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H^–  D^–  T^–
H₂  D₂  T₂
HD  HT  DT
He

Search for s-HOMO Lewis bases species in The Chemical Thesaurus

Nearly all of the elements are able to form hydride compounds:

When s-HOMO Lewis bases interact with Lewis acids to form a complex they are deemed to reduce the Lewis acid: s-HOMO Lewis bases are reducing agents.

The hydride ion is a strong proton abstracting base and nucleophile. H₂ is only very weakly basic such that H₂, D₂, T₂, HD, HT and DT hardly seem to be basic at all, yet as they can be protonated to [H₃]⁺ (in the gas phase) they must be Lewis bases.

H^– and H₂ form metallic complexes.

Helium can be protonated under gas phase conditions to [HHe]⁺ so it is a Lewis base, an s-HOMO Lewis base.

H^–  D^–  T^–
H₂  D₂  T₂
HD  HT  DT

Search for complex anion Lewis bases species in The Chemical Thesaurus

Complex anion Lewis base species behave as charged hard spheres that form ionic charge-controlled complexes (ie act as non-nucleophilic counter ions), or they behave as donors of hard/soft ligands, X^–.

Ligand substitution – in which a nucleophilic Lewis Base displaces a nucleofugal ligand – is common and ligand symbiosis considerations/effects are very important.

There are four subclasses of complex anion Lewis base:

X = Halogen anion which gives rise to the synthetically useful non-basic, non-nucleophilic, non-interfering anionic spectator counter ions:

[BF₄]^–    [SbF₆]^–    [AsF₆]^–    [FeBr₄]^–

X = Hydride ion which gives rise to species which act as donors of nucleophilic hydride ion, [BH₄]^– and [AlH₄]^–, as long as there is not a Brønsted Acidic proton available or H₂ is generated.

M = Heavy metal (Fe or Cr as opposed to B or Al). Such complex anions are much studied in classical inorganic coordination chemistry. Transition metals centres often exhibit multiple oxidation states. These are better considered as type 23 Lewis acid/base complexes.

X = Oxygen heavy metal species with oxygen ligands are commonly used as oxidising agents.

Families of ligand replacement congeneric series are common:

Search for Lobe-HOMO Lewis base species in The Chemical Thesaurus

Typical electron lone pair species in which the Lewis base HOMO is a px FMO orbital.

In valence bond terms, the electron pair is in an sp³, sp² or sp hybrid orbital.

On complexation species show directional bonding which implies that the HOMO is directional, ie lobe shaped.

Species range from hard to soft. The hardness of anionic Lobe-HOMO Lewis bases can be defined with respect to the methyl anion, H₃C^– the carbon-hydrogen bond length in methane (109pm).

All Lobe-HOMO Lewis bases can by definition be protonated and the conjugate Brønsted acid's proton-to-Lewis base bond length, along with its pKa value, serves to probe the Lewis base's chemistry and behaviour. Bond-length and pKa data are linear over congeneric series and planars.

Methane, CH₄, is a convenient reference Lobe-HOMO Lewis base due to the importance of carbon in organic chemistry and that so many reactions occur at carbon centres.

Congeneric species with base-to-H⁺ bond lengths shorter than 109pm, such as the hydroxide ion HO^– (HO–H bond length = 96pm), are deemed to be harder than the methyl carbanion. Longer bond-length as seen with iodide, I^–, (HI bond length = 161pm) equates with softness.

Lobe-HOMO Lewis bases are the classic electron lone-pair donor species that can act as:

• Proton abstracting Brønsted bases
• Ligands around metal complex ions
• Attacking nucleophiles
• Nucleofugal leaving groups
• Non-interfering spectator anions
• Co-complexing or solvating agents

Lewis acid/base complexes formed by Lobe-HOMO Lewis bases range from FMO controlled covalent carbon-carbon bonds to charge-controlled ionic species such as cesium fluoride, CsF.

