundamental Chemistry, Kyoto UniVersity, Kyoto 606-8103, Japan ReceiVed: October 25, 2009; ReVised Manuscript ReceiVed: December 8, 2009

The equilibrium geometries and bond dissociation energies of the 14 valence electron (VE) complexes [(PMe3)2M-BeCl2], [(PMe3)2M-BeClMe], and [(PMe3)2M-BeMe2] with M ) Ni, Pd, and Pt have been calculated using density functional theory at the BP86/TZ2P level. The nature of the M-Be bond was analyzed with the NBO charge decomposition analysis and the EDA energy decomposition analysis. The theoretical results predict the equilibrium structures with a T-shaped geometry at the transition metal where the PMe3 ligands are in the axial positions. The calculated bond dissociation energies show that the M-E bond strengths are in the range of donor-acceptor complexes of divalent beryllium compounds with ammonia. The bond strength decreases when the substituent at beryllium changes from Cl to CH3. The NBO analysis shows a negative charge at the BeX2 fragment, which indicates a net charge flow from the transition metal fragment to the beryllium fragment. The energy decomposition analysis of the M-Be bonds suggests two donor-acceptor bonds with σ and π symmetry where the transition metal fragment is a double donor with respect to the beryllium ligand. The π component of the [Ni]fBeXX′ donation is much smaller than the σ component. Introduction The coordination chemistry of many transition metal (TM) complexes can be suitably described with the Dewar-ChattDuncanson (DCD) model in terms of ligandfmetal σ-donation and metalfligand π-backdonation (Figure 1a).1 The reverse situation where the σ-donation takes place between a metal donor and a ligand acceptor, metalfligand, while there is ligandfmetal π-backdonation (Figure 1b) has recently been found in a theoretical study of TM complexes with group-13 ligands [(PMe3)2M-EX3] (M ) Ni, Pd, Pt; E ) B, Al, Ga, In, Tl; X ) H, F, Cl, Br, I).2 We want to point out that the π-bonding in the latter species does not only come from ligandfmetal π-backdonation but also from metalfligand π-donation because the metals have a d10 electron configuration. Recent theoretical studies suggest that there are carbon ligands CL2 which may serve as a σ-donor and as a π-donor in TM complexes because they possess two energetically high-lying electron lone pairs at the carbon donor atom.3 Prominent examples are carbodiphosphoranes4 and the recently synthesized carbodicarbenes.5 The principle bonding situation in complexes with double-donor ligands is shown in Figure 1c. The reverse situation where the ligand acts as a double acceptor is schematically shown in Figure 1d. Such a scenario can be expected for highly electron-deficient ligands like beryllium species BeX2 which are bonded to d10 transition metals. Several donor-acceptor complexes of neutral and anionic beryllium chloro compounds with main-group elements have been reported in the literature,6 and their bonding situation was investigated with theoretical methods;7 however, a well-defined TM complex with a ligand BeX2 which is characterized by X-ray structure analysis could only recently become isolated. Braunschweig and co-workers †

Part of the “Klaus Ruedenberg Festschrift”. * Corresponding author. E-mail: [email protected]. ‡ Philipps-Universita¨t Marburg. § Kyoto University.

Figure 1. Schematic representation for donor-acceptor interactions in transition metal complexes with different directions: (a) DCD bonding, (b) reversed DCD bonding, (c) double donations from ligand to transition metal, (d) double backdonation from transition metal to ligand.

synthesized the 14 valence electron (VE) complexes [(PCy3)2Pt-BeCl2] and [(PCy3)2Pt-BeClCH3] in which the TM fragment is bonded to a tricoordinated beryllium atom.8 The structure of the molecules suggests that the metal-ligand bonding takes place through electron donation from occupied metal orbitals into the formally empty σ and π valence orbitals of beryllium in BeCl2 and BeClCH3. The latter orbitals may also become partially occupied by donation from the chlorine and/or methyl substituents. Hence it is interesting to probe the nature of the transition metal-beryllium bond in

10.1021/jp910181q  2010 American Chemical Society Published on Web 12/28/2009

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Parameswaran and Frenking

Figure 2. Optimized geometries (BP86/TZ2P) of 1M-3M. Bond angles and torsion angles in degrees and bond distances in Å. Experimental values are in italics.

