School and university students learn that electron pair donor Lewis bases "react with" or "complex with" or "interact with" electron pair acceptor Lewis acids.

There are six distinct types of Lewis acid and four distinct types types of Lewis base, where distinction is by frontier molecular orbital (FMO) topology. It follows that the the six types of Lewis acid and the four types of Lewis base interact to give 24 distinct types of Lewis acid/base complex.

The range of chemistry encompassed and described by the Lewis Acid/Base Interaction Matrix ranges across organic, inorganic and organometallic reaction chemistry. The Lewis acid/base interaction matrix is the core finding of the chemogenesis analysis.

  • The Proton

  • s-LUMO Lewis Acids

  • Onium Ion Lewis Acids

  • Lobe-LUMO Lewis Acids

  • π-LUMO Lewis Acids

  • Heavy Metal Lewis Acids

  • s-HOMO Lewis Bases

  • Complex Anion Lewis Bases

  • Lobe-HOMO Lewis Bases

  • π-System Lewis Bases

Lewis Acid/Base Complexes

Lewis acid/base interaction chemistry can be stated in two ways:

  • Electron pair donor Lewis bases "react with" or "complex with" or "interact with" electron pair acceptor Lewis acids to give Lewis acid/base complexes.

  • The highest occupied molecular orbital (HOMO) of a Lewis base "reacts with" or "complexes with" or "interacts with" the lowest unoccupied molecular orbital (LUMO) of a Lewis acid to give a Lewis acid base complex with a bonding molecular orbital. The contributions of +/– charge and orbital overlap is described by the Klopman equation, here.

The six distinct types of Lewis acid and the four distinct types of Lewis base – where distinction is by frontier molecular orbital (FMO) topology, ie the shape, phase and geometry of the participating HOMOs and LUMOs – interact to give 24 distinct types of Lewis acid/base complex. This process can be visualised with the aid of the Lewis acid/base interaction matrix graphic:

matrix03.jpg

Type 1 Lewis Acid/Base Complexation Chemistry

2 Complexes:

Type_1.png

Bonding:

Type 1 complexes, typified by hydrogen H2, are covalently bonded. The bonding is frontier molecular orbital (FMO) controlled. Hydrogen, H2, has a 1σ2 MO structure, ie they have two electrons in their 1σ molecular orbital.

Charge:

Complexes can be neutral, H2, or positively charged [H2]+.

Chemistry:

Protons complex with hydride ions to form molecular hydrogen, H2, a uniquely simple and much studied diatomic molecule.

The H+ + H ––> H2 reaction is not reversible: H2 does not act as a proton donor (although at high temperature, when exposed to high energy UV radiation or when absorbed onto a metallic surface, H2 can homolytically dissociate: radical cleavage).

As protons and hydride ions do not exist as independent species, they require "delivery" by donor complexes, ie reagents. Protons, H+ ions, are supplied by Brønsted acids and hydride ions by hydride donor complexes.

For example, hydrogen chloride, an H+ donor, reacts with sodium hydride an H donor to give diatomic hydrogen and sodium chloride:

HCl + NaH ––> H2 + NaCl

[H3]+, the product of H+ and H2, is the simplest possible triatomic molecular ion – it has only two electrons – and is of considerable theoretical interest.

The [H3]+ molecular ion occupies an important position in theoretical models of interstellar chemistry as the [H3]+ forms in hydrogen-rich interstellar gas clouds. The [H3]+ ion can protonate carbon, oxygen and other atoms, thereby initiating the interstellar synthesis of nearly 100 molecules including: hydroxyl radicals (HO), carbon monoxide, ethanol, linear polyacetylenes and cyclopropenylidene.

(In this author's opinion the [H3]+ ion should be called the 'hydronium ion', and [OH3]+ should be the 'oxonium ion'.)

Congeneric Arrays:

Few series.


Type 2 Lewis Acid/Base Complexation Chemistry

George Olah Superacids

Screen Shot 2018-07-26 at 10.33.13.png

Bonding:

Protons do not complex efficiently with complex anion Lewis bases as the proton must disrupt the anion’s high symmetry HOMO when a 1:1 complex forms. Hence, the resulting complex is a very powerful proton donor.

Complexes are ionic/highly polar covalent.

Charge:

Superacids are neutral.

Chemistry:

Proton plus complex anion complexes must be prepared in ‘exotic’ solvents such as liquid sulfur dioxide and where the complex anion has fluoride ion ligands.

