Structure and Bonding Analysis of the Cationic Electrophilic

Article pubs.acs.org/JPCA

Structure and Bonding Analysis of the Cationic Electrophilic Phosphinidene Complexes of Iron, Ruthenium, and Osmium [(η5‑C5Me5)(CO)2M{PNiPr2}]+, [(η5‑C5H5)(CO)2M{PNR2}]+ (R = Me, iPr), and [(η5‑C5H5)(PMe3)2M{PNMe2}]+ (M = Fe, Ru, Os) Krishna K. Pandey,* Pradeep Tiwari, and Pankaj Patidar School of Chemical Sciences, Devi Ahilya University Indore, Khandwa Road Campus, Indore 452 001, India S Supporting Information *

ABSTRACT: Quantum-chemical DFT calculations for the electronic, molecular structure and M−PNR2 bonding analyses of the experimentally known cationic electrophilic phosphinidene complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+ and of the model complexes [(η5-C5H5)(CO)2M{PNR2}]+ (R = iPr, Me) and [(η5-C5H5)(PMe3)2M{PNMe2}]+ were carried out using BP86/ TZ2P/ZORA level of theory. The calculated geometrical parameters of the studied complexes are in good agreement with the reported experimental values. The short M−P bond distances and calculated Pauling bond orders (range of 1.23−1.68), suggest the presence of M−P multiple bond characters. The Hirshfeld charge analysis shows that the overall charge flows from phosphinidene ligand to metal fragment. The M−P σ-bonding orbitals are well-occupied (>1.80e). The energy decomposition analysis revealed that the contribution of the electrostatic interaction ΔEelstat is, in all studied complexes, significantly larger (55.2−62.6%) than the orbital interactions ΔEorb. The orbital interactions between metal and PNR2 in [(η5-C5H5)(L)2M{PNR2}]+ arise mainly from M ← PNR2 σ-donation. The π-bonding contribution (19−36%) is much smaller than the σ-bonding. The interaction energies, as well as bond dissociation energies, depend on the auxiliary ligand framework around the metal and decrease in the order (η5-C5H5) > (η5-C5Me5) and CO > PMe3. Upon substitution of R = iPr with smaller group R = Me, the M−PNR2 bond strength slightly decreases.

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characterization of first stable, terminal, electrophilic phosphinidene complexes [(η5-C5Me5)(CO)3M{PNiPr2}]AlCl4 (M = Mo, W) have been reported rather recently by Carty et al.31 Since then, the chemistry of both classes of terminal phosphinidenes has expanded greatly, with new examples, synthetic applications and reactivity being reported extensively.32−52 In particular, a series of cationic, terminal electrophilic phosphinidene complexes including group 6 metals [(η5-C5Me5)(CO)3M{PNiPr2}]+ (Cr, Mo, W), group 8 metals [(η5-C5Me5)(CO)2M{PNiPr2}]+, the rhenium complex [(CO)5Re{PNiPr2}]+, and the cobalt complex [(PPh3)(CO)3Co{PNiPr2}]+ has been reported over the past few years.31−36 Recently, the first examples

INTRODUCTION Considerable attention has been attracted to the transition metal complexed terminal phosphinidenes LnM = PR, because they are considered as phosphorus analogues of carbene complexes and used as viable reagents in organophosphorus chemistry.1−28 Like the metal−carbene complexes, terminal phosphinidene complexes also display electrophilic (Fischer type) or nucleophilic (Schrock type) nature on the basis of reactivity of its phosphorus atom. The first stable phosphinidenes are nucleophilic complexes which were reported by Lappert and co-workers in 1987,1 but isolation of stable electrophilic complexes has proven to be more elusive. For a long time, the terminal electrophilic phosphinidenes were known as transient species generated and trapped in situ.17−28 Despite the earlier spectroscopic evidence of aminophosphinidene complexes of molybdenum and tungsten,29,30 the isolation and structural © 2012 American Chemical Society

Received: September 13, 2012 Revised: November 3, 2012 Published: November 5, 2012 11753