TABLE 1: Calculated Bond Dissociation Energies at BP86/TZ2P and Charge Distribution Given by the Natural Population Analysis at BP86/def2-TZVPP//BP86/TZ2P molecule

De

P(M-Be)

q(M)

q(Be)

q(BeXX′)

2s(Be)σ

2pz(Be)σ

2px(Be)π|

2py(Be)π⊥

1Ni 1Pd 1Pt 2Ni 2Pd 2Pt 3Ni 3Pd 3Pt

25.0 23.8 28.5 17.8 16.0 20.2 12.0 9.4 12.8

0.29 0.26 0.29 0.23 0.21 0.24 0.16 0.15 0.18

-0.29 -0.41 -0.51 -0.30 -0.42 -0.51 -0.29 -0.41 -0.52

1.34 1.36 1.35 1.37 1.38 1.38 1.40 1.41 1.40

-0.18 -0.16 -0.17 -0.14 -0.13 -0.14 -0.11 -0.10 -0.12

0.45 0.44 0.43 0.47 0.47 0.47 0.50 0.50 0.49

0.093 0.080 0.086 0.063 0.055 0.060 0.038 0.033 0.038

0.069 0.069 0.069 0.059 0.061 0.061 0.047 0.051 0.052

0.051 0.051 0.050 0.028 0.027 0.026 0.012 0.009 0.009

[(PCy3)2PtBeCl2] and [(PCy3)2PtBeClCH3]. In this paper, we report a comprehensive theoretical study of the bonding situation and bond dissociation energies of the 14 VE model complexes [(PMe3)2M-BeClMe], and [(PMe3)2M-BeCl2],

[(PMe3)2M-BeMe2] (1M-3M) where M ) Ni, Pd, and Pt. The molecules belong to a special category of complexes featuring a transition metal-ligand multiple bond where the transition metal fragment is a 2-fold electron-pair donor.

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Figure 3. Plot of important molecular orbitals of 1Ni. MO energy in electronvolts.

Computational Details The geometries of the complexes have been optimized at the gradient-corrected DFT level using the exchange functional of Becke9a in conjunction with the correlation functional of Perdew9b (BP86). Uncontracted Slater-type orbitals (STOs) were employed as basis functions for the SCF calculations.10 The basis sets have triple ζ-quality augmented by two sets of polarization functions. The (n-1)s2 and the (n-1)p6 core electrons of the main group elements and the (1s2s2p)10 core electrons of nickel, the (1s2s2p3s3p3d)28 core electrons of palladium, and the (1s2s2p3s3p3d4s4p4d)46 for platinum were treated by the frozen core approximation.11 This level of theory is denoted BP86/ TZ2P. An auxiliary set of s, p, d, f, and g STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle.12 Scalar relativistic effects have been considered using the zero-order regular approximation (ZORA).13 The nature of the stationary points on the potential energy surface has been verified by calculating the Hessian matrices. Some species have very small imaginary modes, which correspond to methyl group rotations. We followed these modes for a number of cases by lifting the symmetry constraints and noticed only minor effects on the geometries and energies. The calculations were carried out with the program package ADF 2007.14 The bonding interactions between two molecular fragments A and B forming a molecule AB have been analyzed with the energy decomposition scheme of the program package ADF,15 which is based on the work by Morokuma16 and Ziegler and Rauk.17 The bond dissociation energy (-∆E ) BDE) between the fragments A and B is partitioned into several contributions which can be identified as physically meaningful quantities. First, ∆E is separated into two major components, ∆Eint and ∆Eprep

