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