dx.doi.org/10.1021/jp309092t | J. Phys. Chem. A 2012, 116, 11753−11762

The Journal of Physical Chemistry A

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between phosphinidene and metal fragments in their triplet state (Figure 1a). However, the bonding in electrophilic phosphinidene complexes is best conceptualized by considering donor− acceptor interaction between phosphinidene fragment and metal fragment in singlet state (Figure 1b). The free phosphinidene moiety in the singlet state contains two lone pair and an empty pz orbital. One lone pair forms an σ-donor interaction to metal, and a second lone pair remains stereoactive, leading to the bent geometry at phosphorus center. The empty pz orbital serves as acceptor orbital and results electrophilicity at phosphorus atom. Further in aminophosphinidenes, the empty pz orbital stabilizes by the π-donation from the heteroatom substituent (N) as well as from filled metal d-orbital (Figure 1b). Besides the remarkable synthetic development, several theoretical studies on unsaturated phosphorus analogues of Fischer type carbene complexes have been performed so far.53−62 Theoretical calculations of the model complexes (OC)nMPR have been presented using the extended Huckel theory.53−56 The influences of the π-donor substituents on the electronic properties of (OC)5MPR complexes of chromium, molybdenum, and tungsten have been investigated via energy decomposition analysis.57 To distinguish the electrophilic from nucleophilic phosphinidene complexes, Ehlers, Baerends, and Lammertsma presented a qualitative study on MLnPH (M = Ti, Zr, Hf, V, Nb. Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir; L = CO, PH3, Cp).58 They concluded that the philicity is influenced by the type of spectator ligand L rather than the nature of the transition metal M.58 Quantum-chemical calculations on the model compounds (E/Z)-[(η5-C5H5)(L)M = PH) (M = Co, Rh, Ir; L = CO, PH3)59 and on the four coordinated phosphinidene complexes of titanium60 and vanadium61 have also been reported. An extensive study on linear versus bent bonding in metal phosphinidene complexes [(η5-C5H5)(CO)nMPMe]+ (M = Cr, Mo, W; n = 2 or 3) has been presented recently.62 To tune the properties of these compounds, it is helpful to understand the nature of the M−PR interactions. Herein, we present a comprehensive theoretical study of original electrophilic phosphinidene complexes of group 8 metals [(η5-C5Me5)(CO) 2M{PN iPr2}]+ (I, M = Fe; II, M = Ru; III, M = Os) and model complexes [(η5-C5H5)(CO) 2M{PNR2}]+ (IV, M = Fe, R = iPr; V, M = Ru, R = iPr; VI, M = Os, R = iPr; VII, M = Fe, R = Me; VIII, M = Ru, R = Me; IX, M = Os, R = Me) and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (X, M = Fe; XI, M = Ru; XII, M = Os) using the BP86/TZ2P/ZORA level of theory. The pivotal issue we address in this investigation is how the nature of M−P bonds of electrophilic phosphinidene complexes may be altered by modifying the spectator ligands on metal and alkyl substituents of phosphinidene ligand. The contribution of orbital interactions and electrostatic interactions (covalent and ionic characters, respectively) to the total M−PNR2 bonds strength as well as the extent of M → P π-back-donation evaluated by energy decomposition analysis. Finally, alteration of M−PR bond strength is discussed when M changes from M = Fe to M = Os, R changes from R = methyl (Me) to isopropyl (iPr) and L changes from L = CO to L = PMe3.

of neutral stable electrophilic phosphinidene complexes of vanadium [(η5-C5H5)(CO)3V{P(NR2)}] (R = iPr, Cy) and their reactivity have also been described.37 All the structurally characterized electrophilic phosphinidene complexes and their important structural parameters are summarized in Chart 1. Chart 1. Overview of the Structurally Characterized Terminal Electrophilic Phosphinidene Complexes

Figure 1. Schematic representation of the M−PR orbital interactions: (a) in nucleophilic phosphinidene complexes (triplet state) and (b) in electrophilic phosphinidene complexes (singlet state).

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COMPUTATIONAL DETAILS Calculations on the cationic electrophilic phosphinidene complexes of iron, ruthenium and osmium [(η5-C5Me5)(CO)2M{PNiPr2}]+ (I−III) and on model complexes [(η5-C5H5)(CO)2M{PNR2}]+ (R = iPr, IV−VI; R = Me, VII−IX) and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (X−XII) have been performed at the gradient-corrected DFT level of theory using

The bonding in terminal phosphinidene complexes depends on their electronic state, i.e., either singlet state or triplet state, schematically presented in Figure 1. It is well-known that the phosphinidenes much prefer a triplet ground state. The transition metal−phosphorus bonding in nucleophilic complexes is described by electron sharing bonding model (covalent bond) 11754



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the exchange functional of Becke63 and the correlation functional of Perdew64 (BP86). Scalar relativistic effects have been considered using the ZORA formalism.65 Uncontracted Slater-type orbitals (STOs) using triple-ζ basis sets augmented by two sets of polarization functions were employed for the SCF calculations.66 The (1s)2 core electrons of carbon, nitrogen and oxygen, (1s2s2p)10 core electrons of phosphorus, (1s2s2p3s3p)18 core electrons iron, (1s2s2p3s3p3d)28 core electrons of ruthenium, and (1s2s2p3s3p3d4s4p4d)46 core electrons of osmium were treated by the frozen-core approximation.67 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.66 Nalewajski−Mrozek (N−M) bond order68 and Hirshfeld charges69 were also calculated. The natural hybrid orbital analyses of the complexes (I−XII) were examined by NBO analysis.70 All the calculations were performed utilizing the ADF-2010.01 package.71 The molecular orbitals were visualized using MOLDEN program.72 The energy decomposition analysis for M−P bonds between the interacting fragments [(η5-C5Me5)(CO)2M]+ or [(η5-C5H5)(L)2M]+ and [PNR2] in all studied complexes I−XII were carried out using Cs symmetry with the energy decomposition scheme of the ADF package,73 based on the methods of Morokuma74 and Ziegler and Rauk.75 The overall bond energy ΔE between the fragments is divided into two major components as: ΔE = ΔE int + ΔEprep