∆E()-BDE) ) ∆Eint + ∆Eprep ∆Eprep is the energy that is necessary to promote the fragments A and B from their equilibrium geometry and electronic ground state to the geometry and electronic state, which they have in the molecule. It is also called deformation energy because the fragments become deformed with respect to the equilibrium

geometry. The instantaneous interaction energy ∆Eint is the focus of the bonding analysis and can be decomposed into three components

∆Eint ) ∆Eelstat + ∆EPauli + ∆Eorb The term ∆Eelstat gives the electrostatic interaction energy between the fragments, which are calculated with a frozen density distribution in the geometry of the complex. The Pauli repulsion (∆EPauli) arises with the energy change associated with the transformation from the superposition of the unperturbed electron densities of fragments FA + FB to the wave function ˆ {ΨA · ΨB}, which properly obeys the Pauli principle Ψ0 ) NA ˆ ) and renormalization (N) through explicit antisymmetrization (A of the product wave function. It comprises the destabilizing interactions between electrons on either fragment with the same spin. The stabilizing orbital interaction term ∆Eorb is calculated in the final step of the analysis when the orbitals relax to their final form. The latter can be decomposed into contributions from each irreducible representation of the point group of the interacting system. The EDA makes it thus possible to quantitatively estimate the contributions of orbitals, which possess different symmetry from the overall metal-ligand orbital interactions. Note that the ∆Eorb term not only comprises the stabilization which comes from the mixing of the occupied and vacant orbitals of the interacting fragments but also includes the stabilizing contribution which comes from the relaxation (polarization) within each fragment. We have calculated three different types of complexes, viz., 1M where substituents X on beryllium are two Cl atoms, 2M where substituents on beryllium are Cl and CH3, and 3M where substituents on beryllium are two CH3 groups. The molecules 1M and 3M and their fragments which are used for the EDA have C2V symmetry, whereas the molecule 2M has Cs symmetry. To perform the NBO analysis18 of our model complexes, we made single-point calculations at the BP86/TZ2P optimized geometries with the Gaussian 0319 program using the BP86 functional and a basis set of triple-ζ quality with two polarization functions (def2-TZVPP).20

-7.3 e(15.8%) 15.2 -20.2 -6.4e (16.1%) 13.5 -16.0 -7.9e (18.1%) 13.1 -17.8

The value in parentheses gives the percentage c a All energies in kcal mol-1. b The value in parentheses gives the percentage contribution to the total attractive interactions (∆EElstat + ∆EOrb). contribution to the total orbital interactions. d Contribution from a′ orbitals. e Contribution from a′′ orbitals.

-24.4 59.6 -47.4 (56.5%) -36.6 (43.5%) -28.3(77.4%) -0.59 (1.6%) -2.7 (7.4%) -5.0(13.6%) 11.7 -12.8 -19.0 48.3 -37.4(55.6%) -29.9(44.4%) -23.2(77.5%) -0.47 (1.6% -2.0 (6.8%) -4.2(14.1%) 9.7 -9.4 -21.3 53.3 -39.8 (53.3%) -34.8 (46.7%) -26.3(75.6%) -0.34 (1.0%) -2.2 (6.2%) -6.0(17.3%) 9.4 -12.0 -35.4 66.3 -55.7(54.8%) -46.0(45.2%) -38.7d (84.2%) -29.5 56.2 -45.9 (53.5%) -39.8 (46.5%) -33.4d (83.9%) -30.8 58.9 -46.0(51.3%) -43.7(48.7%) -35.8d (82.0%)

