Coordination Chemistry Lecture 5 Part 1
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Questions and Answers

Which metal carbonyl requires the harshest conditions for preparation?

  • Mo(CO)6
  • Ni(CO)4
  • Fe(CO)5 (correct)
  • Ru3(CO)12

What characteristic is common among most metal carbonyls?

  • They are classified as organometallic compounds. (correct)
  • They exhibit more inorganic characteristics than organic.
  • They have very strong intermolecular forces.
  • They are generally solid at room temperature.

What is the boiling point of nickel tetracarbonyl (Ni(CO)4)?

  • 42°C (correct)
  • 150°C
  • 80°C
  • 103°C

Which statement about mononuclear carbonyls is true?

<p>They have weak intermolecular forces between molecules. (D)</p> Signup and view all the answers

What is the coordination number of a metal in a typical metal carbonyl complex?

<p>5 (C)</p> Signup and view all the answers

Which carbonyl complex is known to violate the 18-electron rule?

<p>V(CO)6 (C)</p> Signup and view all the answers

What is the coordination number of V(CO)6 if dimerization were to occur?

<p>7 (D)</p> Signup and view all the answers

Which metal in the provided content has 9 valence electrons?

<p>Cobalt (A)</p> Signup and view all the answers

How do Mn(CO)5 and Co(CO)4 achieve an 18-electron configuration?

<p>Through dimerization (D)</p> Signup and view all the answers

What temperature does V(CO)6 decompose at?

<p>70°C (B)</p> Signup and view all the answers

What did Ludwig Mond contribute to the field of chemistry?

<p>First metal carbonyl complex recognition (C)</p> Signup and view all the answers

Which of the following carbonyl compounds contains a zero oxidation state for the metal?

<p>Ni(CO)4 (A)</p> Signup and view all the answers

What overall electron configuration do carbonyl complexes aim to achieve?

<p>18 electron configuration (D)</p> Signup and view all the answers

What type of complex is formed when carbon monoxide acts as a Lewis acid by accepting electrons?

<p>p-acceptor complex (A)</p> Signup and view all the answers

What occurs as a result of backbonding in metal carbonyl complexes?

<p>Weaker C-O bonds (C)</p> Signup and view all the answers

How many valence electrons do many low oxidation state organotransition metal complexes aim for?

<p>18 electrons (C)</p> Signup and view all the answers

Which of the following statements about CO stretching frequencies is correct?

<p>Frequencies measure bond length inversely (D)</p> Signup and view all the answers

What defines an 18-electron configuration in organotransition metal complexes?

<p>Retaining total valence electrons like noble gases (C)</p> Signup and view all the answers

In the bonding picture of p-acceptor complexes, what type of orbital is considered antibonding?

<p>t1u orbital (C)</p> Signup and view all the answers

What is the result of strong overlap between metal and carbon monoxide in complexes?

<p>Synergic bonding between s and p components (D)</p> Signup and view all the answers

What is the oxidation state of the transition metal when determining electron counts for carbon monoxide complexes?

<p>Zero oxidation state (D)</p> Signup and view all the answers

Which of the following is an example of the transition metals in the carbonyl complexes discussed?

<p>Fe and Co (C)</p> Signup and view all the answers

Which orbital type does a filled metal d orbital contribute back to in a p-acceptor complex?

<p>p* orbital (D)</p> Signup and view all the answers

What is the primary reason for ligand dissociation in a dissociative mechanism?

<p>The strong electron σ-donor ability of one ligand (B)</p> Signup and view all the answers

Which types of ligands contribute to the trans effect according to their influence?

<p>Both σ-donors and π-acceptors (C)</p> Signup and view all the answers

Which statement correctly describes the nature of the trans effect?

<p>It is predominantly a kinetic phenomenon. (B)</p> Signup and view all the answers

What is the role of a strong π-acceptor in the associative mechanism?

<p>To remove electron density from the orbital lobe trans to it (B)</p> Signup and view all the answers

Which ligand mentioned in the content is characterized as a poor Lewis base?

<p>Carbon monoxide (CO) (A)</p> Signup and view all the answers

What is the key interaction involved in backbonding for transition metal carbonyl complexes?

<p>Ï€ back-donation from the ligand to the metal (C)</p> Signup and view all the answers

In the context of p-acceptor complexes, which statement is true regarding ligands?

<p>Ligands can be neutral or anionic. (D)</p> Signup and view all the answers

What effect does a strong s-donor ligand have on the stability of the metal-ligand complex?