The atomic lone pair centre may be embedded in a π-system, for example the allyl ion, in which case the species can be dual classified as a Lobe-HOMO and a π-HOMO Lewis base. When the species reacts via a single atomic centre it is classified as a Lobe-HOMO Lewis base.

There are several subclasses of Lobe-HOMO Lewis bases, including:

• Carbanion centres: H₃C^–, enolates, etc., are of huge importance in organic chemistry because the carbanion can act as a nucleophile at a carbon-nulceofuge centre, C-Nfg, such as an alkyl bromide or tosylate. The effect is to increase the length of the carbon skeleton.
  
  The rule is that π-stabilisation makes a carbanion centre less basic but more nucleophilic, as discussed in more detail elsewhere in this chemogenesis webbook, here.

• α-Effect ("alpha effect") bases like hydrazine and hydrogen peroxide are rendered more basic & more nucleophilic by an adjacent lone-pair of electrons. Find α-effect ("alpha effect") Lewis bases in The Chemical Thesaurus.
  
  
• Bidentate and polydentate bases have two or more similar Lewis base centres and are commonly employed as ligands in transition metal chemistry. Find bidentate and polydentate bases in The Chemical Thesaurus.
  
  
• Ambident (ambidentate) bases have two dissimilar Lewis base centres and show selectivity. Find ambident and polydentate bases in The Chemical Thesaurus.

There are two important congeneric planars to be found within electronegative main group elements and their anions. One is formed by the X^– anions and the other by X: neutral lone pair species.

Read more about the chemistry of these congeneric planars elsewhere in this webbook, here:

A number of Group 14 elemental hydrides: CH₄, SiH₄, GeH₄& SnH₄, are rather inert towards Lewis acid and Lewis base reagents. (Species can be oxidised and they are susceptible to attack by radicals and diradicals.)

However, methane can be protonated by super acids to the carbonium ion:

H⁺       +        CH₄        –>       [CH₅]⁺

So methane is a Lewis base but, like helium, it is an exceedingly feeble proton abstractor. It follows that: CH₄, SiH₄, GeH₄, SnH₄ and related compounds are Lewis bases.

Search for inert species in The Chemical Thesaurus

If an extended electron rich π-system species reacts via a single atomic centre, for example when an allyl anion is protonated to give propene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-HOMO Lewis base.

However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic complex, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-HOMO and π-HOMO classification.

Search for π-HOMO Lewis base species in The Chemical Thesaurus

π-HOMO Lewis bases have their electrons in their highest occupied molecular orbital, or HOMO, delocalised over two or more p-orbitals.

Species require modelling by Hückel-FMO techniques as well as by VB-resonance structure methods. Hückel MO modelling gives rise to whole families of π-structure: polyene ribbons, aromatics, etc.:

Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the carbanion, allyl anion & pentatrieneyl anion and alkene, 1,3-diene & 1,3,5-triene π-HOMO Lewis bases:

Soft when the entire π-system is acting as the Lewis base, but harder when a single atomic centre is involved and the species is behaving as a Lobe-HOMO Lewis base.

The allyl anion can behave as (or be considered as) 2 π-electrons delocalised over a 3p orbital function, or as a stabilised 2p orbital carbanion. In this latter case it is better to consider the allyl anion to be behaving as a (harder) Lobe-HOMO Lewis base.

Species behave as π-species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:

• Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl anion, etc.
  
  
• Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
  
  
• Formation of π-organometallic complexes, Type 24 Lewis acid/base complexation.

Search for proton Lewis acid species in The Chemical Thesaurus

The proton is the smallest, lightest, hardest and most versatile Lewis acid.

However, the proton is never observed free (in chemistry at least, high energy high vacuum physics is different). The proton is always passed or transferred from one Lewis base to another in a concerted Brønsted acid/base proton transfer reaction.

The proton has so little mass that it (partially) quantum tunnels between complexed states, and the ability of a species to complex with a proton defines Lewis base character.

Brønsted acids are all proton/Lewis bases complexes: the proton is the agent of Brønsted acidity.