-46.3 71.6 -62.1 (52.7%) -55.7 (47.3%) -42.2 (75.7%) -1.3(2.2%) -4.2 (7.6%) -8.1 (14.5%) 16.8 -28.5 -40.3 61.4 -52.0(51.2%) -49.7(48.8%) -38.0 (76.5%) -1.1(2.3%) -3.4 (6.8%) -7.2 (14.5%) 16.6 -23.8 -41.2 62.2 -50.6 (48.9%) -52.9 (51.1%) -39.9 (75.5%) -1.3(2.4%) -3.2 (6.1%) -8.5 (16.0%) 16.3 -25.0 ∆Eint ∆EPauli ∆EElstatb ∆EOrbb ∆a1(σ)c ∆a2(δ)c ∆b1(π|)c ∆b2(π⊥)c ∆EPrep -De

1Pd 1Ni

1Pt

2Ni (Cs)

2Pd (Cs)

2Pt (Cs)

3Ni

3Pd

3Pt

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TABLE 2: Energy Decomposition Analysis at BP86/TZ2P of the M-Be Bond in 1M-3Ma

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Parameswaran and Frenking Geometries and Bond Dissociation Energies The optimized geometries of the 14 VE complexes 1M-3M are shown in Figure 2. All complexes possess equilibrium structures with a T-shaped geometry at the transition metal where the PMe3 groups are in the axial positions. The M-P bonds become shorter and the M-Be bond becomes longer from 1M to 3M. This indicates that the substituents at beryllium have a significant influence on the M-Be and M-P bonding. Note that the PMe3 ligands in 1M-3M are slightly tilted away from the beryllium ligands and that the calculated P-M-Be angle becomes more obtuse from 1M to 3M. The X-Be-X′ angle becomes wider from 1M to 3M, while it becomes more acute for each series from Ni to Pt except for 3Pd which has a slightly larger value than 3Ni (Figure 2). The geometrical parameters of 1Pt and 2Pt are in good agreement with experimental values of substituted homologues.8 The calculated bond dissociation energies (BDE) at the BP86/ TZ2P level of theory for the M-Be bonds of 1M-3M are given in Table 1. The fragments have been calculated at the respective electronic ground state. The metal fragments (PMe3)2M and BeX2 of 1M-3M have a singlet ground state with linear or quasilinear geometry. The DFT calculations predict that the transition metal-beryllium bond energies have the order 1M > 2M > 3M. The calculated De(M-Be) values are in the range of 28.5-9.4 kcal/mol. The strongest bond (28.5 kcal/mol) is calculated for 1Pt, and the weakest bond (9.4 kcal/mol) is predicted for 3Pd. Note that the experimentally isolated complexes 1Pt and 2Pt have PCy3 as substituents instead of PMe3.8 The BDE decreases when the substituent at beryllium changes from Cl to CH3. This trend correlates with the gradual increase in the M-Be bond length from 1M to 3M. The higher electronegativity of chlorine atoms enhances the Lewis acidity of BeX2 fragments and in turn strengthens the donor-acceptor M-Be bond. Our previous calculations at BP86/TZ2P predict that the BDE of the complex [Cl2Be-NH3] is 27.4 kcal/mol, while that of [Cl3B-NH3] is 19.9 kcal/mol at the same level of theory.7g A bonding analysis showed that the higher BDE of the beryllium complex comes from less deformation energy of the fragments compared with the boron complex. The intrinsic interaction energy between the frozen fragments in [Cl2Be-NH3] is slightly weaker (39.0 kcal/mol) than in [Cl3B-NH3] (41.4 kcal/mol).7g The calculated bond dissociation energies suggest that the BDE for the beryllium complexes 1M are in the same range as those of ammonia complexes of BeCl2. The trend in the BDE of M-Be for M ) Pt > Ni > Pd is similar to many metal-ligand bonds of transition metals, where the second TM row atom has a weaker bond than the first TM row element, while the third TM row atom binds stronger than the first TM row atom.21 The order Ni > Pd follows the general trend that heavier elements of the same group of the periodic system have weaker bonds than the lighter homologue because the valence electrons are further away from the nucleus and thus are less strongly bonded.24 The stronger bonds of Pt come from relativistic effects which lead to a significant expansion of the valence 5d orbitals and thus enhance the orbital interactions.25 Analysis of the Bonding Situation Table 1 gives the results of the charge decomposition analysis using the NBO method. The Wiberg bond order for the M-Be bonds has essentially the same trend as the BDE values. It is interesting to note that the atomic partial charges of the transition metals Ni, Pd, and Pt change very little from 1M to 3M and that the positive partial charges at Be slightly increase from

