terminology

Question 1

Antiferromagnetism

Answer 1

Antiferromagetism is exhibited by materials in which spins on neighbouring centers interact with each other in an antiparelle fashion
Ex. MnO is a compound that exhibits antiferromagnetic behaviour

Question 2

Coordination isomerism

Answer 2

Coordination isomers are possible only for salts in which both cations and anion are complex ions; the isomers arise from interchange of ligands between the two metal centeres.
[Pt(NH₃)₄][PtCl₆] and [Pt(NH₃)₄Cl₂][PtCl₄]

Question 3

A sigma-hold bond involving a tetrel element

Answer 3

Electrophilic sigmal-hole arise on molecules when electron distribution shifts toward the more electronegative element in a covalent bond, thereby producing a region of positive electrostatic potential on the less electronegative element. That area is called a sigma hold because it arises opposite a sigma bond, Its positive electrostatic potential can attract a nucleophile, forming a non-colvalent interaction called a sigma-hole bond. An example of such a bond involving an electrophilic tetrel atom (ie. C) and a nucleophile A is shown below

Question 4

The laporte selection rule

Answer 4

One of the rules that governs the relative intensities of absorption bands exhibited in the electronic spectra of coordination compounds. It states that transitions between states of the sample parity (symmetry with respect to a centre of inversion) are forbidden.

Hence, transitions between d orbitals are forbidden (g=>g transition) since d orbitals are symmetric with respect to inversions

Question 5

LFSE

Answer 5

The ligand filed stabilization Energy is the difference in energy between the (1) total energy of a coordination complex with the electron configuration resulting from ligand-field splitting of the orbitals and (2) the total energy of the same complex with all the d orbitals if they were equally populated
Hence, the octahedral LFSE for a low-spin d4 complex is -1.9 Do + P where P is the pairing energy

Question 6

a substitutionally labile complex

Answer 6

A complex that undergoes ligand–substitution reactions readily (i.e. t=1/2 < 1 min)

Question 7

Binary Nitrosyl

Answer 7

A compound that contains only one kind of metal and NO ligands.

e.g Cr(NO)₄

Question 8

migratory insertion reaction

Answer 8

Question 9

A π-basic ligand

Answer 9

a chemical entity that can provide a pair of electrons in a pi orbital while atached to a metal centre, e.g. Cl^-

Question 10

The principle of microscopic reversibility

Answer 10

states that both the forward and reverse reactions proceed via the same mechanism, e.g. olefin insertion into M-H and β-H elimination

Question 11

A stereospecific reaction.

Answer 11

involves one pure stereoisomer being formed from another, e.g. trans substitution reactions of square-planar Pt(II) complexes

Question 12

ΔS^‡.

Answer 12

is the entropy of activation for a chemica transformation. It can be determined from a Eying plot of ln(K/T) vs. I/T. e.g ΔS^‡ is positive for a dissociative process

Question 13

Ungerade orbital

Answer 13

a non-centrosymmetric, one electron wave function, e.g. a p orbital

Question 14

Agostic interaction

Answer 14

Complexes with an intramolecular interaction between a C-H bond and a metal centre have been termed “agostic” complexes. Examples of four classes of agostic interactions are shown below.

Question 15

trans-influence

Answer 15

The trans influence is a thermodynamic, ground-state effect, typically exhibited by square-planar Pt(II) complexes. It reflects the weakening of a M-L bond by the strong sigma bonding in the M-T link trans to the it, thereby raising the ground-state energy of the M-L linkage and leading to a smaller activation energy for the breaking of this bond. Phosphines, PR3, and the hydride anion, H-, exert strong trans influence.

Question 16

term symbol

Answer 16

a term symbol is a label composed of a letter relating to the value of L and a left superscript for the spin muliplicity, 2S+1. The value of J is given as a subscript. Hence, the term symbol ⁵F₂ marks a state in which L=3, 2S+1 =5 and J=2

Question 17

sigma complex

Answer 17

one in which at least one of the ligands is bound to the metal via a sigma bond, e.g. metal dihydrogen complexes that involve M-η²-H₂

Another explanation: sigma complex contains a ligands that ultizes an electron pair a sigma bond to attach to the metal centre

Question 18

Pauling’s electroneutrality principle

Answer 18

states that the distribution of charge in a molecule or ion is such that the charge on any signal atom is within the range +1 to -1 (ideally close to zero). Hence, the NH₄⁺ cation is best represented as shown below.

