18-Electron Rule: Myth or Reality? An NBO Perspective

Transcription

1 18-Electron Rule: Myth or Reality? An NBO Perspective Eric CLOT UMR CNRS, UM2, ENSCM, UM1

2 Interaction Experiment/Theory Experimental Chemists have Questions Where are the Nucleophilic and Electrophilic Sites in a Molecule? ow Strong are the Bonds within a Molecule? What is the Nature of the Bonds between the Atoms? Why is this Molecule more Stable than this Other? ow can we Describe the Interaction between two Reacting Molecules? They use Concepts to Rationalize the Results Lewis structure, Resonance, yperconjugation Electonegativity, ybridation

3 Computational Chemistry Geometry Method Basis Set Optimization Energy Ψ = E Ψ Wavefunction STRUCTURE, PROPERTIES, REACTIVITY

4 Complex with octahedral geometry L L L L z M L M L L L y L L L L x 3 non bonding orbitals : up to 6 d electrons possible. 18 electrons rule!

5 Complex with square planar geometry L L z M L M L L L y L L x 3 non bonding orbitals : up to 6 electrons possible. 1 quasi non bonding orbital : up to 2 electrons possible. stable with 16 electrons!

6 Natural Bonding Orbitals: Concept Use Information in Wavefunction to Build a Lewis Structure Electronic Configuration for the Atoms in the Molecule : Natural Atomic Orbitals (NAO) Mixing of Natural Atomic Orbitals to Build ybrids : Natural ybrid Orbitals (NO) Covalent Bond as Overlap between ybrids : Natural Bond Orbitals (NBO) Departure From Stricktly Localized Lewis Structure : Natural Localized Molecular Orbitals (NLMO)

7 Bibliography NBO Web Site : nbo5/ Valency and Bonding A Natural Bond Orbital Donor-Acceptor Perspective Frank Weinhold University of Wisconsin, Madison Clark R. Landis University of Wisconsin, Madison (ISBN-13: ISBN-10: ) DOI: /

8 NBO Procedure: Summary

9 Description of an NBO The Overlap between two ybrids to Create a Bonding Orbital Automatically Generates an Antibonding Orbital. σ CO σ = h C h O σ CO σ = h C h O

10 yperconjugation Interactions The Empty AntiBonds can Accept Electron Density from the Occupied Bonds : Departure from Lewis Structure σ C Overlap : LP O Resonance Forms : C O ow Stabilizing is this Resonance? C O

11 Donor-Acceptor Interactions The Departure from the Lewis Structure is Due to Donor-Acceptor Electron Transfers between Occupied and Vacant NBOs. 2 nd Order Perturbation Theory 2 E = 2 σ F σ σ ε σ ε σ LP 0 σc = 20.1 kcal mol 1 E σ = σ + λσ σ The Perturbed NBO Gives Access to Natural Localized Molecular Orbitals which Are Occupied stricktly by 2 Electrons LP 0 = LP σ C σ C

12 MO vs. NLMO MO NLMO -11 ev -7.3 ev -11 ev -8.7 ev ev ev ev ev ev ev ev ev

13 NBO and Transition Metal Complexes Group 3-12 binary oxides, chlorides and alkyl compounds that are avalaible most cheaply and in largest quantities follow a simple relation. G M 6 + n X V X + e u = 6 G M : n X : V X : e u : Group Number of the Metal Number of X Ligands Valency of the X Ligand Number of Unpaired Electron chlorides ScCl 3 TiCl 4 NbCl 5 WCl 5 RuCl 3 RhCl 3 PdCl 2 oxides Sc 2 O 3 TiO 2 Nb 2 O 5 WO 3 RuO 2 Rh 2 O 3 PdO alkyls ZrMe 4 NbMe 5 WMe 6 Ru(Mes) 4 Rh(Mes) 3

14 Dodectet Rule Each bond linkage to a monovalent ligand X is associated with a shared pair of electrons : e bp = 2n X V X Each bond linkage to a Lewis base L is associated with a dative pair of electrons : e dp = 2n L For a closed shell compound with G M 6, the empirical rule gives : G M n X V X + e bp + 2n L = 12 with the non-bonded lone pair electrons e lp = G M n X V x The dodectet rule holds : e bp + e lp + e dp = 12