George Olah's 'magic acid' is prepared by mixing antimony pentafluoride and fluorosulfonic acid. Hydrocarbon wax will dissolve in a magic acid solution.

Such complexes are the strongest Brønsted acids known. Super [Brønsted] acids, 'superacids', have pKa values in the region -15 to -25, ie they are up to 20 orders of magnitude more [Brønsted] acidic than sulfuric acid.

Superacids are able to fully protonate all Lewis base organic functional groups (as opposed to protonating a low equilibrium concentration). Alkenes, benzene, carbonyls and nitro functional groups are all protonated by superacids. For example:

02_1.png

Congeneric Arrays:

Few series.


Type 3 Lewis Acid/Base Complexation Chemistry

Common Brønsted Acids

Screen Shot 2018-07-26 at 10.34.01.png

Bonding:

The point-charge of the proton, H+, is able to efficiently penetrate and complex with the directional lobe shaped sp3/sp2/sp orbitals of all Lobe-HOMO Lewis bases. The resulting complexes are either covalent or polar-covalent.

Charge:

Complexes may be negatively charged, positively charged or they may be neutral.

Chemistry:

The vast majority of Brønsted acids are Type 3 Proton/Lobe-HOMO Lewis acid/base complexes, including all of the common mineral acids H2SO4, HCl, HNO3, H3PO4 etc.

Congeneric Arrays:

Complexes show regular Brønsted acid behavior across congeneric series, ie across and down the periodic table:

03_1.png

Brønsted acidity correlates with Lewis base to proton bond length, elsewhere in this webbook, here. CH4, methane and NH3, ammonia are the weakest Brønsted acids and have the shortest bond lengths while HI, hydrogen iodide and protonated xenon, [HXe]+, are the most acidic and have the longest bond lengths.

Brønsted acidity increases with increasing conjugation and resulting π-stabilisation.

03_2.png

Read more elsewhere in this webbook, here


Type 4 Lewis Acid/Base Complexation Chemistry

Protonation of π-Systems

Screen Shot 2018-07-26 at 10.41.33.png

Bonding:

When a proton complexes with a hydrocarbon π-system (as opposed to complexing with a single heteroatom atomic centre) the conjugated π-system is reduced in length by one p-orbital to form a Hückel distinct MO system: 2palkenes are protonated to give give 1p carbenium ions, 6p benzene is protonated gives the corresponding 5p carbenium ion, etc.

Bonding in the complexes is covalent.

Charge:

The charge on a Type 4 complex can be negative, neutral or positive.

Chemistry:

When a proton complexes with a hydrocarbon π-system the conjugated π-system is reduced in length by one p-orbital to form a Hückel distinct MO system: 2p alkenes give 1p carbenium ions, 6p benzene gives the corresponding 5p carbenium ion, etc.

04_2.png

If there is a heteroatom Lewis base centre present, it is likely that protonation will occur at that centre. For example, 2p carbonyl functions protonate on oxygen and 6p (aromatic) pyridine protonates on the nitrogen lone pair which is not part of the conjugated π-system:

04_3.png

While the action of a strong acid on benzene is null, benzene is deuterated with deutero sulfuric acid, D2SO4. In time the benzene will become fully (or per) deuterated:

04_4.png

Congeneric Arrays:

There are few congeneric series of real interest as each species is Hückel/FMO unique.


Type 5 Lewis Acid/Base Complexation Chemistry

Saline Hydrides

Screen Shot 2018-07-26 at 10.46.42.png

Bonding:

Group I & II alkali and alkaline earth hydrides exist as ionic lattice solids. However, molecular 1:1 (LiH) and 1:2 (MgH2) complexes are formed in the vapor phase, although with difficulty as the compounds are liable to decompose back to elemental form.

LiH is a much theoretically studied diatomic species. Studies show that bonding involves more than just s-LUMO/s-HOMO overlap. In valence bond (VB) terminology, the lithium’s 2s-LUMO mixes with (ie hybridizes with) a higher energy empty 2p orbital (ie the LUMO + 1 MO) to generate a directional sp hybrid bond.

MO calculations show that the bonding in LiH involves 66% Li 2s/H1s overlap and 34% Li 2p/H 1s overlap.

Charge:

The charge on a Type 5 complex is always neutral.

