L1: Intro; hapticity and binary metal carbonyls, Flashcards
1
Q
Hapticity
A
The number of contiguous (C) atoms directly bound to metal in a ligand
Denoted by ‘etan’
2
Q
Carbon monoxide as a carbonyl ligand
A
- Isoelectronic with N2
- As a ligand, binds through the C atom, donates 2 electrons
- The free CO has a net dipole moment very close to zero
3
Q
18 electron rule
A
- The sum of the d-electrons possessed by the metal plus those donated by the ligands must total 18
- Allows prediction of no. ligands attached to transition metals in organometallic complexes
- Also obeyed by anionic and cationic carbonyl complexes
- Goes toward explaining metal-metal bonds
4
Q
Bridging carbonyls
A
- In species with M-M bonds, carbonyls may act as bridging ligands donating one electron to each metal)
5
Q
Neutral ligand method
A
- Consider anionic ligands as uncharged (e.g Chloride, Bromide)
- Allows species to obey 18 electron rule
6
Q
Oxidation state/donor ligand method
A
- Yields same results as neutral ligand method
- X- ligands are formally anionic and count as 2 electron donors
7
Q
Examples that disobey 18 electron rule
A
- V(CO)6 is a stable 17 electron compound; it is paramagnetic and readily undergoes a 1 electron reduction to the 18 electron anion
- Cluster bonding (CHEM224) - 4d and 5d metals in grps 8 and 9
8
Q
Methods for synthesis of metal carbonyls
A
- Direct reaction of metal w/ CO gas
Mainly Ni(CO)4, Fe(CO)5, Co2(CO)8
Binary metal carbonyls are typically liquids or volatile solids, highly toxic and soluble in organic solvents - Reductive carbonylation of metal salts
Treatment of metal salts w/ CO in the presence of suitable reducing agent
…Practice writing equations
9
Q
CO bonding interactions w/ metals
A
- C lone pair (5sigma) interacts w/ a vacant metal orbital (sigm-donation) AND the pi* orbital (2pi) are available to accept electron density back from occupied metal d-orbitals (pi-backbonding)
10
Q
Metal backbonding effect on C-O bond
A
- Metal backbonding populates the pi* orbitals of CO
- This progressively weakens the C-O bond
-> stretching frequency decreases - Extent of it depends on how electron-rich the metal is (ie. low OS)
-> low OS metals form complexes w/ CO much more readily than metals in high OS
11
Q
Interpreting IR spectra
A
- Metal backbonding weakens C-O bond, resulting in a lower stretching frequency
- Terminal and bridging carbonyls have v(CO) stretches in separate regions of the IR spectrum
- Terminal: approx. triple bond
- Bridging: approx. double bond
12
Q
Examples of reactions of binary carbonyl complexes
A
- Thermal or photochemical substitution of CO by other ligands (write equations…)
- Reaction with halogens or H2
- Reduction to carbonyl anions
- Nucleophilic attack at the carbon
13
Q
Reaction w/ halogens or hydrogen
A
- Many metal carbonyls react with halogens to give carbonyl halides
- Reaction with halogens or hydrogen can cleave the M-M bonds in some dimeric/polymeric metal carbonyls
…write example equations
14
Q
Reduction to carbonyl anions
A
- Powerful reducing agents (e.g. Na) will reduce metal carbonyls to carbonylmetallate anions
- e.g. Tetrahydrofuran (THF)
15
Q
Syngas, water gas shift reaction
A
- Syngas is a CO/H2 mixture
- Produced by coal ‘gasification’; reaction of powdered coal w/ steam at high T and P
- Alternatively, made from natural gas by ‘reforming’
- Used industrially to make mixtures of alkanes, alkenes, methanol and higher alcohols by heterogenous catalysis with metal oxide catalysts at high T and P
*Fischer-Tropsch Reactions - Water gas shift reaction is industrially important for H2 enrichment of syn gas
- Heterogenous catalysts (typically Fe/Cu mixtures) are used and high T is required; metal carbonyls have since been found to catalyse WGS and provide mechanistic insight
