18 electron rule: EAN rule (Effective Atomic Number)

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18 electron rule EAN rule (Effective Atomic
Number)
In 1927, developed by Sidgwick
d electrons of metal electrons of ligand 18
electrons

• 4 5 6 7 8 9
  10 11 12
• d3      d4     d5     d6      d7      d8     d9
      d10
• Sc     Ti     V     Cr     Mn     Fe     Co    
  Ni  Cu Zn
• Y      Zr    Nb     Mo    Tc     Ru    
  Rh    Pd Ag Cd
• Ta      W     Re     Os    
  Ir      Pt  Au Hg

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Ni 1s22s22p63d84s2 it is better to promote 4s
electron to 3d, therefore Ni(0) is
d10. Cu(I), Ag(I), and Au(I) is d10, Zn2,
Cd2, and Hg2 is d10
Ti(IV) serves as a good index for memorizing the
d electrons Because it is a d0 metal ion. TiCl4
is colorless, diamagnetic liquid TiCl3 is violet
color
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tetrahedral
octahedral
Square planar
Trigonal bipyramid
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Electrons Ligand
1 ?????(H radical), ?? ???(alkyl radical), ?? ???(aryl radical) , ??? ???(halogen radical), NO (?? ??)
2 CO, CS, RCN, R3N, R3P, R3As, RCN, R2S, ?????(H-), ?????(R-), ?? (alkene), ?3-?? ???(?3-allyl), ??(alkyne), ???? ???(NO), ??? ???(X-)
3 ?3-?? ???(?3-allyl), NO (???)
4 ?3-?? ???(?3-allyl-), ?4-C4R4(?4-cyclobutadiene), ?4-?????(nonconjugate diene), ?4-????(conjugate diene)
5 ?5-C5R5 ???(?5-cyclopentadienyl radical)
6 ?5-C5R5 ???(?5-cyclopentadienyl anion), ?6-C6R6, ?7-C7R7 ???(cycloheptatrienyl cation)
7 ?7-C7R7 ???(cycloheptatrienyl radical)
8 ?8-C8R8 (cyclooctatetraene)
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Ni(CO)4, Fe(CO)5, Cr(CO)6, Ni(CO)4, For Mn,
Mn(CO)5 17 electrons Mn(CO)6 19
electrons Mn(CO)5 ? (CO)5Mn-Mn(CO)5
HMn(CO)5, CH3Mn(CO)5, ClMn(CO)5
CH3Mn(CO)5 CH3- Mn(CO)5 
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18 Electron Rule strong field ligand such as CO,
Hydride, Cycanide anion. Not good for Aqua
complex Weak Field Ligand such as H2O is not
matched with 18 electron rule. Late transition
metal is better than early transition metals.  
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Exceptions
Early trantion metals (?5-C5H5)2ZrCl2
5ex24e2e16e,  (CH3)3TaCl2? 1ex35e2e10e (CH3
)6W? 1ex66e12e For early transition metals,
there is not enough room to attach many ligands
to satisfy 18 electron rule. Coordination number
number of ligand to bind to metal. Coordination
number cannot be larger than the maximun
oxidation number or the group number of
element. For Late transition Metals (PPh3)3Pt
2ex310e16e (?5-C5H5)2Ni5ex210e20e
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d8complex 16 electrons Pt(II), Pd(II), Cu(III),
Ir(I), Rh(I) square planar For example,
Cl(PPh3)3Rh(I) (Wilkinsons complex),
Cl(PPh3)2(CO)Ir(I) (Vaska complex),
(PPh3)2(CCPh)2Pt(II) even though they contains
strong field ligand
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1.3 Mechanism in Organometallic Chemistry

• oxidative addition (???????) and
• reductive elimination (??? ???? )
• 2. insertion (????) and deinsertion
• (????)
• 3. Oxidative coupling (???????)and
• Reductive Cleavage (???????)

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1. oxidative addition (???????) and reductive
elimination (??? ???? )
two electron oxidative addtion (??? ???????) and
one electron oxidative addition (??? ???????)
A 16 electron complexes B 18 electron complexes
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H2Fe(CO)2-4 20 electron complex
Na2Fe(CO)4 18 electron complex
RFe(CO)4X- 18 electron complex
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Rh(II), Co(II) d7 complex
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1965, Chatt and Davidson
Kinetic Factor Thermodynamic Factor
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In 1982, Bergman, Graham, Jones
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Endo methyl migration aromatic stablization
energy
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Reductive Elimination spontaneous
To do reductive elimination, two ligands should
be placed at cis-position
Concerted Mechanism
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Transphos Ligand Pd(II) is dsp2 (square
planar) no reductive elimination Addition of
CH3I allows to make cis-dimethyl to undergo
reductive elimination.
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2. Elimination of one of ligand to make T-shape
to Y shape.
3. Reduce the electron density of central metal
Ligand off from metal by heat or light, oxidize
the metal, addition of strong pi-acceptor ligand
such as CO, maleic anhydride, quinone,
tetracyanoethylene
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1. Insertion (????) and Deinsertion (????)
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Migratory Insertion cis position and concerted
mechanism
Order h3-allyl Et gtMe gtPhCH2 gtvinyl aryl,
ROCH2 gtHOCH2 Hard to migrate to CO  
Hydride(H-), acyl (CH3CO), CF3 ,Heteroatome
RO-, R2N
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Decarbonylation
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Hydride Insertion cis-addition,
4-centered transition state For example
hydroboration, hydrosilylation,
hydroformylation
Reverse Reaction b-Hydride Elimination The
reason why it is hard to make a long chain
alkylmetal complex
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Alkyl Migration into olefin olefin polymerization
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Order of Migration of sigma liand-metal complex
to Olefin H gtgt R, vinyl, arylgt RCOgtgtRO, R2N
Heteroatom is hard to migrate because of strong
bond of heteroatom bearing lone pair to metal
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Alkyne undergoes migratory insertion, but
further successive reaction make polymer
compounds, which make complication.
Other Insertion, deinsertion substrate isocyanide
(CNR), carbene(CR2), SO2 , etc
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Nucleophilic Addition Reaction (??????)
reverse sterechemistry to migratory insertion
High valent metal species electron deficient
metal
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Trans-Addition Product
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Order of Reactivity
1.4.3  Oxidative coupling (???????)
Reductive Cleavage (???????)
M 2 Increase
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Electron withdrawing or strained molecules
For alkyne, electron-withdrawing is no necessary
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