<p>It can weaken the bond with adjacent ligands leading to instability. (C)</p> Signup and view all the answers

Which mechanism allows for nucleophilic attack facilitated by a strong π-acceptor?

<p>Associative mechanism (D)</p> Signup and view all the answers

Which example demonstrates a less stable isomer forming preferentially before a more stable isomer?

<p>Isomerization of cis-platin to trans-platin (A)</p> Signup and view all the answers

Flashcards

18-electron rule in metal carbonyls

Metal carbonyl complexes tend to have 18 valence electrons for stability.

Vanadium hexacarbonyl (V(CO)6)

A metal carbonyl complex with 17 electrons, violating the 18-electron rule.

Metal carbonyl dimerisation

Some metal carbonyls with an odd number of metal electrons combine; forming a bond to share electrons in order to achieve 18 electrons.

Metal carbonyls

Chemical compounds containing metals bonded to carbon monoxide (CO) molecules.

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Nickel carbonyl (Ni(CO)4)

A metal carbonyl complex that adheres to the 18 electron rule; stable compound.

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Electron count of Vanadium (V)

The metal vanadium has 5 valence electrons.

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Electron count of Manganese (Mn)

Manganese (Mn) has 7 valence electrons.

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Electron count of Cobalt (Co)

Cobalt (Co) has 9 valence electrons.

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Trans Effect

The preferential formation of a particular isomer in a reaction, even if it's not the most stable.

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Dissociative Mechanism

Ligand substitution reaction where a strong donor ligand weakens the metal-ligand bond, causing dissociation.

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Associative Mechanism

Ligand substitution reaction where a strong acceptor ligand weakens a trans ligand bond, allowing attack by new nucleophiles.

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p-Acceptor Ligand

A ligand that withdraws electron density from the metal center, often by accepting electron density from the metal d-orbitals.

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Backbonding

A type of bonding in transition metal carbonyl complexes where electrons from filled d-orbitals on the metal are donated to empty antibonding orbitals on the ligands.

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Bonding in transition metal complex

Transition metal bonding is primarily electrostatic interaction between metal cation and ligand electrons

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Ligand

An atom or a group of atoms attached to a metal center in a coordination complex.

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Strong s-donor

Ligand capable of facilitating substitution through the dissociative mechanism due to strong electron donation.

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Metal carbonyl complex

Complex involving a metal bonded to carbon monoxide (CO) ligands.

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Nucleophile

A chemical species that donates an electron pair to form a new covalent bond.

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Sigma (σ) component

A bonding interaction in metal carbonyls where a lone pair on carbon monoxide donates electrons to a vacant metal s orbital.

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Pi (Ï€) component

A bonding interaction in metal carbonyls where electrons back-donate from a filled metal d orbital to an antibonding p* orbital on the carbon monoxide ligand.

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Synergic bonding

The combined effect of sigma and pi components in metal carbonyls, leading to very strong bonds.

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18-electron rule

In organotransition metal complexes, a stable configuration often involves a total of 18 valence electrons on the metal atom.

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p-acceptor complex

A metal complex that demonstrates pi acceptor behavior, in which the metal receives electrons via a pi interaction.

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CO stretching frequency

The frequency of the carbon-oxygen bond vibration in an infrared spectrum; used to determine the bond length and strength in metal carbonyls.

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Oxidation state

The hypothetical charge of an element if all bonds are treated as ionic bonds

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18-electron rule

A rule in organometallic chemistry that predicts the stability of a transition metal complex. Many metal carbonyls fulfill this rule to achieve a stable configuration.

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Metal Carbonyls Preparation

Metal carbonyls are prepared by direct reaction of a metal with carbon monoxide (CO) or reductive carbonylation (using CO in excess).

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Ni(CO)4 Preparation Conditions

Nickel tetracarbonyl (Ni(CO)4) is prepared under mild conditions (30°C, 1 atmosphere pressure).

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Fe(CO)5 Preparation Conditions

Iron pentacarbonyl (Fe(CO)5) requires harsher conditions (200°C, 200 atmospheres) for preparation.

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Metal Carbonyl Properties

Metal carbonyls, like Ni(CO)4, Fe(CO)5, and Mo(CO)6, are volatile liquids/ solids, easily purified using distillation/sublimation, and often exhibit organometallic properties.

