Organometallic Intro

CHEM966 (Tunge) Coordination number

Electron Counting

CN = (# of monodentate) + 2(# of bidentate) +…

Description of a transition metal

In the example Rh is 4-coordinate. Other cases may be less obvious.

1) 2) 3) 4)

Formal Oxidation State d Electron Configuration Total Electron Count at the Metal Coordination Number

Common Ligands Anionic Ligands Cl

R

Consider Wilkinson’s Catalyst:

Ph3P Ph3P

Rh

H R2 N

RO

Tr ident ate ( 6 e-)

Bidentate (4 e-)

Monodent ate ( 2 e-)

=

M

Cl PPh3

O

M

O

M

=

O

Zr

M

M

O

O

M

H Cl

O

Neutral Ligands Monodent ate ( 2 e-)

Assignment of Oxidation State

Bidentate (4 e-)

R

a) Remove ligands in closed shell configuration b) The charge on the metal is the oxidation state [Rh(I)] Ph 3P Ph 3P

Rh

Cl PPh3

Tr ident ate ( 6 e-)

CO PR3

Ph 3P Ph 3P

Rh

Cl

R

N

N R

R2P

Cr(CO)3 N

n = Valence of neutral metal – charge on the metal Rh(0) has 9 valence electrons, so Rh(I) has (9-1= 8) Rh(I) has 8 d-electrons and is said to be d8 Total electron count The total electron count at the metal is given by: (metal electrons + ligand electrons). In this case: the phosphines and chloride are all 2 e- donors so, Total e- = 8 + 2 + 2 + 2 + 2 = 16 e-

O PR 2

N

PR2

Variable coordination modes

Ph 3P

Cl

vs.

Pd

d-Electron count (d )

R

R

PPh3

n

PR 2

NR 3

PPh3

Pd Ph3 P

PPh 3

M O O vs. S M S

O M

O M vs.

O vs.

Coordination number and electron count dictate geometry. d6, 6 coordinate metal = octahedral d8, 3-coordinate = T-shaped d8, 4-coordinate metal = square planar d8, 5-coordinate metal = square pyramidal or trigonal bipyramidal d10, 3-coordinate metal = trigonal planar d10, 4-coordinate metal = tetrahedral Coordination mode and electron count affect reactivity d6, 6 coordinate metal is substitutionally inert

M

Organometallic Intro

CHEM966 (Tunge) On the 18 e- “rule”:

Practice Me O

O Me O

Rh

Rh

O O O Me O

O

R PMe3 Zr

N

TMS

Mes

N Cl

Ph

Cl

H

Me

N Ru PCy3

Mes Ph H

R H3C(F3C)2CO H3C(F 3C)2CO

N W

CMe3

Where "formal" oxidation state f ails

Mo

Mo(0)

Mo

Mo(VI)

In fact, formal oxidation states often do not correlate with ionization energies : Green, M. L. H. J. Organometallic Chemistry 1995,500, 127-148. Consider WMe6 :the calculated charge on W is +0.4 not +6

Graphs generated by the program mlx (Version 1), Copyright©Cary Zachmanoglou, 1998 which was graciously provided by Professor Ged Parkin, Columbia University

The 18 e- rule works well for groups 6-8, but in general you should regard 18 e- as a maximum (not as you view the octet rule)

= 16 e- compounds

Organometallic Intro

CHEM966 (Tunge)

Where do the electrons go? Approximate d-orbital MO’s for common coordination geometries. z dz2 d x2 -y2

M

d xz

dyz

x M

d xy

dxz

d xz

dyz

M x

d z2 dx2- y2

dxy

complex

dz2

H 3C C O

2299

C O

2143

dyz

z

d xy

2060

d x2-y2

Ni(CO) 3PPh3

2068, 1990

dz2 dxy

Ni(CO) 3PtBu 3

2056, 1971

M

dz2

dz2

M

dxy

Cy N (O3C)Ni

increasing backbonding

2049, 1965

N Cy Co(CO) 4 2 Fe(CO) 4

1890 1770

Consequence of backbonding: HCo(CO)4 is as acidic as HCl!

M

d yz

νCO cm-1

Ni(CO) 4

d xz d yz

d xz

M C O

donation into the π* orbital

dx2- y2

z x

M C O

C O

d x2 -y2

dxy

d x2-y2

d xz

dyz

Note: Metals have lone pairs that are often not shown These lone pairs can be involved in π-backbonding

Phosphines and phosphites can be π-acceptors OR M P OR OR

OR M P OR OR

OR donation P OR into the σ* OR orbital

H H M

donation into the π* orbital

M H H

Backbonding in metal alkene complexes 1) lengthens the C-C bond 2) makes carbons more tetrahedral 3) formally oxidizes the metal by 2 e-

π-aciditiy NHC's