Chemistry:

Saline hydride complexes either act as strong proton abstracting Bases or as donors of nucleophilic hydride ion. Organic chemists employ saline hydride complexes as proton abstracting Bases, with H2 being the conjugate Brønsted acid.

Sodium hydride readily abstracts a proton from dimethyl sulfoxide (DMSO), pKa 35, to form sodium dimsyl:

05_1.png

Inorganic chemists are more likely to use saline hydride reagents as a source of nucleophilic (and reducing) hydride ion, for example in the synthesis of sodium borohydride:

05_2.png

Congeneric Arrays:

Screen Shot 2018-07-26 at 10.50.28.png

Type 6 Lewis Acid/Base Complexation Chemistry

Ionic Salts

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Bonding:

Type 6 complexes are, in Pearson-Klopman HSAB terms, classic hard/hard complexes which give rise to charge-controlled ionic salts.

Charge:

The charge on a Type 6 complex is always neutral.

Chemistry:

In polar solvents species dissociate to give independently solvated ions. While in solution, the complex anion part of a Type 6 complex is susceptible to ligand transfer or substitution. The s-LUMO ion part of the complex usually acts as a spectator ion, however, choice of ion influences solubility.

Reagents such as lithium aluminium hydride and sodium borohydride act as hydride ion donor complexes. Type 6 hydride complexes are weaker proton abstracters than the very basic Type 5 complexes and they are correspondingly more suitable for use by organic chemists as sources of reducing nucleophilic hydride ion:

06_1.png

Congeneric Arrays:

Many important ligand replacement congeneric series are known, for example:

06_2.png

Type 7 Lewis Acid/Base Complexation Chemistry

Common Strong & Weak Brønsted Bases and Ionic Salts

Screen Shot 2018-07-26 at 10.55.58.png

Bonding:

The s-LUMO Lewis acids like to be multiply complexed by Lobe-HOMO Lewis base ligands to achieve maximum spherical symmetry about the cation.

Complexes form as ionic crystal lattice (cesium fluoride) or they may generate polar-covalent bonds (methyl lithium).

Charge:

The charge on a Type 7 complex can be negative, neutral or positive.

Chemistry:

With a few important exceptions – for example, amines and the saline hydrides – most of the proton abstracting (Brønsted base) reagents are Type 7 Lewis acid/base complexes:

NaOH, KNH2, LiCH3, CH3COONa NaHCO3, Na2CO3, etc.

Other Type 7 complexes, in which the Lobe-HOMO Lewis Base is the conjugate base of a strong Brønsted acid, are non-basic ionic salts:

      KCl     NaI     LiBr   etc.

In polar solvent solution (water, THFDMF or DMSO), Type 7 complexes may show differing degrees of ion pairing:

07_1.png

Polar solvation requires the solvent to have lone pair Lobe-HOMO centres which compete with the Type 7 Complexes’ Lobe-HOMO Lewis base centre.

Methyl lithium, for example, is usually considered to be a 1:1 polarised covalent compound, however, in diethyl ether or THF solvents the lithium cation is co-complexed with solvent molecules.

In alkane solvents alkyl lithium reagents aggregate into hexamers and higher structures, Wikipedia.

Congeneric Arrays:

There are many important congeneric series: alkyl carbanion reagents become more basic as the alkyl groups are replaced by hydrogen ligands, and as the metal counter ion becomes congenerically heavier.

Regular changes in physical properties occur as pairs of congeneric Lewis acid/base series interact to form congeneric series, planars and volumes of complexes:

07_2.png

Type 8 Lewis Acid/Base Complexation Chemistry

Stabilised π-Anion Complexes

Screen Shot 2018-07-26 at 11.09.58.png

Bonding:

s-LUMO Lewis acids complex poorly with π-HOMO Lewis bases:

  • The s-LUMO Lewis acid is "looking for" ligands, the π-HOMO Lewis base provides a large delocalised anionic π-system.

Thus, the bonding interaction is charge controlled and ionic even though the participating species are "looking for" FMO controlled bonding. The problem is that the FMOs are of dissimilar geometry.

Species may only be stable in [non-aqueous] solution where the s-LUMO Lewis acid is complexed by the solvent. Typical solvents are diethyl ether or THF. 

Charge:

The charge on a Type 8 complex is always neutral.

Chemistry:

s-LUMO Lewis acids are employed as non-electrophilic spectator counter ions to π-HOMO systems with net negative charge: ie the allyl, benzyl and cyclopentadienyl anions. The effect of such complexation is to make the π-HOMO anion to appear ‘naked’ for spectroscopic and reactivity studies.