(R = iPr, V; R = Me, VIII) and [(η5-C 5H5)(PMe3)2Ru{PNMe2}]+ (XI). The structures of the iron and osmium complexes are very similar to those presented in this figure and therefore not depicted. The full set of calculated bond distances and bond angles, given in Table 1, shows that our calculated geometries reproduced the solid-state parameters of the complexes [(η5-C5H5)(CO)2M{PNiPr2}]+ (M = Fe, Ru, Os) expectedly well. The M−PNR2 bond distances determined computationally for the complexes I−XII are shorter than those expected from MP single bond covalent radii predictions (FeP = 2.27 Å; RuP = 2.36 Å; OsP = 2.40 Å),82 but slightly longer than expected MP double bonds (2.11, 2.16, and 2.18 Å for FeP, RuP, and OsP, respectively). The calculated Pauling bond orders83 for the M−PNR2 bond distance of the studied complexes are 1.39 (I), 1.26 (II), 1.37 (III), 1.31 (IV), 1.23 (V), 1.33 (VI), 1.39 (VII), 1.30 (VIII), 1.39 (IX), 1.57 (X), 1.59 (XI), and 1.68 (XII), and thus, the Pauling bond orders support the presence of MP double bond characters in these complexes. It is noticeable that the values of MP Pauling bond orders for carbonyl complexes (I−IX) are significantly smaller than the corresponding values for phosphine (PMe3) complexes (X−XII). As expected from periodic changes on going from M = Fe to M = Os, the M−PNR2 bond distances increase in each set of complexes. When the first two sets (I−III and IV−VI), where the (η5-C5Me5) group is replaced by a (η5-C5H5) moiety, are compared, a small M−P bond lengthening is observed in latter complexes. Similarly, if we consider the effect of phosphinidene ligand substituents (R) in the complexes IV−IX, it is observed that the better electron donor isopropyl groups (iPr) of the complexes IV−VI enhance the N → P π-donation and consequently, M → P π-back-donation is expected to reduce in these complexes. Thereby, the M−P bond distances of the complexes VII−IX are shorter than that of the complexes IV−VI. Further, the M−PNR2 bond distances decrease significantly (∼0.05 Å) upon replacing the auxiliary carbonyl ligands with strong σ-donor trimethylphosphane (PMe3) ligands. It is also noteworthy that the M−PNR2 bond distances of the complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+ (I−III) and [(η5-C5H5)(CO)2M{PNMe2}]+ (VII−IX) are almost equal; however, these bonds are relatively stronger in the latter complexes (vide infra). As shown in Table 1, no significant changes are observed in the P−N bond distances of all studied carbonyl complexes whereas it becomes shorter in the PMe3 complexes (X−XII). The P−N bond distances of the complexes I−XII are markedly shorter than those expected from single bond covalent radii prediction (P−N = 1.82 Å)82 and correspond to the Pauling bond order of ∼1.72 for the carbonyl complexes I−IX and of ∼1.57 for the phosphine complexes (X−XII). Moreover, the average M−C(η5‑C5H5) bonds of the carbonyl complexes IV−IX are ∼0.01 Å shorter than the M-C(η5‑C5Me5) bonds; however, these bonds are significantly longer (to an extant about 0.45 Å) in trimethylphosphane complexes (X−XII). As expected from bonding model (Figure 1b) and observed in Figure 2, the bond angle ∠M−P−N in all complexes is bent due to the stereoactive lone pair of phosphorus and possesses the value of ∼120°. Upon replacing the carbonyl ligands with the PMe3 ligands, the bond angles, i.e., M−P−N, L−M−P, and L−M−L are wider by ∼10°. Bonding Analysis. We begin the analysis of M−PNR2 bonding in the studied electrophilic phosphinidene complexes of iron, ruthenium and osmium, I−XII, with a discussion of

(1)

where ΔEprep 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: ΔEprep = Etotal(distorted fragments) − Etotal(fragments in the equilibrium structure) (2)

In eq 1 ΔEint is the instantaneous interaction energy between the two fragments of the molecule. It can be decomposed into three main components: ΔE int = ΔEelstat + ΔE Pauli + ΔEorb

