Organometallic Structures

Question 1

Electron Counting

Answer 1

1. Count Number of anionic ligands
2. Determine Oxidation state of Metal
3. Count donor electrons (mindful of haptic bonding)
4. sum valence electrons and d electrons

Question 2

18 electron rule exceptions

Answer 2

1. Steric exceptions
   - high oxidation or early transition
   - small radii increases sterics of ligands
   - steric energy prevents coordination of more ligands to reach 18 electrons
2. Electronic exceptions
   - late transition or low oxidation
   - high atomic number means stable d orbitals
   - dz2 no longer bonds
   - d8 froms square planar
   - d10 froms trigonal planar

Question 3

Metal Bonding

Answer 3

1. Assume complex achieves 18 electrons
2. count valence electrons for each metal
3. if less tahn 18 add metal metal bonds

one metal bond equals one electron to each metal centre

Question 4

Molecular orbitals of Geometries

Answer 4

1. Octahedral
   - ligands approach along axes
   - eg bonding sigma
   - t2g non-bonding sigma
2. Square planar
   - ligands approach along xy axes
   - dx2-y2 bonding
   - dz2 interacts
   - t2g non bonding
3. Tetrahedral
   - ligands approach in between axes
   - t2g bonding sigma
   - eg non-bonding sigma

Question 5

Square Planar or Tetrahedral

Answer 5

Depends on Electronic configuration then sterics. Config with lowest energy is preferred even if higher sterics.

Question 6

pi acceptors and pi donors

Answer 6

1. pi donors
   - have filled p orbitals
   - donate electron density via pi interactions
   - eg amines, halides, oxygen
2. pi acceptors
   - have vacant p orbitals
   - accept electron density from metal d orbitals
   - eg CO, phosphines, alkenes

Question 7

pi backbonding

Answer 7

metal donates electron density into π* orbital of ligand.

• weakens ligand bond (decreased frequency)
• increases sigma donation
• increase metal-ligand bond
• strong sigma donors (NHC, PPh3) increase electron density and thus π backbonding

Question 8

Hard Soft Acid Bases

Answer 8

1. Hard
   - small radius
   - high charge density
   - non-polarisable
   - eg F, Cl, amines, O
2. Soft
   - large
   - low charge density
   - polarisable
   - eg low valent metals, CO, R-, PPh3

like goes with like

Question 9

Metal Carbonyl Synthesis

Answer 9

1. Direct Ligation of unoxidised metal
2. Reductive Carbonylation with CO gas
3. Reductive carbonylation with organic carbonyl

Question 10

Metal Phosphine Synthesis

Answer 10

1. Coordination to unsaturated metal
2. Coordination to low valent
3. ligand substitution

Question 11

Metal-Alkyl Synthesis

Answer 11

1. Direct ligation with alkylating agent
   e. g AlMe3, SnMe4, Mao, LiMe

Question 12

Metal-CP synthesis

Answer 12

1. Ligand Exchange and in-situ deprotonation with metal salt

Question 13

Metal-NHC Synthesis

Answer 13

1. Free carbene metallation
2. in situ deprotonation and metallation
3. silver-NHC transmetallation

Question 14

Substitution Mechanisms

Answer 14

1. Associative
2. Dissociative

Question 15

Associative Sub Increase Rate

Answer 15

1. decrease sterics
2. trans effect

Question 16

Dissociative Sub increase rate

Answer 16

1. Increase sterics

Question 17

Oxidative Addition non-polar

Answer 17

1. metal coordinates to bond
2. donation into π* breaks bond
3. coordinates cis

Question 18

Oxidative Addition Polar

Answer 18

1. metal nucleophilic attacks electrophilic atom, breaking bond froming anion
2. becomes electrophilic
3. anion attacks trans due to sterics

Question 19

Oxidative Addition Factors

Answer 19

1. Electron Density
   - donates electron density to ligand
2. coordinatively unsaturated

Question 20

Oxidative Addition Rate

Answer 20

1. π acceptors reduce electron density and thus rate eg CO, suppress oxidative addition
2. sigma donors increase electron density and thus increase rate eg NHC, PPh3

Question 21

Reductive Elimination Requirements

Answer 21

1. 2 ligands cis for orbital overlap
2. product makes sense

Question 22

Reductive elim factors

Answer 22

1. locked cis means faster rate
2. low valent metals are unstable by reduction

Question 23

Migration Reactions

Answer 23

1. Alkene insertion to form alkyl ligand
2. CO insertion to from carbonyl

Question 24

Hydride Elimination Requirements

Answer 24

1. Beta hydrogen present
2. M-C-C-H in syn coplanar
3. adjacent vacant/psuedo-vacant site
   (4. Metal has d electron density to cleave C-H)

Question 25

Rules for polyenes

Answer 25

1. Even before odd 2. Open before closed (after even or odd) (shorter before longer) (least anionic/most electrophilic carbon/largest orbitals)

Question 26

Direct CO ligation

Answer 26

CO ligates to unoxidised metal

Question 27

Reductive carbonylation with CO gas

Answer 27

1. coordination to metal salt 2. reduction with reducing agent e.g. Al, NaOMe

Question 28

Reductive Carbonylation with organic carbonyl

Answer 28

1. oxidative addition of acyl halide 2. reverse migration of alkyl 3. reductive elimination of R-X

Question 29

Metallation of Preformed Carbene

Answer 29

1. form carbene by deprotonation 2. Metallation

Question 30

In-situ deprotonation metallation of NHC

Answer 30

requires metal with basic ligands e.g OAc

Question 31

Silver-NHC transmetallation

Answer 31

labile Ag-NHC bond due to high d electron density transfers readily to other metals

Question 32

Complex geometry factors

Answer 32

Coordination number electronics sterics (metal oxidation state, ligands, sterics)

Question 33

Dative vs Covalent Ligands

Answer 33

Dative donates 2 electrons (neutral) covalent donates 1 electron (anionic)

Question 34

Determining anionic neutral

Answer 34

put both dative bond electrons on ligand determine electron configuration of ligand

Question 35

Enhancing insertion rate

Answer 35

1. increase steric bulk insertion decreases coordination 2. lewis acid additives/metals e.g. AlCl3 stabilises metal acyl transition 3. electrophilic metals withdraw electron from carbon activating electrophilicity