Bonding in Type 8 complexes is strongly influenced by environment: polar solvents compete with the π-system ligand at complexing the hard s-LUMO Lewis acid cation. s-LUMO/π-anion chemistry is usually carried out in diethyl ether, THF or glyme (polyether) solvents where the ethereal Lewis bases are the true s-LUMO complexing agents.

Most type 8 complexes are strong proton abstracting bases because the conjugate Brønsted acids of the π-HOMO systems are weakly [Brønsted] acidic π-hydrocarbons. The conjugate Brønsted acid of benzyl lithium, for example, is toluene, pKa of 41.

Congeneric Arrays:

The nature of the s-LUMO cation/π-system bond changes congenerically with cation: a Li+ complex is less ionic (therefore more covalent and therefore more strongly ion-paired in solution) than the equivalent Cs+ complex.

08_2.png

Type 9 Lewis Acid/Base Complexation Chemistry

Reduction of Onium Ion

Screen Shot 2018-07-26 at 11.14.47.png

Bonding:

Hydride ions do not form 1:1 complexes with onium ions because the Brønsted basic, nucleophilic, reducing hydride ion will initiate a proton abstraction reaction.

Charge:

A reaction always occurs, rather than forming a Lewis acid/base complex so the idea of a charge on a complex is not applicable.

Chemistry:

If the onium ion has a proton ‘ligands’ the onium ion will act as a Brønsted acid and donate a proton to the hydride ion to form H2:

H3O+ + H ––> H2 + H2O:

If the onium ion has alkyl ligands (such as a trialkyl oxonium ion or a tetraalkyl ammonium ion) a nucleophilic substitution will occur in which electropositive nucleophilic hydride ion reduces one of the onium ion ligands to the corresponding alkane:

09_1.png

Congeneric Arrays:

The concept of congeneric series is not so useful with onium ion/s-HOMO Lewis acid/base interactions.


Type 10 Lewis Acid/Base Complexation Chemistry

Ionic Salts

Screen Shot 2018-07-26 at 11.20.18.png

Bonding:

Onium ion/complex anion complexes are charge-controlled ionic salts. In polar solvents these species dissociate to give independently solvated ions which show minimal ion-pairing.

Charge:

The charge on a Type 10 complex (salt) is always neutral.

Chemistry:

Trimethyl and triethyl oxonium ions are powerful alkylating agents (ie they act as methyl cation and ethyl cation donor complexes) which require non-nucleophilic complex anion counter ions such as:

[BF4], [SbF6] or [PF6]

Congeneric Arrays:

Few series of much interest.


Type 11 Lewis Acid/Base Complexation Chemistry

Ionic Salts, Proton Transfer or Alkylation

Screen Shot 2018-07-26 at 12.51.03.png

Bonding:

Charge controlled ionic complexes or some type of ligand transfer reaction pathway.

Charge:

The charge on a Type 11 complex is always neutral.

Chemistry:

Onium ions exhibit three types chemistry with Lobe-HOMO Lewis bases:

  • Ionic salt formation:

[NH4]+ + Cl ––> NH4Cl

  • Proton transfer from the onium ion to lobe-HOMO Lewis base:

[NH4]+ + OH ––> NH4OH

  • Alkyl carbenium ion transfer from the onium ion to the lobe-HOMO Lewis base:

[(CH3)3O]+ + :NR3 ––> (CH3)2O: + [CH3–NR3]+

Congeneric Arrays:

There are many congeneric series and two dimensional congeneric planars, for example the alkyl ammonium halide planar:

Screen Shot 2018-07-26 at 12.58.41.png

Type 12 Lewis Acid/Base Complexation Chemistry

Proton Transfer or Alkylation

Screen Shot 2018-07-26 at 12.59.53.png

Bonding:

Charge controlled ionic bonding, but a proton transfer reaction is likely to occur rather than the formation of a complex.

Charge:

A reaction always occurs, rather than forming a Lewis acid/base complex so the idea of a charge on a complex is not applicable.

Chemistry:

Onium ions generally do not often form complexes with π-HOMO Lewis bases because H+ or R+ transfer (from the onium ion to the π-HOMO Lewis base) is likely to occur.