(3)

where ΔEelstat describes the classical Coulombic interaction between the fragments which is attractive in most cases. The term ΔEPauli (the exchange repulsion or Pauli repulsion) takes into account the destabilizing two-orbital, three- or four-electron interactions between the occupied orbitals of both fragments, and ΔEorb represents orbital interactions between the occupied and virtual orbitals of the two fragments. ΔEorb can be further partitioned into contributions from orbitals which belong to different irreducible representations of the point group of the system. It has been shown that the results of EDA give a quantitative insight into the nature of the metal−ligand interactions.76−79 The various theoretical groups have proved that the BP86 method is more reliable than B3LYP for organometallic systems.80 Several theoretical calculations have been performed on the osmium complexes using BP86 functional in which results are quite reliable and closer to the experimental observations.81

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RESULT AND DISCUSSION Structural Analysis. Figure 2 shows the optimized structures of the experimentally known electrophilic phosphinidene complex [(η5-C5Me5)(CO)2Ru{PNiPr2}]+ (II) together with the model complexes of ruthenium [(η5-C5H5)(CO)2Ru{PNR2}]+ 11755



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Figure 2. Optimized structures of the electrophilic phosphinidene complexes of ruthenium [(η5-C5Me5)(CO)2Ru{PNiPr2}]+ (II), [(η5-C5H5)(CO)2Ru{PNiPr 2}]+ (V), [(η5-C5H5)(CO)2Ru{PNMe 2}]+ (VIII), and [(η5-C5H5)(PMe3)2Ru{PNMe2}]+ (XI).

Table 1. Selected Optimized Geometrical Parameters of the Electrophilic Phosphinidene Complexes [(η5-C5Me5)CO)2M{PNiPr2}]+, [(η5-C5H5)(CO)2M{PNR2}]+ (R = Me, iPr), and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (M = Fe, Ru, Os) bond angles (deg)a

bond distances (Å) complexes [(η5-C5Me5)(CO)2Fe(PNiPr2)]+ (I) [(η5-C5Me5)(CO)2Ru(PNiPr2)]+ (II) [(η5-C5Me5)(CO)2Os(PNiPr2)]+ (III) [(η5-C5H5)(CO)2Fe(PNiPr2)]+ (IV) [(η5-C5H5)(CO)2Ru(PNiPr2)]+ (V) [(η5-C5H5)(CO)2Os(PNiPr2)]+ (VI) [(η5-C5H5)(CO)2Fe(PNMe2)]+ (VII) [(η5-C5H5)(CO)2Ru(PNMe2)]+ (VIII) [(η5-C5H5)(CO)2Os(PNMe2)]+ (IX) [(η5-C5H5)(PMe3)2Fe(PNMe2)]+ (X) [(η5-C5H5)(PMe3)2Ru(PNMe2)]+ (XI) [(η5-C5H5)(PMe3)2Os(PNMe2)]+ (XII)

M−P

M−CO

2.168 [2.1515(6)]b 2.287 [2.2654(5)]c 2.302 [2.278(2)]d 2.186 2.295 2.311 2.168 2.279 2.298 2.131 2.216 2.240

1.758

M−PMe3

P−N

C−O

M−P−N

L−M−P

L−M−L

1.157

120.0 [118.66(7)]b 120.6 [119.58(6)]c 120.3 [119.0(2)]d 119.5 120.1 119.9 118.9 119.5 119.3 129.5 128.8 128.2

94.4

95.3

94.9

92.5

95.2

91.5

2.246 2.336 2.341

1.657 [1.629(2)]b 1.655 [1.627(2)]c 1.656 [1.634(5)]d 1.652 1.651 1.651 1.656 1.654 1.654 1.682 1.679 1.680

94.0 94.2 94.6 93.9 94.1 94.5 100.3 100.5 100.6

95.0 91.9 90.9 94.6 91.8 90.8 101.5 100.1 99.7

1.896 1.898 1.761 1.893 1.896 1.764 1.897 1.901

1.156 1.158 1.154 1.153 1.155 1.152 1.152 1.154

a

L = CO or PMe3. bX-ray structural data for [(η5-C5Me5)(CO)2Fe(PNiPr2)]+. cX-ray structural data for [(η5-C5Me5)(CO)2Ru(PNiPr2)]+. dX-ray structural data for [(η5-C5Me5)(CO)2Os(PNiPr2)]+.