The Ka and pKa of are a measure of Brønsted acid strength with respect to water. As the Lewis acid H⁺ remains constant, the terms Ka and pKa are a measure of a conjugate (Lewis) base's affinity for H⁺ with respect to the standard Lewis base water, :OH₂.

Search for s-Lewis acid species in The Chemical Thesaurus

The s-Lewis acids are the cations of the Group I alkali and Group II alkaline earth metals.

The shell-like LUMO (2s, 3s 4s 5s & 6s AOs) is superimposed upon a sphere of closed electron shells which defines the ionic radius of the cation.

There are two s-LUMO series:

Group I alkali metals: Li⁺  Na⁺   K⁺   Rb⁺   Cs⁺
Group II alkali earth metals: Be²⁺   Mg²⁺  Ca²⁺   Sr²⁺   Ba²⁺

Search for onium ion Lewis acid species in The Chemical Thesaurus

The onium ion Lewis acids have a central electronegative atom saturated with Lewis acid "ligands", usually H⁺ or alkyl⁺.

Onium ion Lewis acids are all proton/X Lobe-HOMO or carbenium ion/X Lobe-HOMO complexes where:

X = N, O, F, Ne, P, S, Cl, Ar, As, Se, Br, Kr, Sb, Te, I, Xe

Onium ions either form charge-controlled (ionic) complexes or they react by transferring a ligand to a nucleophilic /basic Lewis base.

If the transferred ligand is H⁺, the onium ion acts as a Brønsted Acid.

If the transferred ligand is a carbenium ion Lewis acid, the onium ion is said to be an alkylating agent.

High symmetry tetraalkyl ammonium ions, such as [(CH₃)₄N]⁺, can act as spectator cations.

Methane can be protonated to the five valent carbonium ion: [CH₅]⁺

Second order nucleophilic substitution reactions at carbon pass through a five valent carbonium ion transition state:

There is one onium ion Lewis acid planar:

Ammonium, phosphonium, oxonium and sulfonium ions give rise to many ligand replacement congeneric series, for example:

[R₄N]⁺   [R₃NR']⁺   [R₂NR₂]⁺   [RNR'₃]⁺   [NR'₄]⁺

where R and/or R' = H, CH₃, alkyl, C₆H₅ etc.

Search for lobe-LUMO Lewis acid species in The Chemical Thesaurus

Hard to soft. Hardness of Lobe-LUMO Lewis acids is here defined with respect to the carbon-hydrogen bond length in methane (109pm).

All Lobe-LUMO Lewis acids can complex with hydride ion and the corresponding Type 11 Complex's Lewis acid to hydride bond length serves to probe the Lewis acid's chemistry.

It transpires that bond-length data is linear along congeneric series and over planars. It is convenient to nominate methane as a reference Lobe-LUMO Lewis acid because of its importance in organic chemistry.

Methane can be deconstructed to the carbenium ion Lobe-LUMO Lewis acid and a Hydride ion Lewis base. The H₃C⁺-to-H^– bond length, ie methane's C-H bond length of 109pm, can be used as a reference point with which to compare to other Lobe-LUMO Lewis acids.

Congeneric species with 'Lewis acid-to-H^–' bond lengths shorter than 109pm (such as the hydroxy cation HO⁺, HOH bond length = 96pm) are deemed to be harder than the carbenium ion.

Many Lobe-LUMO Lewis acids react via concerted S_n2 mechanisms. These reactions exhibit transition state symbiosis. Hard nucleofuges, such as fluorosulfonate FSO₂O^–, render the Lobe-LUMO centre harder, and soft nucleofuges, such as iodide ion I^–, render the centre softer.

Species are susceptible to attack by nucleophilic Lewis bases and they may be actively electrophilic. Lobe-LUMO Lewis acids increase the extent of the sigma-skeleton when they complex with nucleophiles.

