TREATMENT OF VARIABLE VALENCE IN ORGANIC NOMENCLATURE
(λ CONVENTION)

Recommendations 1983

CONTENTS

Recommendations
Lm-1.0. Terminology
Lm-1.1. Bonding Number
Lm-1.2. Standard Bonding Number
Lm-2.0. Designation of Nonstandard Bonding Numbers
Lm-3.0. Indicated Hydrogen
Lm-3.1. Maximum Number of Noncumulative Double Bonds
Lm-3.2. Indicated Hydrogen Symbolism
Lm-4.0. Derivatives of Parent hydrides
Lm-4.1. Saturation of Double Bonds
Lm-4.2. Multiple Bonds with a Saturated Parent Hydride
Lm-4.3. Derivatives
Lm-4.4. Prefixes
References

RECOMMENDATIONS

The extension of substitutive nomenclature to compounds containing heteroatoms of variable valence requires a method for distinguishing between the various valence states of each atom. These recommendations, designated by Lm (lambda), provide a general method for indicating nonstandard valence states of formally neutral skeletal atoms in parent hydrides (but see Note).

Note. Cyclic parent hydrides having cumulative double bonds in addition to the maximum number of noncumulative double bonds in the rest of the structure will be included in a forthcoming comprehensive treatment of cumulative double bond systems.
Lm-1.0. Terminology

Lm-1.1. Bonding Number

The bonding number, "n", of a skeletal atom in a parent hydride is the sum of the total number of valence bonds to adjacent skeletal atoms, if any, and the number of attached hydrogen atoms. The relevance of these recommendations to naming radicals and ions is not considered in this report.

Examples:

1.SH2n = 2
2.SnH2n = 2
3.NH2-NH2n = 3
4.PH4-NH-PH4N, n = 3; P, n = 5
5.N, n = 3; C, n = 4
6.SiH2=SiH2n = 4
7.SH6n = 6
8.IH7n = 7

Lm-1.2. Standard Bonding Number

The bonding number of a neutral atom in a parent hydride is standard when it has the value given in the following table of elements that occur more or less frequently in organic compounds. It is nonstandard when its value is either larger or smaller.

Bonding number   3   4   3   2   1

                 B   C   N   O   F
                     Si  P   S   Cl
                     Ge  As  Se  Br
                     Sn  Sb  Te  I
                     Pb  Bi  Po  At
Lm-2.0. Designation of Nonstandard Bonding Numbers

A nonstandard bonding number of a neutral skeletal atom in a parent hydride is indicated by the symbol λn, where n is the bonding number as defined in Lm-1.1.

If the locant for an atom with a nonstandard bonding number is used in the name of the normal (standard) parent hydride, the λn symbol is cited immediately after this locant. If the locant for such an atom is not expressed in the name, the locant, if necessary and the λn symbol are cited in front of the name of the parent hydride, but after any indicated hydrogen (see Lm-3.0).

Numbering of parent hydrides with heteroatoms in nonstandard valence states follows the rules of Sections B and C of the IUPAC Organic Nomenclature Rules (Ref. 1) for numbering heteroatoms as far as possible. When a choice is needed between the same skeletal atom in different valence states, the one in a nonstandard valence state is preferred for assignment of the lower locant. If a further choice is needed between the same skeletal atom in two or more nonstandard valence states, preference for lower locant is given in order of the decreasing numerical value of the bonding number, i.e., λ6 is preferred to λ4.

Examples:

1.SH4λ4-Sulphane [the name sulfurane has been suggested (Ref. 7)]
2.IH5λ5-Iodane [the name periodinane has been suggested (Ref. 8)]
3.2λ6-Trisulfane
4.1λ5,2λ5,3λ5-Triphosphane (not Tri-λ5-phosphane. An abbreviated form (λ5)3-triphosphane (in speech tri-λ5-triphosphane is perhaps better) is permissible.)
5.1,3λ5-Oxaphosphole
6.1λ4,3-Thiazine
7.1,4λ4-Oxathiepane
8.5λ5-Phosphaspiro[4.4]nonane
9.2,4-Dioxa-3λ2-plumbabicyclo[3.3.1]nonane
10.7λ4-[1,2]Dithiolo[1,5-b][1,2]dithiole or 1,6,6aλ4-Trithiapentalene

Note: Nonstandard valence states in fused ring systems are indicated only in the complete ring system, not in component rings.
11.2H-5λ5-phosphinino[3,2-b]pyran
12.1λ4,5-Benzodithiepin (the λ4 sulfur atom is preferred for lowest locants)

Lm-3.0. Indicated Hydrogen.

Lm-3.1. Maximum Number of Noncumulative Double Bonds

When the maximum number of noncumulative double bonds is assigned to a parent ring structure, account must be taken of the modified valence of ring atoms with nonstandard bonding numbers.

Lm-3.2. Indicated Hydrogen Symbolism.

After the maximum number of noncumulative double bonds has been assigned according to Rule Lm-3.1, any ring atom with a bonding number of three or higher connected to adjacent ring atoms by single bonds only, and carrying one or more hydrogen atoms, is designated by the "indicated hydrogen" symbolism as described in Rule A-21.6 (Ref. 1). If the preceding rules leave a choice, such ring atoms are preferred for low locants.

Note: Designation of indicated hydrogen at nonbridgehead ring positions between two bivalent ring atoms is often omitted.
Examples:

1.1H-1λ4-Thiepine
2.2H-1λ4-Thiepine
3.1H-1λ4,3λ4-Dithiepine (the -SH2- λ4-sulfur atom is preferred for low numbering)
4.2H-5λ4-Dibenzothiophene
5.3H-3λ4,2,4-Benziodadioxepin
6.3H-2λ4-Cyclohepta[c]thiopyran (not 2H-3λ4-cyclohepta[c]thiopyran)

Lm-4.0. Derivatives of Parent hydrides

Derivatives of parent hydrides with skeletal atoms in nonstandard valence states are named according to the established rules of organic nomenclature (Ref. 1) as exemplified below.

Lm-4.1. Saturation of Double Bonds

Saturation of double bonds in a ring system whose parent name requires the maximum number of noncumulative double bonds is indicated by hydro prefixes as prescribed by Rules A-23.1 and B-1.2 (Ref. 1).

Examples:

1.1,2-Dihydro-1λ5-phosphinine
2.Decahydro-1λ4-benzothiopyran

Lm-4.2. Multiple Bonds with a Saturated Parent Hydride

The presence of multiple bonds in a parent hydride whose parent name requires saturated skeletal atoms at all positions, is described by subtractive suffixes such as "ene" and "yne" (see Rules A-3, A-11.3, A-31.2, A-41.3 in Ref.1).

Examples:

1.2λ4,7λ4-Dithia-3,8-diazaspirol4.4]nona-2,7-diene
2.
5-Oxa-3,14,16λ4-trithia-9,10-diaza-8λ5,11λ5-diphosphaoctadeca-7,9,11,15,16-pentaene

Lm-4.3. Derivatives

Derivatives formed by substitution of hydrogen atoms of the parent hydride are named by means of prefixes and/or suffixes in the established manner (Rule C-0.l, Ref. 1).

Examples:

1.1,1,2,2-Tetramethyl-1,2-dithioxo-1λ5,2λ5-diphosphane
2.1λ4-Thiopyran-1-carboxylic acid

Lm-4.4. Prefixes

Prefixes for describing substituents derived from parent hydrides having heteroatoms in nonstandard valence states are formed in the usual manner by using endings such as "yl", "ylidene", "diyl", etc. (Ref. 1).

Examples:

1.(Pentafluoro-λ6 -sulfanyl)acetyl chloride
2.N-(1H-1λ4,2,3-Thiadiazol-1-ylidene)carbamic acid
3.N-[Bis(phenylimino)-λ6-sulfanylidene]anthranilic acid

REFERENCES

1. International Union of Pure and Applied Chemistry, Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, Pergamon Press, Oxford, 1979.

2. International Union of Pure and Applied Chemistry, The Designation of Nonstandard Classical Valence Bonding in Organic Nomenclature (Provisional), Pure Appl. Chem. 54, 217-227 (1982).

3. International Union of Pure and Applied Chemistry, Revision of the Extended Hantzsch-Widman System of Nomenclature for Heteromonocycles (Recommendations 1982), Pure Appl. Chem. 55, 409-416 (1983).

4. A. F. Clifford, J. Chem. Doc. 5, 91-95 (1965); 10, 180-185 (1970).

5. International Union of Pure and Applied Chemistry, Nomenclature of Inorganic Chemistry, 2nd Edition, Butterworths, London, 1979, (a) Rule 0.1, p. 5; (b) Rule 1.31, p. 11.

6. International Union of Pure an d Applied Chemistry, Nomenclature of Organic Chemistry, Section D (Tentative), IUPAC Information Bulletin Appendixes on Tentative Nomenclature, Symbols, Units, and Standards, No. 31, August, 1973.

7. J. C. Martin and R. J. Arhart, J. Am. Chem. Soc. 93, 2339-2342 (1971).

8. R. L. Amey and J. C. Martin, J. Am. Chem. Soc. 100, 300-301 (1978).

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