Nalewajski−Mrozek (N−M) bond orders,68 Hirshfeld atomic charges,69 and natural hybrid orbitals (NHO),70 which are presented in Table 2. The tabulated results reveal that the values of (N−M) bond orders of the M−PNR2 bonds are in the range 0.93−1.41 and

suggest the presence of strong M−PNR2 bonds in the studied complexes. Upon substitution of ancillary CO ligands with PMe3 ligands, the values of the M−PNR2 bond orders consistently increase (in a manner consistent with the decrease in the calculated M−PNR2 bond distances). Note that the Ru−P 11756



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Table 2. N−M Bond Order, Hirshfeld Charges, and Natural Hybrid Orbital Analysis for Terminal Electrophilic Phosphinidene Complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+, [(η5-C5H5)(CO)2M{PNR2}]+ (R = Me, iPr), and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (M = Fe, Ru, Os) [(η5-C5Me5)(CO)2M{PNiPr2}]+ I

a

II

III

[(η5-C5H5)(CO)2M{PN iPr2}]+ IV

V

VI

[(η5-C5H5)(CO)2M{PNMe2}]+ VII

VIII

IX

X

XI

XII

0.97 1.50 0.98

1.06 1.50 1.13

1.38 1.38 0.73

1.31 1.36 0.69

1.41 1.36 0.79

0.26 0.02 0.27 0.17 0.26 0.43

0.14 0.04 0.34 0.19 0.25 0.44

−0.07 0.35 0.15 0.10 0.12 0.22

0.15 0.30 0.08 0.06 0.11 0.17

0.03 0.32 0.15 0.08 0.10 0.18

1.862

1.874

1.869

1.864

1.874

58.49 20.40 12.83 66.77 0.00

57.33 21.50 14.27 64.22 0.01

55.92 27.37 14.73 57.88 0.02

49.95 19.13 23.47 57.39 0.00

51.40 17.94 21.19 60.85 0.02

49.70 27.06 25.61 47.70 0.03

41.51 15.80 83.84 0.35

42.67 16.75 82.81 0.44

44.08 19.12 80.43 0.45

50.05 26.57 73.25 0.18

48.60 25.64 74.06 0.30

50.30 25.67 73.93 0.39

1.865 71.20 28.80

1.889 69.74 30.26

1.899 66.84 33.16

1.841 65.06 34.94

1.839 64.28 34.72

1.864 61.39 38.61

Nalewajski−Mrozek (N−M) Bond Order 1.01 0.94 1.03 1.04 1.51 1.52 1.52 1.49 1.04 0.99 1.14 1.02 Hirshfeld Charges 0.01 0.26 0.14 0.01 0.04 0.02 0.03 0.05 0.38 0.25 0.34 0.40 0.24 0.18 0.20 0.23 0.29 0.27 0.26 0.26 0.53 0.45 0.46 0.49 NHO Analysis of M−P σ-Bond 1.819 1.854 1.863 1.827

M−P P−N M−(CO/PMe3)

1.00 1.49 1.02

0.92 1.51 0.97

1.02 1.50 1.12

M La (η5-C5H5) P NR2 PNR2

0.00 0.02 0.49 0.23 0.24 0.47

0.25 0.00 0.34 0.18 0.23 0.41

0.14 0.01 0.42 0.19 0.23 0.42

occupancy M %M %s %p %d %f P % %s %p %d

1.588

1.851

1.869

42.34 32.08 44.21 23.71 0.00

55.67 21.64 15.01 63.34 0.01

55.15 23.01 16.82 60.15 0.02

58.11 20.43 13.32 66.25 0.00

57.66 17.07 82.64 0.29

44.33 17.92 81.70 0.39

44.85 20.83 79.36 0.40

41.89 15.31 84.35 0.34

occupancy %s %p

1.854 69.20 30.80

1.877 67.85 32.15

1.895 65.16 34.84

1.862 71.08 28.92

57.04 21.54 14.63 63.82 0.01

54.67 26.31 20.84 52.82 0.02

42.96 45.33 16.21 18.61 83.36 80.94 0.43 0.45 Lone-Pair Orbital on P 1.882 1.898 69.76 66.91 30.24 33.09

[(η5-C5H5) (PMe3)2M{PNMe2}]+

L = CO or PMe3.

As seen in Table 2, the calculated Hirshfeld charge distribution indicates that the phosphorus atom of PNR2 ligands always carries a significant positive charge in the complexes I−IX which become reduced in the complexes X−XII. One interesting observation is that the interacting moieties, i.e., PNR2 ligands and metal fragments, bear positive charge densities in all studied complexes. Here, we want to point out that the atomic partial charges contain no information about the electronic charge distribution around the atoms in a molecule, which is responsible for the electrostatic interaction between two species. Atoms that have an overall positive charge may have local areas of negative charge concentration that may attract negatively charged atoms that have local areas of charge depletion.84,85 The net positive charge on PNR2 ligands indicates that the overall charge flow from phosphinidene ligand to metal fragment. The small charges of PNR2 fragments in the complexes X−XII with PMe3 ligands than that in the studied carbonyl complexes suggest the relatively weaker M ← PNR2 σ-donation. One can infer from these observations that the M ← PNR2 σ-donation decreases upon replacing the CO ligands with the PMe3 ligands. All above observations are justified with the results of energy decomposition analysis (Table 3). To get better insight into the nature of M−PNR2 bonds in the complexes I−XII, the results of natural hybrid orbital (NHO) analysis are also presented in Table 2. The M−PNR2 σ-bonding orbitals are well-occupied (>1.80e) in all studied