The acetate ion can be dual classified as both as a Lobe-HOMO and π-HOMO Lewis base (the acetate ion is isoelectronic with the allyl anion). Thus, ammonium acetate can be considered to be a Type 12 complex:

[NH4]+ [CH3COO]

(but it is probably better to consider it as a Type 11 complex.)

Congeneric Arrays:

A reaction always occurs, rather than forming a Lewis acid/base complex so the idea of congeneric series is not applicable.


Type 13 Lewis Acid/Base Complexation Chemistry

Reduction of Lewis Acid

Screen Shot 2018-07-26 at 13.03.48.png

Bonding:

s-HOMO Lewis bases are reducing agents, so complexation with hydride or hydrogen is also classed as reduction.

However, with respect to carbenium ions and organic chemistry, as C-H bonds are polarised Cδ––Hδ+, ie as weak Brønsted acids, Type 13 complexes reverse their C-H bond polarisation:

C+ + H ––> C–H ––> Cδ––Hδ+

and so become Type 3 complexes.

Charge:

Carbenium ion (ie where the Lewis acid = R3C+ + hydride ion) complexation reactions are rare as both species interact strongly with their counter ions.

  • The true X + Y ––> X-Y complexation reaction would have to be performed under high vacuum vapor phase conditions with the reactive species generated by an experimentally exotic process such as laser induced dissociation.

  • Alkyl halides, such as 3-bromohexane, are readily reduced by H2 plus a catalyst, or by hydride ion donor reagents, as are carbonyl functions.

  • Proton abstraction may occur if there is an acidic proton present and a strongly basic hydride donor reagent is used.

Congeneric Arrays:

Few series.


Type 14 Lewis Acid/Base Complexation Chemistry

Friedel-Crafts Reagents

Screen Shot 2018-07-26 at 13.10.38.png

Bonding:

Charge controlled ionic complexes are highly reactive (transient) species which are prepared in solution where they show little or no ion pairing.

Charge:

The charge on a Type 14 complex is always neutral.

Chemistry:

Type 14 complexes include electrophilic haloenium ion or carbenium ion reagents with non-electrophilic counter ions.

Lobe-LUMO Lewis acids have a strong symbiotic affinity for halogen anion ligands, close in type to those they already possess. Thus:

  • AlCl3 has a high affinity for Cl to give [AlCl4]

  • FeBr3 has a high affinity for Br to give [FeBr4]

Type 14 complexes are not prepared from the cation plus anion, but from the dihalogen, organic halide or acyl halide plus halophilic Lewis acid:

  • Cl2 + AlCl3 ––> Cl+ + [AlCl4]

  • R–Cl + AlCl3 ––> R+ + [AlCl4]

The resulting Type 14 complexes act as sources of naked electrophilic Cl+, Br+ and R3C+, etc. that are able to undergo electrophilic aromatic substitution with benzene and other aromatics:

14_01.png

Congeneric Arrays:

Few series.


Type 15 Lewis Acid/Base Complexation Chemistry

Classical Organic Chemistry: SN1, SN2, SE1, SE2 Pathways

Screen Shot 2018-07-26 at 13.27.10.png

Bonding:

The normal strong covalent and polarised covalent s-bonds of main group chemistry are generally Lobe-LUMO/Lobe-HOMO complexes:

  • C-C
  • C-N
  • C-O
  • C-F
  • C-Br
  • C-Nu
  • C-Nfg
  • Cl-Cl
  • O-O
  • S-Cl
  • P-Br
  • B-N+
  • etc.

In solution Lobe-LUMO/Lobe-HOMO complexes may be covalently bonded, polar-covalently bonded or strongly ion-paired, weakly ion-paired or solvent separated.

Charge:

Complexes may be negatively or positively charged or they may be neutral.

Chemistry:

Prochiral tertiary carbenium ions – carbenium ions with three different ligands and a solvent separated Lobe-HOMO Lewis Base counter ion – react with nucleophiles to give racemic mixtures of enantiomers:

15_1.png

Chiral sp3 carbon/nucleofuge complexes react with nucleophiles via a concerted SN2 mechanistic process in which the chiral centre inverts ‘like an umbrella’, a Walden inversion.

15_2.png

Strong ion-pairing makes concerted second order substitution more favorable than step-wise first order reactivity.

