5 Electron Counting,

7/27/2019 5 Electron Counting,

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M

Formalisms Oxidation State & Electron Counting

Oxidation State The charge left on the metal after allligands have been removed in their normal,

closed shell, configuration.

There are two ways to figure out the oxidation state. The one we will use involves breakingthe metalligand bonds heterolytically with both electrons going with the ligand.

Understanding the nature of the metal center is critical in understanding how it reacts.

There are four things we need to know:

(1) The oxidation state of the metal(2) The number of delectrons on the metal in that oxidation state(3) The coordination number of the metal(4) The availability (or lack thereof) of any coordination sites on the metal

M Cl

Examples:

M Cl

(+1) M H M H(+1)

M CO M CO(0)

M PPh3 M PPh3(0)

M M(0)

OR

R

OR

R

M(+1)

M M(0)

HH

H H

HH

H H


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Now that we know the oxidation state of the metal we can figure out the d-electron count. This canbe done easily by consulting the periodic table.

The 4sand 3dorbitals are very close in energy and for complexesof transition metals it is agood approximation that the dorbitals are filled first.

First row

Second row

Third row

3d

4d

5d

TiZr

Hf

4

VNb

Ta

5

CrMo

W

6

MnTc

Re

7

FeRu

Os

8

CoRh

Ir

9

NiPd

Pt

10

CuAg

Au

11Group #

4

3

2

5

4

3

6

5

4

7

6

5

8

7

6

9

8

7

10

9

8

10

91

0

2

1

3

2

4

3

5

4

6

5

7

6

8

7

0

I

IIIII

IV

oxidation

state

# of d-electrons


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Why is knowing the number of d-electrons important?

18 electron rule In mononuclear diamagnetic complexes, the toal number of electrons in thebonding shell (the sum of the metal delectrons plus those contributed by the ligands) neverexceeds 18. This determines the maximumnumber of ligands allowable for any metal in anyoxidation state.

If a metal complex has 18 electrons in the bonding shell it is coordinatively saturated.

If a metal complex has fewer than 18 electrons it is coordinatively unsaturated

and typically has at least one vacant coordination site.

In general, catalytic processes require the metal catalyst to have a vacant coordination siteso that the substrate can bind.

Knowing the oxidation state of the metal and the number of delectrons is only part of the story.In order to properly count the electrons in the bonding shell we also need to know how many

electrons each ligand supplies. It is not always two.


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Classes of ligands There are three classes of ligands: (1) formalanions (X-Type); (2) formalneutrals (L-Type); (3) formalcations.

All of these ligand classes have one pair of electrons for bonding the metal that are used todetermine the metal's oxidation state. Often the ligand will also contain additional unsaturation thatcan donate addition electron pairs to the bonding shell. The manner in which these unsaturatedligands binds is called the "hapticity" ().

Some Simple X-Type Donors monohapto (1); only donate two electrons to bonding shell

alkyl R aryl Ar hydride H halide X cyanide CN

acyl CR

O

alkenyl

H H

H

alkynyl R

Allyl X-Type ligand with multiple coordination modes; can donate 2 or 4 electrons

M

1

2e ()

1 coordination site

M

3

4e ()

2 coordination sites

M

3

4e (, )


oxo M=O

O2


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M

M


Cyclopentadienyl X-Type ligand with multiple coordination modes; can donate 2, 4, or 6 electrons

1

2e

1 coordination site

3

4e


5

6e

3 coordination site

(most common)

M

M

Some common L-Type ligands and their hapticity

phosphine R3P M 1 2e

amine R3N M 1 2e

nitrile RCN M 1 2e

carbonyl OC M 1 2e

carbene M 1 2e

R

X

alkene/alkyne M 2 2eM

diene M 4 4e

arene 6 6e

Note: For polydentate ligands, count eachattachment separately.


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Carbene Complexes

Several types are known. How they contributes to the overall electron counting is dependent on thesubtituents and metal involved.

LnM

XR1

R2

Fischer carbenes

X = O, NR, SM = low-valent, middle or

late transition metals L-type ligand donating 2 electrons

LnM

R1

R2

Schrock carbenes

R1, R2 = alkyl or HM = high-valent carbyl or

middle transition metals 2X-type ligand (2 charge) donating 4 electrons

LnM

R1

EWG

Carbenoids

LnM = Rh2(O2CR)4, N4Ru,(N2O2)Ru, or (N,N)Cu

L-type ligand donating 2 electrons

LnM C

Vinylidenes

R1, R2 = alkyl, aryl, or H L-type ligand donating 2 electrons

R1

R2LnM

N-Heterocyclic carbenes

R = alkyl or aryl generally a spectator ligand L-type ligand donating 2 electrons

NR

RN


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Examples For each complex determine the oxidation state of the metal, the number of delectronssupplied by the metal, the overall electron count, and the number of vacant coordination sites.