Figure 3. Trends of Pauling bond orders and Nalewajski−Mrozek (N−M) bond orders of the M−P bond in the complexes I−XII.

bonds have smallest values of N−M bond orders as well as Pauling bond orders for the studied complexes (Figure 3). The Nalewajski−Mrozek bond order values for the P−N bonds of the respective carbonyl complexes are relatively higher, ranging from 1.49 to 1.52, which is decreased in the PMe3 complexes, X−XII, to the value 1.38 for X, 1.36 for XI, and 1.36 for XII. Again, the trends of bond orders of P−N bonds and the M−CO or M−PMe3 bonds are in line with the trends reported for the calculated geometries. 11757


Energy contribution in kcal/mol. bThe values in parentheses are the percentage electrostatic contribution to the total attractive interaction reflecting the ionic character of the bond. cThe values in parentheses are the percentage σ-contribution to the total orbital interaction ΔEorb.

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phosphinidene complexes. In the complexes I−IX with CO auxiliary ligands, the σ-bonding orbital slightly polarized toward metal atoms and contribute more than 55% to the M−PNR2 σ-bond, whereas in the bis(phosphine) complexes X−XII, the contribution of metal and phosphorus is almost equal. Further, the main contribution to the M−P σ-bonds arises from d-characters of the metal (∼60%) and p-characters of the phosphorus atom (∼80%). The stereoactive lone pair on the phosphorus resides in a spn hybrid orbitals with s character >61% in all the studied complexes. The % s contribution to the lone pair orbital of phosphorus decreases from M = Fe to M = Os in each set of complexes I−XII (Table 2). Energy Decomposition Analysis of Electrophilic Phosphinidene Complexes I−XII. To further probe the nature of M−PNR2 bonds in the complexes I−XII, we have carried out an energy decomposition analysis considering the fragments [(η5-C5Me5)(CO)2M]+ or [(η5-C5H5)(L)2M]+ and [PNR2] (singlet) in Cs symmetry. Table 3 gives the calculated energy values of various energy terms, and their trends are depicted in Figure 4. The tabulated bond dissociation energies (BDEs) reveal the expected periodic trend in the bond strengths due d-orbital extent; i.e., the Os−PNR2 bonds are stronger than the corresponding Fe−PNR2 and Ru−PNR2 bonds. Interestingly, the absolute values of BDEs for each set of all studied carbonyl complexes I−IX display a V-like trend (Figure 4a), with the minimum at the ruthenium. Such V-like trends in bond energies, with a minimum at the second row transition metal complexes are common, caused by the relativistic effects of 5d elements.86,87 The interaction energies, ΔEint, show the same trends as calculated BDEs (Figure 4b), with the discrepancies between two values (i.e., ΔEprep), amounting to about 7 kcal/mol. The nature and properties of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the fragments [(η5-C5Me5)(CO)2M]+ or [(η5-C5H5)(L)2M]+ and [PNR2] play an important role in explaining the orbital interaction difference, depicted in Figure 5 and exemplified for the complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+ (I−III). The energies of the HOMOs and LUMOs of the metal fragments vary as [(η5-C5Me5)(CO)2Fe]+ (HOMO = −9.676 eV; LUMO = −8.332 eV), [(η5-C5Me5)(CO)2Ru]+ (HOMO = −9.821 eV; LUMO = −8.008 eV), and [(η5-C5Me5)(CO)2Os]+ (HOMO = −9.898 eV; LUMO = −8.131 eV), whereas the corresponding energies of the [PNR2] moiety are calculated as [PNiPr2] (HOMO = −3.657 eV; LUMO = −3.073 eV). Evidently, the LUMO of the metal fragment [(η5-C5Me5)(CO)2Ru]+ is closer into the HOMO of the ligand fragment, allowing better M ← PNR2 σ-donation. On the other hand, the relatively larger energy gap between HOMO of the metal fragment [(η5-C5Me5)(CO)2Ru]+ and LUMO of the ligand fragment results weaker M → PNR2 π-back-bonding. The latter effect appears to outweigh the aforementioned σ-bond interaction, and thus, the M−P bond strength of ruthenium complexes is relatively weaker than the iron and osmium complexes. The effect of substitution of relatively larger pentamethylcyclopentadienyl ring (η5-C5Me5) on metal with smaller one (η5-C5H5) is also clearly observed in Table 3 where the interaction energy values as well as BDEs of the complexes [(η5-C5Me5)(CO)2M(PNiPr2)] (I−III) are nearly 10 kcal/mol smaller than their [(η5-C5H5)(CO)2M(PNR2)] counterparts. Despite the shortest M−PNR2 bond distances in the complexes X−XII, their corresponding values of the ΔEint and BDEs are also smallest among the studied complexes. Moreover, the M−P bond strengths of the complexes