Acyl chlorides and other carbonyl/Nfg complexes also undergo concerted nucleophilic substitution, although the mechanism is rather different:

15_3.png

Congeneric Arrays:

There are many congeneric series and planars, for example:

15_4.png

Type 16 Lewis Acid/Base Complexation Chemistry

Electrophilic Addition and SEAr Reactivity

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Bonding:

Halogen/alkene Type 16 complexes exhibit extensive back-bonding.

Charge:

Positively charged complex, such as the bromonium ion, or some reaction pathway may yield products with a variety of charges.

ChemistryClassical Organic Chemistry

Many vacant p orbital Lobe-LUMO Lewis acids, particularly carbenium ions and acylium ions, are aggressive electrophiles, E+, able to react with π-HOMO organics, such as alkenes, via electrophilic addition or electrophilic addition-followed-by-elimination. For example:

16_2.png

An electrophile, E+, may react with benzene and other aromatics to give (Friedel-Crafts or nitration) electrophilic aromatic substitution products:

16_3.png

Congeneric Arrays:

Few congeneric series or planars of interest.


Type 17 Lewis Acid/Base Complexation Chemistry

Reduction of the Organic π-System

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Bonding:

π-System Lewis Acids form complexes with hydride ions which exist as C-H bonds.

Charge:

Complexes may be negatively charged or they may be neutral.

Chemistry:

π-System Lewis acids are reduced by s-HOMO Lewis bases:

Hydride ion donor reagents, such as lithium aluminium hydride or sodium borohydride, both Type 6 complexes:

17_2.png

H2 plus transition metal catalyst:

17_3.png

During such reductions the hydrogen is likely to be in the form of a transition metal/hydride Type 21 complex.

The saline hydride reagents, NaH etc., Type 5 complexes, are generally too Brønsted basic to be used as organic reducing agents.

Congeneric Arrays:

Few series or planars of interest.


Type 18 Lewis Acid/Base Complexation Chemistry

Cationic π-System Salts

Screen Shot 2018-07-31 at 09.04.30.png

Bonding:

Charge controlled ionic complexes which form solvent separated ions in polar solvents so making the interesting π-cations appear ‘naked’ for spectroscopic study.

Note: Species can only be truly naked under high vacuum vapour phase conditions or in mathematical in silico computer models.

Charge:

Complexes are neutral.

Chemistry:

Exotic cationic π-systems, such as allyl & pentatrienyl cations:

18_3.png

cyclopropenyl, cyclobutdienyl, tropylium & cyclooctatetrenely cations:

18_2.png

are highly reactive electrophilic entities that require very non-nucleophilic anionic counter ions, and tetrafluoroborate, [BF4], and hexafluorantimonate, [SbF6], type ions are ideal.

Congeneric Arrays:

Few of interest.


Type 19 Lewis Acid/Base Complexation Chemistry

Nucleophilic Attack on π-Systems

Screen Shot 2018-07-31 at 09.12.04.png

Bonding:

Complexation leads to normal covalent and/or polar-covalent bonding.

Chemistry:

Nucleophilic attack, usually in an ambidentate manner, upon the π-LUMO Lewis Acid. One way to form π-LUMO Lewis Acids is to have a nucleofugal leaving group attached to a pro- π-LUMO Lewis Acid. Proton abstracters remove H+ to form the corresponding π-system.

Congeneric Arrays:

A number of interacting congeneric series are known, for example, phenyl methane carbenium ion/halogen anion complexes:

19_2.png

Chloride and other halogen anion nucleofuges are more easily substituted by hydroxide ion nucleophiles as –NO2 electron withdrawing group (EWG) functions are added (ortho and para) to the Nfg:

19_3.png

Type 20 Lewis Acid/Base Complexation Chemistry

π/π Interactions, including Diels-Alder Cycloaddition

Screen Shot 2018-07-31 at 09.15.42.png

Bonding:

If the π-LUMO Lewis acid and π-HOMO Lewis base species have suitable:

  • Shape (geometry)

  • Charge

  • and Phase matched FMOs

an orbital phase-symmetry controlled multi-centre π/π complexation reaction can take place with various the participating atoms rehybridizing to give a largely σ-bonded product. The transition state is deemed to proceed via a pericyclicintermediate, where "peri-" is a prefix meaning around or surrounding here.