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TABLE 3: Energy Decomposition Analysis at BP86/TZ2P of the M-Be Bond in 1Ni by Deleting a1 and b2 Virtual Orbitals of BeCl2 (1Ni-BeCl2-a1, 1Ni-BeCl2-b2) and Ni(PMe3)2 (1Ni-Ni(PMe3)2-a1, 1Ni-Ni(PMe3)2-b2)a ∆Eint ∆EPauli ∆EElstatb ∆EOrbb ∆a1(σ)c ∆a2(δ)c ∆b1(π|)c ∆b2(π⊥)c

1Ni

1Ni-BeCl2-a1

1Ni-Ni(PMe3)2-a1

1Ni-BeCl2-b2

1Ni-Ni(PMe3)2-b2

-41.2 62.2 -50.6 (48.9%) -52.9 (51.1%) -39.9 (75.5%) -1.3 (2.4%) -3.2 (6.1%) -8.5 (16.0%)

-26.1 62.2 -50.6 (57.3%) -37.7 (42.7%) -23.4 (62.0%) -1.2 (3.2%) -3.7 (9.8%) -9.5 (25.0%)

-36.6 62.2 -50.6 (51.2%) -48.2 (48.8%) -34.6 (71.8%) -1.4 (2.9%) -3.5 (7.3%) -8.7 (18.0%)

-37.0 62.2 -50.6 (50.9%) -48.7 (49.1%) -40.8 (83.9%) -1.2 (2.5%) -3.3 (6.7%) -3.4 (7.0%)

-39.0 62.2 -50.6 (49.9%) -50.7 (50.1%) -39.6 (78.1%) -1.4 (2.7%) -3.3 (6.6%) -6.5 (12.7%)

a All energies in kcal mol-1. b The value in parentheses gives the percentage contribution to the total attractive interactions (∆EElstat + ∆EOrb). c The value in parentheses gives the percentage contribution to the total orbital interactions.

1M < 2M < 3M. It is tempting to use the atomic partial charges of the M and Be atoms for estimating the strength of the electrostatic interactions in the M-Be bonds. This is a very unreliable approach because the atomic partial charges do not possess any information about the spatial distribution of the electronic charges, which is crucial for Coulombic interactions. Previous theoretical work has shown that using atomic partial charges can be misleading and that the calculation of electrostatic interactions requires the integration of the electronic charge in the molecular space.22 The calculated partial charges suggest that the beryllium ligands BeXX′ always carry a negative partial charge between -0.18 and -0.10 e. The calculated charges thus agree with the bonding picture of overall (PMe3)2MfBeXX′ donation. The calculated population of the atomic orbitals of the beryllium atom gives further insight into the electronic structure and bonding situation of the complexes. Table 1 shows that the 2s(Be)σ and 2pz(Be)σ values give the population of the atomic orbitals (AOs) of the Be atom that contribute to the M-Be σ bond which is aligned along the z axis. The orbital population for 2s(Be)σ is in the range of 0.43-0.50, while that of 2pz(Be)σ is below 0.1. The calculated values for 2p(Be)π| give the populations in the p(π) AO of Be in the X-Be-X′ plane, while the values for 2p(Be)π⊥ give the populations in the p(π) AO of Be perpendicular to the X-Be-X′ plane. It becomes obvious that the populations in the 2p valence orbitals of Be are very small. It follows that the Be-Cl, Be-M, and Be-C (in 2M) bonds are all strongly polarized away from the beryllium atom which agrees with the electronegativity of Be. Figure 3 shows the ten highest lying occupied and two lowest lying vacant MOs of 1Ni. The valence orbitals of the other complexes 1M-3M are very similar, and thus we use the orbitals of 1Ni as a model to understand the bonding situation. The overall symmetry of the complex is C2V, and thus there are orbitals which have a1, a2, b1, and b2 symmetry. The shape of the occupied a1 valence MOs of 1Ni shows that the HOMO-4 and HOMO-8 contribute to the Ni-Be σ-bonding. The HOMO-4 can easily become identified with [Ni]fBeCl2 charge donation mainly from the dz2 orbital of Ni into the spz (z being the Ni-Be axis) hybridized orbital of Be but also to the pz orbitals of chlorine. The NBO analysis presented above indicates that the Be(pz) participation is rather small. The shape of the HOMO-8 suggests electron flowing which is complementary to the direction that is shown in HOMO-4. There is charge donation from the in-plane lone pairs of chlorine via the pz orbital of Be into the dz2 orbital of Ni. The shape of the HOMO-8 indicates that the a1 orbital interactions in 1Ni comprise mainly [Ni]fBeCl2 σ-donation but also a small amount of [Ni]rBeCl2 σ-backdonation. The occupied orbitals HOMO-3, HOMO-6, and HOMO-9 indicate Ni-Be out-of-plane π⊥-bonding where the plane is