a

XII XI X

−105.7 −76.7 −82.5 −92.8 231.0 173.3 218.4 262.8 −206.3 (61.3%) −143.8 (57.5%) −183.8 (61.1%) −222.8 (62.6%) −130.4 −106.2 −117.1 −132.8 −108.6 (83.3%) −72.9 (68.6%) −83.2 (71.5%) −96.9 (73.0%) −21.8 −33.3 −33.8 −35.9 6.8 11.1 11.2 16.8 −98.9 −65.6 −71.3 −76.0

IX VIII VII

−107.4 −95.2 −95.6 225.1 169.0 195.3 −202.2 (60.8%) −147.8 (55.9%) −173.1 (59.5%) −130.3 −116.3 −117.8 −109.3 (83.9%) −96.5 (83.0%) −97.5 (82.8%) −21.0 −19.7 −20.3 7.6 4.2 6.5 −99.8 −91.0 −89.1

VI V IV III II I

−86.8 −85.6 −96.0 −96.7 −97.1 ΔEint ΔEpauli 174.4 199.8 238.5 164.2 189.4 ΔEelestb −149.9 (57.2%) −173.9 (60.9%) −209.1 (62.5%) −144.6 (55.4%) −168.8 (58.9%) ΔEorb −111.3 −111.5 −125.4 −116.3 −117.7 ΔE(a′)c −91.0 (81.8%) −91.0 (81.6%) −102.9 (82.1%) −97.5 (83.8%) −98.3 (83.5%) ΔE(a″) −20.3 −20.5 −22.5 −18.8 −19.4 ΔEprep 4.8 6.3 7.3 4.6 6.9 ΔE(−BDE) −82.0 −79.3 −88.7 −92.1 −90.2

[(η5-C5H5)(PMe3)2M{PNMe2}]+ [(η5-C5H5)(CO)2M{PNMe2}]+ [(η5-C5H5)(CO)2M{PNiPr2}]+ [(η5-C5Me5)(CO)2M{PNiPr2}]+

Table 3. Energy Decomposition Analysisa of Terminal Electrophilic Phosphinidene Complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+, [(η5-C5H5)(CO)2M{PNR2}]+ (R = Me, iPr), and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (M = Fe, Ru, Os)


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Figure 4. (a) Trends of the absolute values of bond dissociation energies for the phosphinidene complexes I−IX and (b) trends of absolute values of different energy terms (in kcal/mol) for the complexes I−XII.

(Table 1). The reasons for these observed energy trends will be discussed in subsequent section. The breakdown of the interaction energy, ΔEint, into the repulsive term ΔEPauli, and the attractive terms ΔEorb and ΔEelstat, presented in Table 3, shows that the repulsive interaction ΔEPauli, has the largest absolute value. Figure 4b shows that the trends of ΔEelstat and ΔEPauli, from iron to osmium in each set of complexes are roughly parallel to each other. The absolute values of both terms increase sharply on going from M = Fe to M = Os. The electrostatic interaction ΔEelstat, in all complexes, I−XII, is significantly larger than the orbital interactions ΔEorb, which means the ionic contribution to the M−P bonds is greater than the covalent contributions. The values of electrostatic interaction ΔEelstat contribute nearly 55− 62% to the total attractive interactions. However, it is significant to note that the trend of interaction energies, ΔEint is best correlates with the trend of the orbital interactions ΔEorb rather than ΔEelstat (Figure 4b). It is the interplay of the three terms ΔEPauli, ΔEorb, and ΔEelstat that determines the net bond energies of the particular bond. For example, although the M−PNR2 bonds in the phosphine complexes [(η5-C5H5) (PMe3)2M{PNMe2}]+ (X−XII) are shorter than that in the respective carbonyl complexes [(η5-C5H5)(CO)2M(PNR2)]+, the ΔEPauli values in the former complexes (X−XII) are only slightly smaller (in iron complex) or even larger in the ruthenium and osmium complexes. The increase of the Pauli

Figure 5. Frontier orbital energies (in eV) of the (a) PNR2 fragments and (b) metal fragments.