The classic pericyclic interaction is the Diels-Alder cycloaddition between a diene and a dienophile:

20_1.png

The orbital phase symmetry arguments required for cycloaddition to take place are interchangeable. Cycloaddition can either involve the HOMO of the diene + the LUMO of the dieneophile or the LUMO of the diene + the HOMO of the dieneophile:

20_6.png

Pericyclic reactions are concerted: they take place in a single step. As a consequence, concerted processes provide allow for great Stereochemical control, and pericyclic processes are amongst the most useful of all synthetic methodologies available to the synthetic organic chemist.

Pericyclic chemistry is discussed in detail elsewhere in this webbook.

Electron-poor δ+ π-systems can interact with electron-rich δ π-systems. The initial attraction between is electrostatic (ionic) in nature to form a π/π charge transfer complex.

Charge:

Complexes may be negatively charged, positively charged or they may be neutral.

ChemistryFMO Controlled Pericyclic Interactions

Diels-Alder cycloaddition can be considered as multicentre π-LUMO plus π-HOMO Lewis acid/base Complexation chemistry. However, it is sometimes difficult to decide which species is acting as the Lewis acid and which is the Lewis base.

In normal electron demand Diels-Alder cycloaddition chemistry the diene is electron rich, implying Lewis base character, and the alkene, the dieneophile, is electron deficient implying that the species is the Lewis acid:

20_2.png

There are four general classes of pericyclic reaction process:

  • Cycloaddition

  • Electrocyclic Reactions

  • Sigmatropic Rearrangements

  • Group Transfer Reactions

Pericyclic chemistry is discussed in detail elsewhere in this webbook.

Charge Transfer Complexes

π/π-Interactions can also lead to the formation of a charge-transfer complex, for example between the electron poor 1,3,5-trinitrobenzene and electron rich benzidine.

20_4.png

π/π-Interactions can also lead to the formation or conductive organic metalssuch as stacked TTF/TCNQ materials:

20_5.png

Congeneric Arrays:

Few series.


Type 21 Lewis Acid/Base Complexation Chemistry

Heavy Metal Hydrides

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Bonding:

21_2.png
  • Metallic or interstitial hydrides are formed by many d-block and f-block elements when heated with hydrogen under pressure. The hydrides tend to be non-stoichiometric and they may be of variable composition.

  • There is a hydride gap where elements do not form hydrides. This roughly maps to the siderophile elements.

  • Intermediate hydrides have properties between metallic and covalent.

Metallic properties and non-stoichiometric compositions are common. The bonded hydrogen may be mobile. In various models the hydrogen may be present as H + with the lost electrons going to the metal’s d-orbitals. In other models the hydrogen is assumed to have acquired electrons from the metallic conduction band and so be present as H .

Note that even though a metal may not form a hydride, it can still form a hydride complex ion, for example rhenium: [ReH9]2–

Charge:

Complexes may be negatively charged or they may be neutral.

Chemistry:

Hydride and hydrogen donors. Transition metal hydrides, MHn, can sometimes be formed by ‘dissolving’ hydrogen gas into a bulk metal to from a metal/hydride phase. This phase is the active hydrogenating agent employed during catalytic hydrogenation. For example, spongy palladium metal is able to absorb 900 times its own volume of hydrogen gas.

Transition metal/hydride coordination complexes can be formed by nucleophilic displacement of a halogen (or other nucleofugal ligand) by hydride ions supplied by LiAlH4 or NaBH4:

21_1.png

Metallic hydrides materials are usually dark powders or brittle solids. The bonding mechanism is important because heavy metal hydrides are being actively considered as hydrogen storage materials for hydrogen powered automobiles.

Congeneric Arrays:

Few series.


Type 22 Lewis Acid/Base Complexation Chemistry

Heavy Metal Ionic Salts

Screen Shot 2018-07-31 at 10.11.00.png

Bonding:

Compounds are charge-controlled ionic salts. In solution, ions are solvent separated.

Charge:

Complexes are neutral.

Chemistry:

There are few common heavy metal/complex anion complexes. That said, Pearson states in his early HSAB publications that transition metal ions of high oxidation state are harder than those of low oxidation state.

Thus, we would only expect transition ionic metal/complex anion complexes to form with the harder higher oxidation state transition metal ions.

This is what is found: the hard/hard copper(II) tetrafluoroborate complex, Cu[II] (BF4)2, is known, but the mixed soft/hard copper(I) tetrafluoroborate, Cu[I] BF4, is not.

That said, the mixed soft/hard complex silver tetrafluoroborate, AgBF4, is known and is a useful chemical reagent.

Congeneric Arrays:

Few series.