15 Case Study : W x (CO) y In each case, NBO gives a Lewis structure that obeys the dodectet rule. W CO W W CO CO OC W CO CO e bp = 12 e lp = G M -6 = 0 e dp = 0 e bp = 8 e lp = G M -4 = 2 e dp = 2 e bp = 4 e lp = G M -2 = 4 e dp = 4 e bp = 0 e lp = G M -0 = 6 e dp = 6 Only one s and five d orbitals are necessary to build the hybrids on the metal able to accomodate 6 pairs of electrons.

16 sd ybridation Geometry Dictated by ybridation between s and d Orbitals α accute α obtuse sd sd sd sd sd sd d W = C 3v C 5v C 3v C 5v

17 Influence of a π-accepting Ligand The π-back-donation from a lone pair on the metal to a π-accepting orbital on the ligand induces changes in the preferred geometries

18 W 4 CO sd 4 ybridation 65.9 and NLMO : σ W = 0.92σ W 0.33π CO W C O W C O W C O W C + + %NRT O

19 W(CO) 3 and W(CO) 6 W(CO) 3 : Three Lone Pairs Involved in Back-Bonding W(CO) 6 is ypervalent (18-e) with 3-Centers 4-Electrons Bonds σ (W-C) LP(W) LP(C) NBO NLMO

20 ω-bonds The vast majority of transition metal complexes are either octahedral or square planar with 18- or 16-electrons, respectively. The mechanism to override the 12-electron rule and to add extra ligands involves a particular type of bonding, the 3c-4e ω-bond introduce by Pimentel to describe bonding in hypervalent main group compounds. I 3 - σ * (M-L) σ(m-l) LP(L')

21 Kubas Complex The first dihydrogen complex W(CO) 3 ( ) 2 ( 2 ) is a typical octahedral complex following the 18-electron rule. The Lewis structure proposed by NBO contains a W(CO) 2 ( ) 12-electron fragment and three entities (CO,, 2 ) in interaction through 3-center 4-electron ω-bonds to reach an 18-electron configuration. CO OC W OC sd 2 ybridation : three M-L bonds at 90.

22 Analyzing Electron Transfer Upon Coordination The composition of the NLMO allows to estimate the strength of σ-donation from the ligands. σ( 2 ) = 0.92σ( 2 ) σ (WC) LP(P) = 0.86LP(P) σ (WP) LP(C) = 0.87LP(C) σ (WC)

23 Analyzing Electron Transfer Upon Coordination The composition of the NLMO allows to estimate the strength of π-back-donation from the metal. LP(W ) = 0.90LP(W ) + 0.3π (CO) + 0.2σ ( 2 ) LP(W ) = 0.87LP(W ) + 0.3π (CO) + 0.3π (CO) LP(W ) = 0.85LP(W ) π (CO) (π (CO) + π (CO))

24 Analyzing Trans Influence The composition of the σ( 2 ) and LP(W) NLMOs allows to estimate the strength of σ-donation and π-back-donation. L OC P 3 CO W P 3 2 CO NC P 3 = Å σ( 2 ) = 0.92σ( 2 ) σ (WC) LP(W ) = 0.90LP(W ) + 0.3π (CO) + 0.2σ ( 2 ) = Å σ( 2 ) = 0.895σ( 2 ) σ (WC) LP(W ) = 0.86LP(W ) π (CN) σ ( 2 ) = Å σ( 2 ) = 0.886σ( 2 ) σ (WC) LP(W ) = 0.91LP(W ) σ ( 2 ) Observed bond distance results from both σ-donation and π-back-donation.