I−III are weaker than those for the complexes VII−IX whereas the bond distances of two set of complexes are almost equal 11759



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Figure 6. Important Kohn−Sham molecular orbitals of the electrophilic phosphinidene complex [(η5-C5Me5)(CO)2Ru{PNiPr2}]+ (II).

shows the valence s-orbital of phosphorus atom, and the stereoactive lone pair at the phosphorus atom is depicted in Figure 6e, which is the highest occupied molecular orbital (HOMO) of the complex II. Figure 6f shows the M−P π-antibonding orbital.

repulsion compensates for the increase of the attractive interactions and thus, prevents stronger bond at shorter distance. Similarly, when the values of various energy terms of the complexes I−III and the complexes VII−IX are compared, it becomes clear that the relative increase in ΔEPauli is somewhat higher than the relative increase in ΔEelstat for the former complexes and further the values of the orbital interactions also relatively smaller. Thereby, the net bonding interaction in the complexes I−III is weaker than that in the complexes VII−IX. An interesting difference between the bonding in different sets of the complexes I−XII concerns the σ- and π-contributions to the total orbital interactions. Table 3 also gives the breakdown of the ΔEorb into the M ← PNR2 σ-donation and M → PNR2 π-back-donation components. The π-contribution ΔEa″ (19−36%) to the orbital interactions in the studied complexes is significantly smaller than the σ-contribution, ΔEa′, and increases upon going from iron to osmium. Upon substitution of good σ-donor, π-acceptor ligands CO with strong σ-donor PMe3 ligands (X−XII), the strength of M ← PNR2 σ-donation readily decreases while M → PNR2 π-back-donation increases. The substituent R of aminophosphinidene ligands also influences, albeit minor, the M → PNR2 π-back-donation. For example, relatively weaker P−N π-bonding is expected in the complexes VII−IX due to smaller electron donating substituents R = Me than that in the complexes IV−VI with R = iPr. Hence, the M → PNR2 π-back-donation in the former complexes is enhanced slightly but σ-donation also reduces subsequently. As a result, the values of total orbital interactions in the complexes [(η5-C5H5)(CO)2M(PNiPr2)]+ (IV −VI) and [(η5-C5H5)(CO)2M(PNMe2)]+ (VII−IX) are almost equal (Table 3). Thus, the strength of π-back-donation in the electrophilic phosphinidene complexes depends on the nature of auxiliary ligands as well as strength of N−P π-bonding (substituent R). To visualize the M−PNR2 bonding in the studied phosphinidene complexes, the envelope plots of some relevant molecular orbitals of the ruthenium complex [(η5-C5Me5)(CO)2Ru(PNiPr2)]+ (II) are presented in Figure 6. The M−P σ-bonding is depicted in Figure 6a (HOMO−16), and significant contribution of M → PNR2 π-back-donation is shown in Figure 6b (HOMO−6). The (HOMO−3) molecular orbital in Figure 6c is illustrated P−N π-bonding. Figure 6d (HOMO−2)

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SUMMARY AND CONCLUSIONS The following conclusions may be drawn from the theoretical investigation of M−PNR2 bonding in the electrophilic phosphinidene complexes of iron, ruthenium, and osmium [(η5-C5Me5)(CO)2M{PNiPr2}]+ (I−III), [(η5-C5H5)(CO)2M{PNR2}]+ (R = iPr, IV−VI; R = Me, VII−IX), and [(η5-C5H5)(PMe3)2M{PNMe2}]+ (X−XII): (1) The calculated geometry parameters of the complexes [(η5-C5Me5)(CO)2M{PNiPr2}]+ (I−III) are in good agreement with their available experimental values (Table 1). The calculated M−PNR2 bond distances are shorter than the M−P single bonds and further shorten upon replacing the CO ligands with PMe3 ligands. (2) The calculated Pauling bond order and Nalewajski− Mrozek (N−M) bond orders of the M−PNR2 bond are in the range 1.00−2.00, indicating the M−P partial double bond characters. Hirshfeld charges show that the overall charges flow from phosphinidene to metal fragment. The M−P σ-bonding orbitals in all complexes are well-occupied (>1.80e). (3) The binding energies of all studied carbonyl complexes, I−IX, exhibit a V-like trend, with the minimum at the ruthenium. Further, the binding energies of the phosphinidene complexes vary according to nature of the substituent R of the ligand PNR2 as well as auxiliary metal ligand framework. (4) The electrostatic interaction ΔEelstat, in all complexes, I−XII, is significantly larger than the orbital interactions ΔEorb. Moreover, the π-contribution ΔEa″ (19−36%) in the studied complexes is significantly smaller than the σ-contributions ΔEa′. (5) The π-back-bonding increases upon going from iron to osmium in each set of complexes I−XII. Upon substitution of auxiliary CO ligands with strong σ-donor PMe3 ligands (in X−XII), the strength of M ← PNR2 11760



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σ-donation readily decreases while M → PNR2 π-backdonation increases.

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ASSOCIATED CONTENT

S Supporting Information *

Full references of ADF 2010.01 (ref 71) and Cartesian coordinates of all optimized geometries of the cationic electrophilic phosphinidene complexes I−XII. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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