defined by the Ni-BeCl2 atoms. The shape of the orbitals suggests that the HOMO-3 and HOMO-6 orbitals can be identified as [Ni]fBeCl2 π⊥-donation, while the HOMO-9 signifies [Ni]rBeCl2 π⊥-backdonation. The shape of the b1 orbitals (HOMO-1 and HOMO-5) which recognize Ni-Be inplane π|-bonding do not show any mixing between the transition metal fragment and the BeCl2 ligand. The same holds for the a2 orbitals HOMO-2 and HOMO-7, which signify Ni-Be δ-bonding. This means that the latter two types of orbital interactions contribute very little to the Ni-Be bonding. A quantitative estimate of the strength of the orbitals that possess a1, a2, b1, and b2 symmetry is given by the energy decomposition analysis which shall be discussed next. Table 2 shows the EDA results for the complexes 1M-3M. The total interactions ∆Eint between the transition metal fragments (PMe3)2M and the ligands BeXX′ are clearly larger than the bond dissociation energies, but the trends between the different systems remain the same. This is because the deformation energy of the fragments ∆Eprep albeit is rather large but quite uniform for the three systems 1M, 2M, and 3M. The breakdown of the ∆Eint into the three major components suggests that the transition metal-ligand bonds have a slightly stronger electrostatic attraction than attractive orbital bonding except for 1Ni where the ∆Eorb term is a little larger than ∆Eelstat. The EDA values for the ∆Eorb contribution to the transition metal-ligand bonding, which come from orbitals with different symmetry, give a quantitative support of the qualitative interpretation of the shape of the valence orbitals, which is given above. The most important orbital interaction comes from the ∆a1(σ) term, which provides between 75 and 84% of the ∆Eorb term. A minor but non-negligible contribution comes from the b2(π⊥) orbitals which give between 14 and 17% of the total orbital interactions. The stabilization which comes from the orbitals which have a2(δ) or b1(π|) symmetry is negligible. The EDA results thus suggest that the M-Be bonding in (PMe3)2MBeXX′ arises from strong electrostatic attraction and from nearly equally strong donor-acceptor interactions of orbitals. The orbital interactions come mainly from orbitals, which have σ symmetry, while a smaller contribution comes from orbitals which have π⊥-symmetry. To estimate the strength of the donation and backdonation components of the a1(σ)- and b2(π⊥)-orbital interactions in the beryllium complexes, we carried out EDA calculations of [(PMe3)2Ni-BeCl2] (1Ni) where the vacant orbitals with a1 or b2 symmetry in one of the interacting fragments are deleted.23 This means, for example, that by deleting the a1 vacant orbitals in the (PMe3)2Ni fragment of [(PMe3)2Ni-BeCl2] the EDA results would only give the (PMe3)2NifBeCl2 component of the a1(σ)-orbital interactions but not the (PMe3)2NirBeCl2 component. As noted in the Computational Details section, the ∆Eorb term comprises not only the stabilization which comes