25 Guidelines In general, an octahedral ML 6 d 6 complex will have a structure where the strong σ-donors are creating a C 3v like ML 3 12-electron fragment with mutual 90 angles. The three remaining ligands are engaged in non-symmetric ω-bonds and the composition of the NLMOs gives access to the extent of charge transfers from the ligand to the metal and from the metal to the ligand. The best situation for the structure is obtained when the weakest ligand is trans to the strongest one. The same situation holds for ML 4 d 8 complexes with a C 2v -like 12-electron building block. Again the same pattern is valid for ML 2 d 10 systems with a 12-electron M-L unit as the Lewis structure.

26 ydrogenation and Wilkinson s Catalyst Many catalytic cycles are drawn with particular emphasis on the number of electrons but not on the relative positions of the various ligands with different stereoelectronic properties.

27 Phosphine Dissociation Wilkinson s catalyst is hypervalent with 16-electron and upon phosphine dissociation another hypervalent complex is formed (14-electron). + R 3 P Rh Cl R 3 P Rh Cl + Rh Cl The ML 3 intermediate has a T-geometry and not a Y-shape. The phosphine engaged in an ω-bond dissociates.

28 Dihydrogen Activation The vacant site trans to P does not allow too strong ethylene coordination and affords an easy pathway for 2 oxidative addition. C R 3 P Rh Cl R 3 P Rh Cl R 3 P 2 Rh Cl 13.3 R 3 P Rh Cl The ML 5 intermediate is not a trigonal bipyramid. The oxidative addition is easy because the nature of the M-P bond trans to can change smoothly.

29 Olefin Insertion R 3 P Cl Rh C R 3 P Cl C 2 Rh C 2 R 3 P C 2 5 Rh Cl R 3 P Rh Cl R 3 P Rh C C 18.1 C 2 Cl R 3 P Rh Cl The catalyst needs to adapt the relative position of the ligands to open a low energy pathway. Best to avoid situations with two strong donor ligands mutually trans.

30 Overall Catalytic Cycle 20 Gibbs Free Energy (kcal mol -1 ) Oxidative Addition Olefin Insertion Reductive Elimination -20 The easiest pathway preserves a situation where the three strongest ligands in each steps are creating a pseudo C 3v ML 3 fragment with the three remaining ligands engaged in ω-bonds.

31 Muraï s Reaction Selective C C coupling in ortho of the carbonyl group. Ru( 2 ) 2 () 2 ( ) 2 allows to work in mild conditions, contrary to the catalyst developped by Muraï, Ru(CO)() 2 (PPh 3 ) 3. R R O + R' Ru 2 ( 2 ) 2 ( ) 2 Substituting the carbonyl with pyridine impedes the C-C coupling but leads to the characterization of the C activation product. r.t. O R' N + Ru 2 ( 2 ) 2 ( ) 2 N Ru 2

32 Ortho C- activation with chelation Formation of the agostic interaction more difficult than actual C cleavage. σ-cam mechanism preserves the optimal relative position of the ligands

33 Ethylene Insertion into Ru or Ru C?

34 Ethylene Insertion Coordination cis to aromatic ring is mandatory to achieve low barrier for insertion into Ru-. Insertion is associated with transfer from 2 to. ere again the typical ML 3 fragment of strong donors is maintained.

35 C-C Coupling Isomerization to cis phosphines is necessary to achieve a geometry adapted to C-C coupling. cleavage is necessary to keep the cis phosphine complex at low energy

36 Catalytic Cycle decoordination O=C coordination C 2 =C 2 oxidative cleavage of C- reductive coupling of - II Ru O II Ru O insertion of C 2 =C 2 into Ru- II Ru O II O Ru II Ru O O product release O II Ru O IV Ru O coordination of O=C reductive coupling of C-C isomerization to cis P oxidative cleavage of -

37 Conclusions Transition metals use essentially s and d orbitals to create bonds with the ligands: sd n hybridation A stable configuration is reached when 6 pairs of electrons surround the metal: dodectet rule ypervalency is easily obtained through donation from a lone pair into a M-X antibond: ω-bond. 18-electron or 16-electron complexes are hypervalent and the relative positions of the strong donor ligands follow the dodectet rule : sd 2 or sd hybridation. Dodectet rule also explains the mutual position of the ligands along a catalytic cycle.