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Study Notes

Lecture 5 Part 1

  • Topic 5 introduces concepts and new types of complex continued.
  • Ligands trans to each other share a dx2-y2 orbital and the donor-acceptor ability of one ligand affects the other.
  • If ligand D (donor) is a strong electron σ-donor, inter-electron repulsion facilitates the dissociation of the other L* ligand. This is called a dissociative mechanism.
  • A strong D-M bond leads to a weak M-L* bond, causing L* to dissociate from the metal.
  • When L = a strong σ donor, substitution occurs through a dissociative mechanism.
  • A strong Ï€ acceptor (A) removes electron density from the orbital lobe trans to it, making it prone to attack by other ligands. This is called an associative mechanism. This is the (most common) mechanism. Examples of strong Ï€ acceptors are C≡N and H2C=CH2 (ethene).
  • Both σ and Ï€ effects contribute to ligand order in the trans effect series.
  • The trans effect is primarily a kinetic phenomenon, where one isomer forms preferentially before another, regardless of thermodynamic product preference.
  • Example, cis-platin is less stable than trans-platin but forms first in PtCl42- reactions.
  • Ï€-Acceptor complexes are a basis for organometallic complexes.
  • Transition metal complex bonding is governed by electrostatic interactions between metal cations and ligand (L) electrons.
  • Ligands can be neutral or anionic.
  • A strong bond involves efficient overlap between σ orbitals. Example, a dz2 (metal) orbital interacting with an sp3 hybrid orbital of a ligand (e.g., lone pair in ammonia NH3).
  • Carbon monoxide (CO) is a weak σ donor, less effective at attracting electron density than halides.
  • Demonstrated via reactions with Lewis acids
  • Lewis acids (like BF3) have an empty orbital to accept electrons. (A for acid, A for acceptor)
  • An example of a complication is CO acting as a ligand, where extra electrons for Mo (in a Mo(CO)6 complex), are accommodated through backbonding.

Lecture 5 Part 2

  • Electron Counting Rules: Metal is in zero oxidation state, retaining full valence electrons. Consider electrons formally donated by the ligand.
  • In the case of carbon monoxide, :C≡O donates 2 electrons when attached terminally to the metal.
  • Example: Mo(CO)6. Mo electrons (5s14d5) = 1+5=6, CO electrons = 6 x 2=12. Total = 18, obeying the 18-electron rule
  • Transition metal binary carbonyl complexes. The numbers of electrons required are given for complexes ranging from Vanadium (V) to Nickel (Ni).
  • Ni(CO)4, Fe(CO)5 and Cr(CO)6 are straightforward, have the correct number of CO ligands to fill all valence orbitals.
    • These complexes obey the 18-electron rule.
  • Vanadium hexacarbonyl V(CO)6 is the odd one out, for steric reasons it cannot dimerise and has only 17 valence electrons, violating the 18-electron rule.
  • It is more stable than many other complexes, readily accepting an electron in a reaction with sodium.
  • Manganese and Cobalt Carbonyls, Mn(CO)5- and Co(CO)4 each have an odd number of metal electrons, (7 for Mn; 9 for Co).
    • To obtain the desired 18-electron configuration these monomeric fragments dimerise, sharing electrons in the metal-metal bond. For example, their respective formulae are Mn2(CO)10 and Co2(CO)8.
  • Method 1 for preparing metal carbonyls is direct action on metal (elements in the periodic table). Only Nickel tetracarbonyl (Ni(CO)4 ) and Iron pentacarbonyl(Fe(CO)5) can be made using this method.
  • Method 2 for preparing metal carbonyls is reductive carbonylation using a large excess of carbon monoxide (CO). Examples include Mo(CO)6, Ru3(CO)12 and [Ti(CO)6]2-.
  • Metal carbonyls are generally organometallic compounds and possess organic characteristics.
    • Most are volatile (liquids or solids) and are easily purified via standard distillation.
      • Examples: Ni(CO)4 (liquid, r.t. m.p. -19°C, b.p. 42°C );
      • Fe(CO)5(liquid, r.t. m.p. -20°C, b.p. 103°C);
      • Mo(CO)6 (solid, r.t. m.p. 150°C, sublimes around 80°C).
  • Mononuclear carbonyls have simple structures, examples: Ni(CO)4 (tetrahedral) and Fe(CO)5(trigonal bipyramidal).
  • Exercise Example 5 details the structure of and bonding within dimanganese decacarbonyl with particular emphasis on the different interactions involved, the coordination number and oxidation state of the metal, a structural diagram and an electron counting analysis to determine if the complex adheres to the 18-electron rule.

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Explore the complexities of coordination chemistry in this quiz on Lecture 5 Part 1. Delve into the concepts of ligand interactions, dissociative and associative mechanisms, and the trans effect. Ideal for students looking to reinforce their understanding of metal-ligand bonding.

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