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from the mixing between the occupied and vacant orbitals of the fragments but also the stabilizing contribution which comes from the relaxation (polarization) within each fragment. This has to be considered when the calculated numbers are discussed. The EDA results for 1Ni with selected deletion of vacant orbitals are shown in Table 3. The first two entries with omitted orbitals give the EDA results where the vacant a1 orbitals of the fragments BeCl2 (1Ni-BeCl2-a1) and Ni(PMe3)2 (1Ni-Ni(PMe3)2-a1) are deleted. The full strength of the a1 contribution in 1Ni (-39.9 kcal/mol) reduces to -34.4 kcal/mol when the vacant a1 orbitals in Ni(PMe3)2 are deleted. This value can be attributed to the (PMe3)2NifBeCl2 a1(σ) donation but also to the polarization within the (PMe3)2Ni fragment. Deletion of the a1 orbitals in BeCl2 still gives a value of -23.4 kcal/mol which indicates the strength of the (PMe3)2NirBeCl2 a1(σ) backdonation and the relaxation within the BeCl2 fragment. The data suggest that the (PMe3)2NifBeCl2 a1(σ) donation is clearly stronger than the (PMe3)2NirBeCl2 a1(σ) backdonation which is the expected trend. The final two entries indicate the relative strength of the b2(π⊥) orbital interactions. The calculated data suggest that the (PMe3)2NifBeCl2 b2(π⊥) donation (-6.5 kcal/mol) is nearly twice as strong as the (PMe3)2NirBeCl2 b2(π⊥) backdonation (-3.4 kcal/mol). The EDA results thus support the classification of the [Ni]-BeCl2 orbital interactions in the complex 1Ni as double donation from the transition metal fragments to the beryllium ligand. Summary and Conclusion The theoretical results at the BP86/TZ2P level predict that the 14 VE complexes [(PMe3)2M-BeCl2], [(PMe3)2M-BeClMe], and [(PMe3)2M-BeMe2] with M ) Ni, Pd, and Pt have equilibrium structures with a T-shaped geometry at the transition metal where the PMe3 ligands are in the axial positions. The calculated bond dissociation energies suggest that the M-E bond strengths are in the same range of donor-acceptor complexes of divalent beryllium compounds with ammonia. The bond dissociation energy decreases when the substituent at beryllium changes from Cl to CH3. The NBO analysis shows a negative charge for the BeX2 fragments which indicates a net charge flow from the transition metal fragment to the beryllium fragment. The M-E bonding analysis by EDA suggests two donor-acceptor bonds with σ and π symmetry where the transition metal fragment is a double donor with respect to the beryllium ligand. The π component of the [Ni]fBeXX′ donation is much smaller than the σ component. Acknowledgment. P.P. thanks the Alexander von Humboldt foundation for a postdoctoral fellowship. This work was supported by the Deutsche Forschungsgemeinschaft. Excellent service by the computer center of the Philipps-Universita¨t Marburg is gratefully acknowledged. Supporting Information Available: Total energies, the number of imaginary frequencies, and Cartesian coordinates of 1M-3M. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C79. (b) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2929. (c) Frenking, G. J. Organomet. Chem. 2001, 635, 9. (2) Goedecke, C.; Hillebrecht, P.; Uhlemann, T.; Haunschild, R.; Frenking, G. Can. J. Chem. 2009, 87, 1470.

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