There are three subclasses of Lobe-LUMO Lewis acids. Members of each subclass have the property that they complex with, or are attacked by, nucleophilic Lewis bases.

Vacant p-orbital Lewis Acids
Trivalent boron and aluminium species, BF₃ and AlCl₃, and enium ions of the type R₃C⁺, RO⁺, Br⁺. The methyl cation carbenium ion, H₃C⁺, is a useful reference species.

Species undergo A + B -> A-B complexation reactions, where B is a nucleophile, Nu:

There are several congeneric series:

And many congeneric dots:

R-Nfg Complexes
Where Nfg = nucleofuge or Lewis base leaving group.

Species susceptible to S_n1 and S_n2 nucleophilic substitution: H₃C-I, (H₃C)₃C-Cl, H₃C-OTs, epoxides etc.

The (usually carbon) centre attached to the nucleofuge is rendered δ⁺:

Alkyl groups stabilise the carbenium ion centre:

There quite a number of nucleofugal leaving groups, including halide ions, sulfate, tosylate, triflate, etc.

The liability of a leaving group – how easily it is displaced – correlates with the pKa of Lewis base's conjugate acid. Thus, an Nfg with a strong conjugate Brønsted acid, such as bromide ion (HBr) is a good leaving group and is easily displaced.

Further examples are found with three membered rings with an O, N, S, etc. heteroatom, and that are susceptible to nucleophilic attack and ring opening:

π-Heteroatom Functions
Polarised π-bonded functional groups susceptible to nucleophilic addition, or nucleophilic addition-followed-by-elimination, which leads to net substitution.

The δ⁺ carbon centres of imines, carbonyls, alpha,beta-unsaturated carbonyls, etc.:

If an extended electron poor π-system species reacts via a single atomic centre, for example when a benzyl cation is reduced to toluene, the species is better considered behaving as an ambidentate, π-stabilised Lobe-LUMO Lewis acid.

However, when when the species reacts via its extended π-system directly, for example during Diels-Alder cycloaddition or when forming a π-organometallic, the the species should be considered as a π-HOMO Lewis base species. Thus, there is an overlap between π-stabilised Lobe-LUMO and π-LUMO classification.

Search for π-LUMO Lewis acid species in The Chemical Thesaurus

Delocalised cationic hydrocarbon π-systems and those neutral but electron deficient π-functions which participate in concerted multicentre reactions.

Hückel MO theory gives rise to whole families of π-structure: polyene ribbons, aromatics, etc. Each system is FMO unique. π-Species must be considered at the Hückel level, as well as by VB resonance techniques.

Indeed, quantum mechanics is all about patterns. A particularly striking manifestation is seen with the polyene system of: 1, 2, 3, 4, 5, 6... conjugated p-orbital systems and how they give rise to the cabenium ion, allyl cation & pentatrieneyl cation and alkene, 1,3-diene & 1,3,5-triene π-LUMO Lewis acids:

Species behave as π-LUMO Lewis acid species when they undergo FMO controlled multicentre interactions. These most obviously manifest themselves in three situations:

• Stabilisation of the π-system: certain patterns/structures are associated with stability such as 4n+2 π-electrons in a cyclic array, the allyl cation, etc.
  
  
• Diels-Alder cycloaddition and other pericyclic interactions, Type 20 Lewis acid/base complexation.
  
  
• Having non-nucleophilic complex anion Lewis base anions to stabilise the π-LUMO Lewis acid species.

Few congeneric series, but the chloronitrobenzene series can be considered congeneric with respect to the nucleophilic displacement of Cl^– by a nucleophile:

Search for heavy metal Lewis acid species in The Chemical Thesaurus

Multiple vacant lobe-shaped p, d or f orbitals which may rehybridize on complexation. Many orbitals available for back-bonding.

Pearson's original analysis remains excellent.

Hard to soft. Pearson states in his early HSAB publications, see elsewhere in this webbook, that transition metal ions of high oxidation state are harder than those of low oxidation state.

Few congeneric series, although periodicity is seen down groups: