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18-electron rule

January 01, 1970

The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds.[1] The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five (n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or non-bonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period, lending stability to the complex. Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive. The rule is not helpful for complexes of metals that are not transition metals. The rule was first proposed by American chemist Irving Langmuir in 1921.[1][2]

Applicability edit

The rule usefully predicts the formulas for low-spin complexes of the Cr, Mn, Fe, and Co triads. Well-known examples include ferrocene, iron pentacarbonyl, chromium carbonyl, and nickel carbonyl.

Ligands in a complex determine the applicability of the 18-electron rule. In general, complexes that obey the rule are composed at least partly of π-acceptor ligands (also known as π-acids). This kind of ligand exerts a very strong ligand field, which lowers the energies of the resultant molecular orbitals so that they are favorably occupied. Typical ligands include olefins, phosphines, and CO. Complexes of π-acids typically feature metal in a low-oxidation state. The relationship between oxidation state and the nature of the ligands is rationalized within the framework of π backbonding.

Consequences for reactivity edit

Compounds that obey the 18-electron rule are typically "exchange inert". Examples include [Co(NH3)6]Cl3, Mo(CO)6, and [Fe(CN)6]4−. In such cases, in general ligand exchange occurs via dissociative substitution mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand. On the other hand, 18-electron compounds can be highly reactive toward electrophiles such as protons, and such reactions are associative in mechanism, being acid-base reactions.

Complexes with fewer than 18 valence electrons tend to show enhanced reactivity. Thus, the 18-electron rule is often a recipe for non-reactivity in either a stoichiometric or a catalytic sense.

Duodectet rule edit

Computational findings suggest valence p-orbitals on the metal participate in metal-ligand bonding, albeit weakly.[3] However, Weinhold and Landis within the context of natural bond orbitals do not count the metal p-orbitals in metal-ligand bonding,[4] although these orbitals are still included as polarization functions. This results in a duodectet (12-electron) rule for five d-orbitals and one s-orbital only.

The current consensus in the general chemistry community is that unlike the singular octet rule for main group elements, transition metals do not strictly obey either the 12-electron or 18-electron rule, but that the rules describe the lower bound and upper bound of valence electron count respectively.[5][6] Thus, while transition metal d-orbital and s-orbital bonding readily occur, the involvement of the higher energy and more spatially diffuse p-orbitals in bonding depends on the central atom and coordination environment.[7][8]

Exceptions edit

π-donor or σ-donor ligands with small interactions with the metal orbitals lead to a weak ligand field which increases the energies of t2g orbitals. These molecular orbitals become non-bonding or weakly anti-bonding orbitals (small Δoct). Therefore, addition or removal of electron has little effect on complex stability. In this case, there is no restriction on the number of d-electrons and complexes with 12–22 electrons are possible. Small Δoct makes filling eg* possible (>18 e−) and π-donor ligands can make t2g antibonding (<18 e−). These types of ligand are located in the low-to-medium part of the spectrochemical series. For example: [TiF6]2− (Ti(IV), d0, 12 e−), [Co(NH3)6]3+ (Co(III), d6, 18 e−), [Cu(OH2)6]2+ (Cu(II), d9, 21 e−).

In terms of metal ions, Δoct increases down a group as well as with increasing oxidation number. Strong ligand fields lead to low-spin complexes which cause some exceptions to the 18-electron rule.

16-electron complexes edit

An important class of complexes that violate the 18e rule are the 16-electron complexes with metal d8 configurations. All high-spin d8 metal ions are octahedral (or tetrahedral), but the low-spin d8 metal ions are all square planar. Important examples of square-planar low-spin d8 metal Ions are Rh(I), Ir(I), Ni(II), Pd(II), and Pt(II). At picture below is shown the splitting of the d subshell in low-spin square-planar complexes. Examples are especially prevalent for derivatives of the cobalt and nickel triads. Such compounds are typically square-planar. The most famous example is Vaska's complex (IrCl(CO)(PPh3)2), [PtCl4]2−, and Zeise's salt [PtCl3(η2-C2H4)]−. In such complexes, the dz2 orbital is doubly occupied and nonbonding.

 

Many catalytic cycles operate via complexes that alternate between 18-electron and square-planar 16-electron configurations. Examples include Monsanto acetic acid synthesis, hydrogenations, hydroformylations, olefin isomerizations, and some alkene polymerizations.

Other violations can be classified according to the kinds of ligands on the metal center.

Bulky ligands edit

Bulky ligands can preclude the approach of the full complement of ligands that would allow the metal to achieve the 18 electron configuration. Examples:

Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand. For example:

  • W(CO)3[P(C6H11)3]2 has 16 e− but has a short bonding contact between one C–H bond and the W center.
  • Cp(PMe3)V(CHCMe3) (14 e−, diamagnetic) has a short V–H bond with the 'alkylidene-H', so the description of the compound is somewhere between Cp(PMe3)V(CHCMe3) and Cp(PMe3)V(H)(CCMe3).

High-spin complexes edit

High-spin metal complexes have singly occupied orbitals and may not have any empty orbitals into which ligands could donate electron density. In general, there are few or no π-acidic ligands in the complex. These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands (e.g., oxygen), or addition of a strong field ligand can cause electron-pairing, thus creating a vacant orbital that it can donate into. Examples:

  • CrCl3(THF)3 (15 e−)
  • [Mn(H2O)6]2+ (17 e−)
  • [Cu(H2O)6]2+ (21 e−, see comments below)

Complexes containing strongly π-donating ligands often violate the 18-electron rule. These ligands include fluoride (F−), oxide (O2−), nitride (N3−), alkoxides (RO−), and imides (RN2−). Examples:

  • [CrO4]2− (16 e−)
  • Mo(=NR)2Cl2 (12 e−)

In the latter case, there is substantial donation of the nitrogen lone pairs to the Mo (so the compound could also be described as a 16 e− compound). This can be seen from the short Mo–N bond length, and from the angle Mo–N–C(R), which is nearly 180°. Counter-examples:

  • trans-WO2(Me2PCH2CH2PMe2)2 (18 e−)
  • Cp*ReO3 (18 e−)

In these cases, the M=O bonds are "pure" double bonds (i.e., no donation of the lone pairs of the oxygen to the metal), as reflected in the relatively long bond distances.

π-donating ligands edit

Ligands where the coordinating atom bear nonbonding lone pairs often stabilize unsaturated complexes. Metal amides and alkoxides often violate the 18e rule

Combinations of effects edit

The above factors can sometimes combine. Examples include

  • Cp*VOCl2 (14 e−)
  • TiCl4 (8 e−)

Higher electron counts edit

Some complexes have more than 18 electrons. Examples:

  • Cobaltocene (19 e−)
  • Nickelocene (20 e−)
  • The hexaaquacopper(II) ion [Cu(H2O)6]2+ (21 e−)
  • TM(CO)8− (TM = Sc, Y) (20 e−)

Often, cases where complexes have more than 18 valence electrons are attributed to electrostatic forces – the metal attracts ligands to itself to try to counterbalance its positive charge, and the number of electrons it ends up with is unimportant. In the case of the metallocenes, the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal. Somewhat satisfying are the two following observations: cobaltocene is a strong electron donor, readily forming the 18-electron cobaltocenium cation; and nickelocene tends to react with substrates to give 18-electron complexes, e.g. CpNiCl(PR3) and free CpH.

In the case of nickelocene, the extra two electrons are in orbitals which are weakly metal-carbon antibonding; this is why it often participates in reactions where the M–C bonds are broken and the electron count of the metal changes to 18.[9]

The 20-electron systems TM(CO)8− (TM = Sc, Y) have a cubic (Oh) equilibrium geometry and a singlet (1A1g) electronic ground state. There is one occupied valence MO with a2u symmetry, which is formed only by ligand orbitals without a contribution from the metal AOs. But the adducts TM(CO)8− (TM=Sc, Y) fulfill the 18-electron rule when one considers only those valence electrons, which occupy metal–ligand bonding orbitals.[10]

See also edit

References edit

  1. ^ a b Langmuir, I. (1921). "Types of Valence". Science. 54 (1386): 59–67. Bibcode:1921Sci....54...59L. doi:10.1126/science.54.1386.59. PMID 17843674.
  2. ^ Jensen, William B. (2005). "The Origin of the 18-Electron Rule". Journal of Chemical Education. 82 (1): 28. Bibcode:2005JChEd..82...28J. doi:10.1021/ed082p28.
  3. ^ Frenking, Gernot; Shaik, Sason, eds. (May 2014). "Chapter 7: Chemical bonding in Transition Metal Compounds". The Chemical Bond: Chemical Bonding Across the Periodic Table. Wiley-VCH. ISBN 978-3-527-33315-8.
  4. ^ Landis, C. R.; Weinhold, F. (2007). "Valence and extra-valence orbitals in main group and transition metal bonding". Journal of Computational Chemistry. 28 (1): 198–203. doi:10.1002/jcc.20492. PMID 17063478.
  5. ^ Frenking, Gernot; Fröhlich, Nikolaus (2000). "The Nature of the Bonding in Transition-Metal Compounds". Chemical Reviews. 100 (2): 717–774. doi:10.1021/cr980401l. PMID 11749249.
  6. ^ Zhao, Lili; Holzmann, Nicole; Schwerdtfeger, Peter; Frenking, Gernot (2019). "Chemical Bonding and Bonding Models of Main-Group Compounds". Chemical Reviews. 119 (14): 8781–8845. doi:10.1021/acs.chemrev.8b00722. PMID 31251603. S2CID 195761899.
  7. ^ Bayse, Craig; Hall, Michael (1999). "Prediction of the Geometries of Simple Transition Metal Polyhydride Complexes by Symmetry Analysis". Journal of the American Chemical Society. 121 (6): 1348–1358. doi:10.1021/ja981965+.
  8. ^ King, R.B. (2000). "Structure and bonding in homoleptic transition metal hydride anions". Coordination Chemistry Reviews. 200–202: 813–829. doi:10.1016/S0010-8545(00)00263-0.
  9. ^ Girolami, Gregory; Rauchfuss, Thomas; Angelici, Robert (1999). "Experiment 20". Synthesis and Technique in Inorganic Chemistry. Sausalito, California: University Science Books. ISBN 978-0-935702-48-4.
  10. ^ Jin, Jiaye; Yang, Tao; Xin, Ke; Wang, Guanjun; Jin, Xiaoyang; Zhou, Mingfei; Frenking, Gernot (2018-04-25). "Octacarbonyl Anion Complexes of Group Three Transition Metals [TM(CO)8]− (TM=Sc, Y, La) and the 18-Electron Rule". Angewandte Chemie International Edition. 57 (21): 6236–6241. doi:10.1002/anie.201802590. ISSN 1433-7851. PMID 29578636.

Further reading edit

  • Tolman, C. A. (1972). "The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis". Chem. Soc. Rev. 1 (3): 337. doi:10.1039/CS9720100337.

January 01, 1970
electron, rule, chemical, rule, thumb, used, primarily, predicting, rationalizing, formulas, stable, transition, metal, complexes, especially, organometallic, compounds, rule, based, fact, that, valence, orbitals, electron, configuration, transition, metals, c. The 18 electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes especially organometallic compounds 1 The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five n 1 d orbitals one ns orbital and three np orbitals where n is the principal quantum number These orbitals can collectively accommodate 18 electrons as either bonding or non bonding electron pairs This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal ligand bonding or non bonding When a metal complex has 18 valence electrons it is said to have achieved the same electron configuration as the noble gas in the period lending stability to the complex Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive The rule is not helpful for complexes of metals that are not transition metals The rule was first proposed by American chemist Irving Langmuir in 1921 1 2 Contents 1 Applicability 1 1 Consequences for reactivity 1 2 Duodectet rule 2 Exceptions 2 1 16 electron complexes 2 2 Bulky ligands 2 3 High spin complexes 2 4 p donating ligands 2 5 Combinations of effects 2 6 Higher electron counts 3 See also 4 References 5 Further readingApplicability editThe rule usefully predicts the formulas for low spin complexes of the Cr Mn Fe and Co triads Well known examples include ferrocene iron pentacarbonyl chromium carbonyl and nickel carbonyl Ligands in a complex determine the applicability of the 18 electron rule In general complexes that obey the rule are composed at least partly of p acceptor ligands also known as p acids This kind of ligand exerts a very strong ligand field which lowers the energies of the resultant molecular orbitals so that they are favorably occupied Typical ligands include olefins phosphines and CO Complexes of p acids typically feature metal in a low oxidation state The relationship between oxidation state and the nature of the ligands is rationalized within the framework of p backbonding Consequences for reactivity edit Compounds that obey the 18 electron rule are typically exchange inert Examples include Co NH3 6 Cl3 Mo CO 6 and Fe CN 6 4 In such cases in general ligand exchange occurs via dissociative substitution mechanisms wherein the rate of reaction is determined by the rate of dissociation of a ligand On the other hand 18 electron compounds can be highly reactive toward electrophiles such as protons and such reactions are associative in mechanism being acid base reactions Complexes with fewer than 18 valence electrons tend to show enhanced reactivity Thus the 18 electron rule is often a recipe for non reactivity in either a stoichiometric or a catalytic sense Duodectet rule edit Computational findings suggest valence p orbitals on the metal participate in metal ligand bonding albeit weakly 3 However Weinhold and Landis within the context of natural bond orbitals do not count the metal p orbitals in metal ligand bonding 4 although these orbitals are still included as polarization functions This results in a duodectet 12 electron rule for five d orbitals and one s orbital only The current consensus in the general chemistry community is that unlike the singular octet rule for main group elements transition metals do not strictly obey either the 12 electron or 18 electron rule but that the rules describe the lower bound and upper bound of valence electron count respectively 5 6 Thus while transition metal d orbital and s orbital bonding readily occur the involvement of the higher energy and more spatially diffuse p orbitals in bonding depends on the central atom and coordination environment 7 8 Exceptions editp donor or s donor ligands with small interactions with the metal orbitals lead to a weak ligand field which increases the energies of t2g orbitals These molecular orbitals become non bonding or weakly anti bonding orbitals small Doct Therefore addition or removal of electron has little effect on complex stability In this case there is no restriction on the number of d electrons and complexes with 12 22 electrons are possible Small Doct makes filling eg possible gt 18 e and p donor ligands can make t2g antibonding lt 18 e These types of ligand are located in the low to medium part of the spectrochemical series For example TiF6 2 Ti IV d0 12 e Co NH3 6 3 Co III d6 18 e Cu OH2 6 2 Cu II d9 21 e In terms of metal ions Doct increases down a group as well as with increasing oxidation number Strong ligand fields lead to low spin complexes which cause some exceptions to the 18 electron rule 16 electron complexes edit An important class of complexes that violate the 18e rule are the 16 electron complexes with metal d8 configurations All high spin d8 metal ions are octahedral or tetrahedral but the low spin d8 metal ions are all square planar Important examples of square planar low spin d8 metal Ions are Rh I Ir I Ni II Pd II and Pt II At picture below is shown the splitting of the d subshell in low spin square planar complexes Examples are especially prevalent for derivatives of the cobalt and nickel triads Such compounds are typically square planar The most famous example is Vaska s complex IrCl CO PPh3 2 PtCl4 2 and Zeise s salt PtCl3 h2 C2H4 In such complexes the dz2 orbital is doubly occupied and nonbonding nbsp Many catalytic cycles operate via complexes that alternate between 18 electron and square planar 16 electron configurations Examples include Monsanto acetic acid synthesis hydrogenations hydroformylations olefin isomerizations and some alkene polymerizations Other violations can be classified according to the kinds of ligands on the metal center Bulky ligands edit Bulky ligands can preclude the approach of the full complement of ligands that would allow the metal to achieve the 18 electron configuration Examples Ti neopentyl 4 8 e Cp 2Ti C2H4 16 e V CO 6 17 e Cp Cr CO 3 17 e Pt PtBu3 2 14 e Co norbornyl 4 13 e FeCp2 17 e Sometimes such complexes engage in agostic interactions with the hydrocarbon framework of the bulky ligand For example W CO 3 P C6H11 3 2 has 16 e but has a short bonding contact between one C H bond and the W center Cp PMe3 V CHCMe3 14 e diamagnetic has a short V H bond with the alkylidene H so the description of the compound is somewhere between Cp PMe3 V CHCMe3 and Cp PMe3 V H CCMe3 High spin complexes edit High spin metal complexes have singly occupied orbitals and may not have any empty orbitals into which ligands could donate electron density In general there are few or no p acidic ligands in the complex These singly occupied orbitals can combine with the singly occupied orbitals of radical ligands e g oxygen or addition of a strong field ligand can cause electron pairing thus creating a vacant orbital that it can donate into Examples CrCl3 THF 3 15 e Mn H2O 6 2 17 e Cu H2O 6 2 21 e see comments below Complexes containing strongly p donating ligands often violate the 18 electron rule These ligands include fluoride F oxide O2 nitride N3 alkoxides RO and imides RN2 Examples CrO4 2 16 e Mo NR 2Cl2 12 e In the latter case there is substantial donation of the nitrogen lone pairs to the Mo so the compound could also be described as a 16 e compound This can be seen from the short Mo N bond length and from the angle Mo N C R which is nearly 180 Counter examples trans WO2 Me2PCH2CH2PMe2 2 18 e Cp ReO3 18 e In these cases the M O bonds are pure double bonds i e no donation of the lone pairs of the oxygen to the metal as reflected in the relatively long bond distances p donating ligands edit Ligands where the coordinating atom bear nonbonding lone pairs often stabilize unsaturated complexes Metal amides and alkoxides often violate the 18e rule Combinations of effects edit The above factors can sometimes combine Examples include Cp VOCl2 14 e TiCl4 8 e Higher electron counts edit Some complexes have more than 18 electrons Examples Cobaltocene 19 e Nickelocene 20 e The hexaaquacopper II ion Cu H2O 6 2 21 e TM CO 8 TM Sc Y 20 e Often cases where complexes have more than 18 valence electrons are attributed to electrostatic forces the metal attracts ligands to itself to try to counterbalance its positive charge and the number of electrons it ends up with is unimportant In the case of the metallocenes the chelating nature of the cyclopentadienyl ligand stabilizes its bonding to the metal Somewhat satisfying are the two following observations cobaltocene is a strong electron donor readily forming the 18 electron cobaltocenium cation and nickelocene tends to react with substrates to give 18 electron complexes e g CpNiCl PR3 and free CpH In the case of nickelocene the extra two electrons are in orbitals which are weakly metal carbon antibonding this is why it often participates in reactions where the M C bonds are broken and the electron count of the metal changes to 18 9 The 20 electron systems TM CO 8 TM Sc Y have a cubic Oh equilibrium geometry and a singlet 1A1g electronic ground state There is one occupied valence MO with a2u symmetry which is formed only by ligand orbitals without a contribution from the metal AOs But the adducts TM CO 8 TM Sc Y fulfill the 18 electron rule when one considers only those valence electrons which occupy metal ligand bonding orbitals 10 See also editElectron counting Formalism used for classifying compounds Ligand field theory Molecular orbital theory applied to transition metal complexes d electron count Description of the electron configuration Tolman s rule Rule describing chemical reactionsReferences edit a b Langmuir I 1921 Types of Valence Science 54 1386 59 67 Bibcode 1921Sci 54 59L doi 10 1126 science 54 1386 59 PMID 17843674 Jensen William B 2005 The Origin of the 18 Electron Rule Journal of Chemical Education 82 1 28 Bibcode 2005JChEd 82 28J doi 10 1021 ed082p28 Frenking Gernot Shaik Sason eds May 2014 Chapter 7 Chemical bonding in Transition Metal Compounds The Chemical Bond Chemical Bonding Across the Periodic Table Wiley VCH ISBN 978 3 527 33315 8 Landis C R Weinhold F 2007 Valence and extra valence orbitals in main group and transition metal bonding Journal of Computational Chemistry 28 1 198 203 doi 10 1002 jcc 20492 PMID 17063478 Frenking Gernot Frohlich Nikolaus 2000 The Nature of the Bonding in Transition Metal Compounds Chemical Reviews 100 2 717 774 doi 10 1021 cr980401l PMID 11749249 Zhao Lili Holzmann Nicole Schwerdtfeger Peter Frenking Gernot 2019 Chemical Bonding and Bonding Models of Main Group Compounds Chemical Reviews 119 14 8781 8845 doi 10 1021 acs chemrev 8b00722 PMID 31251603 S2CID 195761899 Bayse Craig Hall Michael 1999 Prediction of the Geometries of Simple Transition Metal Polyhydride Complexes by Symmetry Analysis Journal of the American Chemical Society 121 6 1348 1358 doi 10 1021 ja981965 King R B 2000 Structure and bonding in homoleptic transition metal hydride anions Coordination Chemistry Reviews 200 202 813 829 doi 10 1016 S0010 8545 00 00263 0 Girolami Gregory Rauchfuss Thomas Angelici Robert 1999 Experiment 20 Synthesis and Technique in Inorganic Chemistry Sausalito California University Science Books ISBN 978 0 935702 48 4 Jin Jiaye Yang Tao Xin Ke Wang Guanjun Jin Xiaoyang Zhou Mingfei Frenking Gernot 2018 04 25 Octacarbonyl Anion Complexes of Group Three Transition Metals TM CO 8 TM Sc Y La and the 18 Electron Rule Angewandte Chemie International Edition 57 21 6236 6241 doi 10 1002 anie 201802590 ISSN 1433 7851 PMID 29578636 Further reading editTolman C A 1972 The 16 and 18 electron rule in organometallic chemistry and homogeneous catalysis Chem Soc Rev 1 3 337 doi 10 1039 CS9720100337 Retrieved from https en wikipedia org w index php title 18 electron rule amp oldid 1133505856, wikipedia, wiki, book, books, library,

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