RhPPh3Ph3P

Ph3P Cl

Wilkinson'scatalyst

Cr

COCOOC

Fe

COOC

Fe

COOC

Zr

H

Cl

Schwartz'sreagent

PdPPh3Ph3P

Ph3P PPh3

Co Co COOC

OC

CO

COOC

OC COMM bonds: contribute 1e to each metal


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d Orbitals and Ligand Bonding

The 18 electron rule is predicated on the notion that it is the d orbitals of the metal that are responsiblefor ligand binding as well as engaging the substrate in chemistry (the substrate can be thought of asnothing more than a particularly reactive ligand). Therefore understanding how the d orbitals of a metal

interact with the sphybrid orbitals of the ligand and/or substrate is important.

-donor bonds formed by overlap of filled sphybrid orbitals (or -bonding orbitals) of ligand withvacant dorbitals of the metal. Orbital symmetry should be kept in mind.

Ligands that are primarily -donors (e.g., R3P, R3N, H and R) increaseelectron density on the metal

+M

one lobe of avacant dorbital

+ P

RR

R

filled sp3

orbital

What about unsaturated ligands?


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-Bonds on the ligand can also serve as a -donor ligands

filleddorbital

vacant* orbitals

+M

one lobe of avacant dorbital

+

+

filled bond

BUT...the * orbital also has the same symmetry and similar energy to the dorbitals

By this model, the metal is donating electron density to the ligand. This is known as "backbonding" and can act as an electron sink. Thus the electron density of the metal will decrease.

+

++

+

M


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Two different types of backbonding are possible depending on the geometry of the ligand orbitals.

+M

+

+

C O++

longitudinal

vacant CO* orbitalfilled metal

dorbital

vacantdorbital

lone pair of COserving as -donor

+M

+

+

C

C

perpendicular

filled metaldorbital

vacantdorbital

+

+

vacant C=C* orbital

++

filled C=C-bond as-donor

Bottom Line...because of these two bonding modes the electron density of the metal (and as aconsequence its reactivity) can be tuned by varying the identity -donor ligands (add electrondensity) and -acceptor ligands (remove electron density).

+

For this course I have chosen to focus more on the reactivity of metals rather than the structures.We will talk about ligand choices more when necessary.


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Phosphine Ligands

Cone Angle: This is a measure of the three-dimensional steric influence a ligand has on the enviromentaround the metal. It can be changed idenpendently from electronics.

M

P

R RR

cone angle ()

PR3 PH3 87

PF3 104

P(OMe)3 107

PMe3 118

PMe2Ph 122

Hartwig, Organotransition Metal Chemistry, 2010, pp 3738.

Phosphine ligands are frequently used ligands for organometallic reactions. While they can be somewhatdifficult to synthesize and are often oxygen sensitive, they are highly tunable. Influences on the metal

can be mediated by electronics, cone angle, monodentate vs. bidentate, and bite angle.

Electronics: The identity of the substituents around the phosphorus atom has a profound influence onhow donating the ligand is.

P

Me MeMe

P

Ph PhPh

P

RO OROR

P

RO NR2OR

PR3P(t-Bu)3

PCy3PMe3

P(C6H4-4-OMe)3PPh3

P(OMe)3

co (cm-1)

2056

2056

2064

20662069

2079

P(OPh)3 2085

trialkylphosphine triarylphosphine

phosphite phosphoramidite

co (cm-1) is determined with

Ni(CO)3L and is a measurement

of the amount of backbonding.More donating L, morebackbonding and co decreases.

PR3 PHPh2 128

P(OPh)3 128

PEt3 132

PPh3 145

PPh2(t-Bu) 157

Ph2

PCH2

CH2

PPh2

123

PR3 PPh(t-Bu)2 170

PCy3 170

P(t-Bu)3 182

PPh3 145

P(o-tol)3 194


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Phosphine Ligands

Bite Angle: This is a measure of the three-dimensional steric influence a ligand has on the enviromentaround the metal. It can be changed idenpendently from electronics.

Ligand n

dppm (1, n = 1) 72

dppe (1, n = 2) 84 (7095)

dppp (1, n = 3) 91

dppb (1, n = 4) 98

DIOP (2) 102 (90120)

Steinborn, Fundamentals of Organometallic Catalysis, 2012, pp 90.

BINAP (3) 92

M

P P

bite angle (n)

Me-DuPHOS (4) 83

Ligand n

BISBI (5) 113 (92155)

TRANSPHOS (6) 111

NORPHOS (7) 123 (110145)

DPEphos (8) 102 (86120)

DBFphos (9) 131 (117147)

Xantphos (10) 112 (97135)dppf (11) 96

PPh2Ph2P

n

Ph2P PPh2

1

5

PPh2Ph2P 6

PPh2

PPh2

7

O

PPh2PPh28

OPPh2Ph2P

9

PPh2

PPh2

3

O

O

PPh2

PPh2

2

PP

4

O

PPh2PPh210

Fe

PPh2

PPh2

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5 Electron Counting,

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