Type 23 Lewis Acid/Base Complexation Chemistry

Classical Inorganic Coordination Chemistry

Screen Shot 2018-07-31 at 10.15.48.png

Bonding:

The nature of bonding in heavy metal complexes, particularly transition metal complexes, is a vast subject with a long history and is of great technological importance: indeed the term ‘complex’ was first used to describe coordinated transition metal ions.

The two most important bonding models are the electrostatic ligand-field theory and molecular orbital (MO) theory.

Ligand-field theory assumes that the metal cation + anionic ligand interaction is primarily ionic. However, the more demanding MO model is computationally more accessible.

The geometry of heavy metal complexes can usually be predicted by the valence shell electron pair repulsion VSEPR method, however, the effect of d and f orbitals must also be considered: [FeCl4] is tetrahedral but the d8 complex [PdCl4]2– is square planar.

Transition metal complex ions may exhibit Jahn-Tellar distortion: Wikipedia, Robert J. Lancashire page & ScienceWorld.

Charge:

Complexes may be negatively charged, positively charged or they may be neutral.

Chemistry:

Lobe-HOMO Lewis bases form a multitude of complexes with heavy metal cations, indeed a great deal of classical inorganic coordination chemistry involves Type 23 complexation. Complexes may be linear, [AgCl2], tetrahedral, Ni(CO)4 or octahedral:

23_1.png

Heavy metals show multiple oxidation states due to loss of different numbers of d and f orbital electrons. Redox considerations can be as important as HOMO/LUMO interactions when predicting reactivity.

Heavy metal complexes are usually coloured due to electronic transitions involving d or f orbitals and spectroscopic study can be used to probe the nature of the bonding in the complex. Sometimes small changes in ligands give rise to large spectral changes, and sometimes not.

As a rule, d-d electronic transitions give rise to pale colors whereas charge-transfer transitions result in complexes with dark colours.

Transition metals are employed in many biological systems where a protein molecule has a heavy metal ion at the active site, including: Hemoglobinzinc finger protein & cytochrome:

 By Smokefoot - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=63789703

By Smokefoot - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=63789703

 By Klaus Hoffmeier - Own work, Public Domain,  https://commons.wikimedia.org/w/index.php?curid=1417229

By Klaus Hoffmeier - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1417229

Industrial process catalysts may be heterogeneous (solid catalyst, liquid or gaseous reaction mixture) or homogeneous with respect to the reaction mixture. Wilkinson's catalyst, chlorotris(triphenylphosphine)rhodium(I), facilitates the homogeneous hydrogenation of alkenes:

200px-Wilkinson's-catalyst-2D.png

Congeneric Arrays:

There are many congeneric series formed by ligand replacement.


Type 24 Lewis Acid/Base Complexation Chemistry

π-Organometallic Chemistry

Screen Shot 2018-07-31 at 11.36.26.png

Bonding:

The bonding in π-organometallics must be considered using LCAO MO theory because the normal rules of valency break down. For example, what is the valency of the Fe(II) ion in ferrocene, Fe(C5H5)5? Is the iron 2 valent or 10 valent?

The hapto nomenclature, ηx, where the hapacity gives the number of conjugated p orbitals which ligate to a metal, rather than the number of electrons.

Charge:

Complexes may be negatively charged, positively charged or they may be neutral.

Chemistry:

There is a very extensive π-heavy metal chemistry known: all heavy metals are known to form π-organometallic compounds.

  Organotitanium  chemistry   Organochromium  chemistry   Organomanganese  chemistry   Organoiron  chemistry   Organocobalt  chemistry   Organonickel  chemistry   Organocopper  chemistry   Organozinc  chemistry   Organopalladium  chemistry   Organosilver  chemistry   Organocadmium  chemistry   Organotin  chemistry   Organoiridium  chemistry   Organoplatinum  chemistry   Organogold  chemistry   Organolead  chemistry

Organotitanium chemistry

Organochromium chemistry

Organomanganese chemistry

Organoiron chemistry

Organocobalt chemistry

Organonickel chemistry

Organocopper chemistry

Organozinc chemistry

Organopalladium chemistry

Organosilver chemistry

Organocadmium chemistry

Organotin chemistry

Organoiridium chemistry

Organoplatinum chemistry

Organogold chemistry

Organolead chemistry

For example, chromium metallocenes:

500px-CrCl3_dibenzenechromium.png

Congeneric Arrays:

Few of much interest.