18 Electron Rule - Crystal Field Theory - Lecture Notes | CHE 3340, Study notes of Inorganic Chemistry

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Coordination

Compounds:

Chapter 22

18 Electron Rule

18 Electron Rule

 Simple e^ counting rules for predicting

structures & formulas of TM complexes

 (^) Justified using MO theory

 Similar to Octet Rule for Main Group

 For TM's in valence shell have set of

five d orbitals + three p 's + one s

orbital = 9 valence orbitals

 (^) To obtain Noble gas e^ configuration need 18 e  's  (^) Hence 18 e^ Rule!!

How Many Electrons Does Each Ligand Donate?

 Monodentate ligands — 2 e^ donors

 :F

 , :Cl  , :Br  , :I  , :H  , :CH 3  , :CR 3  , :SH  , :SR   (^) :CN:  , :CO:, :OH  , :OR  , :OH 2

ROH, :SCN:

  (^) :NH 3

, :NR

3

, :PR

3 , :AsR 3

Bidentate ligands — 4 e



donors

 Tridentate ligands — 6 e^ donors

H 2 N NH 2 R 2 P^ PR (^2) AsR R 2 As (^2) C C O   O O O N N N N N NH NH H 2 N 2

Electron Counting

1. How do you handle Ionic

compounds?

 (^) Add or subtract charge from metal to get M oxidation state.

2. How do you handle M—M bonds?

 (^) Break homolytically—give 1 e^ to each M  (^) Count M—M as 1 e^ donor  (^) as 2 e^ donor  (^) as 3 e^ donor  (^) as 4 e^ donor M M M M M M

Examples

Cr OC OC CO CO CO CO a. Cr d 6 6 CO 6 e  12 e  18 e  Cr(CO) 6 C Re PMe 3 CO CO CO O H 3 C b. (^) Re(PMe 3 ) (CO) 3 (COCH 3 ) Re O O N N N N

CN NC Co CN CN  c. (^) [Re(py) 4 (O) 2 ]

d. (^) [Co(CN) 4 ] 

Examples

Co NO Ph 2 As Ph 2 As AsPh 2 AsPh 2 2+ Co Ph 2 As NCS AsPh 2 NO Ph 2 As AsPh 2

h. [Co(diars) 2 NO] 2+ i. [Co(diars) 2 (NO)SCN]

Fe Fe C C C C OC CO OC CO C O O O O O g. [Fe(CO) 4 ] 2 (-CO)CO)

Ligand Counting

1-electron donors

 (^) H•, Cl•, Br•, I•, R•^ (R = alkyl or phenyl), or RO•

2-electron donors

 CO:, :PR

3

, :P(OR)

3

, R

2

C=CR

2

2 -alkene), Ralkene), R 2

C:

(carbene, M=CR 2

3-electron donors

3 -alkene), RC 3

H

5

• (allyl radical, ), RC : - (carbyne MCR), -alkene), RCl

, -alkene), RBr

, -alkene), RI

, -alkene), R R 2

P

4-electron donors

 (^) ^4 -alkene), Rdiene; ^4 -alkene), RC 4

H

4 (cyclobutadienes) H 2 C H C CH 2

Ligand Counting

5-electron donors

5 -alkene), RC 5

H

5

• (Cp - ),  3 -alkene), RCl - ,  3 -alkene), RBr - ,  3 -alkene), RI - ,  3 -alkene), R R 2

P

6-electron donors

6 -alkene), RC 6

H

6 (benzene or other  6 -alkene), Rarenes e.g.  6 -alkene), R C 6

H

5 Me)

 1 or 3-electron donors

 NO

 (^) 3 e–  (^) 1 e–

M N O M N O

M N

O

18 e



Rule Rigorously

Followed

3. Compounds with high 

o

values &

strongly  -backbonding ligands

 (^) Rigorously conform to 18 e^ Rule  (^) t 2g level is bonding now, so energetically favorable to have completely filled  (^) TM-alkene), RCarbonyl and -alkene), Rbonding organometallic compounds

18 e  Rule Not Rigorously Followed Complexes with d 8 e  configurations  (^) Sometimes 18 valence e's  strongly -alkene), R backbonding ligands  (^) Removes e^ density from M  (^) Fe(CO) 5 , Fe(CNR) 5 , [Pt(SnCl 3 ) 5 ] 3   (^) Sometimes 16 valence e's  NO -alkene), R backbonding ligands  (^) Doesn't remove e^ density from M  (^) [AuCl 4 ]  , [PdCl 4 ] 2  , Ni(dmg) 2 Note: explains why some Metals favor CN = 4 Some CN = 5 Some CN = 6, etc.

Bonding & Electronic Structure in Transition Metal Complexes

A. Crystal Field Theory (CFT)

 (^) Simplest  (^) Purely Electrostatic (ionic) model  (^) Ignores covalent bonding interactions with TM  (^) Ligand lone pair = point negative charge  (^) Repels e  's—s in d orbital on TM  (^) Allows us to understand & correlate all those properties that arise from presence of partly filled shells

Bonding & Electronic Structure in Transition Metal Complexes

B. Ligand Field Theory (LFT)

 (^) Extension of MO theory  (^) More exact  (^) Includes both covalent and electrostatic interaction between ligands and TM  (^) Focus: role of d orbitals on TM overlap with  &  ligand orbitals  (^) More cumbersome

Why are d orbitals split?

1. Place TM ion in spherical ligand field of negative charge = 12 e  's—s Fe 3+ ( d 5 ) free atom/ion spherical negative charge

E

n

e

r

g

y Fe

3 + 12 e 

Why are d orbitals split?

2. Place TM ion in octahedral ligand

field of 6L's each with pair of e



's—s

 (^) Come in along x , y , & z axes

 2 d orbitals are repelled more

 (^) Incoming lone pairs on ligands are pointing directly toward d orbitals containing e  's—s.

 3 d orbitals repelled less d

xy

, d

yz

, d

zx  (^) Incoming lone pairs on ligands pointing in  2 2 x y d  2 z d