Question 19

Tolman’s cone angle

Answer 19

Tolman has defined the cone angle of a ligand as the apex angle, theta, of a cone that encompasses the van der Waals radii of the outermost atoms of a ligand as illustrated for the PMe₃ ligand below.

Question 20

Differentiate.

beta-H elimination and beta-H abstraction

Answer 20

beta-hydrogen elimination from metal-alkyl complexes is the microscopic reverse of migratory insertion of an olefin into a metal-hydride bond and thus occurs by the migratory deinsertion mechanism shown below: see image

beta-hydrogen elimination requires the presence of a open coordination site at the metal prior to the C-H bond cleavage step, and it occurs most reaily from metal-alkyl complexes that can adopt a syn coplanar arrangement of the metal and the hydride groups.

Question 21

differentiate

trans influence and trans effect

Answer 21

trans influence is a thermodynamic effect that applies primarily to the leaving group, and it contrbutes to the overall kinetic result by change the reactant ground state. As shown in the image.

a strong sigma bond between Pt and T weakens the Pt-X bond because both use the same Pt p_x and d_x²-y² orbitals. The weaker Pt-X bond is thus higher in energy in its ground state, thereby leading to a smaller activation enegy for breaking of this bond

Question 22

Thermodynamic instability and kinetic lability

Answer 22

A species is thermodynaimcally unstable if delta G is negative ofr its subsequent conversion to other entities

A species is kinetically labile if it has a low energy of activation (i.e. a rapid rate of reaction) for the particular trnasformation of question.

These are two separate and distinct effects

Thus methane is thermodynamically unstable with respect to its conversion to CO2 and H2O by reaction with dioxygen, but it is kinetically inert under ambient conditions.

Question 23

Fischer carbene complex and Schrock alkylidene complex

Answer 23

The two types of complexes having M=C bonds are best compared as summarized in the image.

Fischer carbyn complexes are electrophilic and thus prone to nucleophilic attack, e.g. nucleophiles such as PMe₃, py, alkyl lithiums etc. react with Fischer carbyne complexes to produce the correponding Fischer carbene complex:

L_nM≡C-R +Nu: => L_nM^-=C(Nu⁺)R

In contrast, Schrock alkylidyne complexes are nucleophilic and prone to attack by electrophiles e.g.

(t-BuO)3W≡C-t-Bu + 2HCl => (t-BuO)2Cl2W=C(H)(t-Bu) + t-BuOH

Question 24

ROMP.

Answer 24

ROMP is Ring-Opening Metathesis Polymerization, i.e. (see image)

The polymerization is also catalyzed by Grubbs’ catalysts of the type Cl2L2Ru=CHR.

Question 25

RDS

Answer 25

RDS is the Rate-Determining (or slow) Step in a mechanism. Thus, during the hydrogenation of olefins catalyzed by Wilkinson’s complex, the rate-determining step is the 1,2-insertion of the olefin into the Rh-H bond.

Question 26

ΔV‡.

Answer 26

ΔV‡ is the volume of activation, i.e. the change in the volume of a system from reactants to transition state, during ligand-substitution reactions. It can be determined by measuring the rate constants at different pressures, and it is positive for dissociative processes and negative for associative processes.

Question 27

γ-H abstraction.

Answer 27

γ-H abstraction involves the removal of a γ-H from an alkyl ligand in a transition-metal complex. This process can occur either intramolecularly (as shown in the example below) or intermolecularly (in the presence of an external abstracting agent).

Question 28

delta H^‡ and delta S^‡

Answer 28

delta H^‡ is the enthalpy of activation for a chemical transformation. it can be determined from the slope of an Eyring plot of ln(k/T) vs. 1/T. delta H^‡ is positive for a dissociation ligand substitution reaction and negative for an associative ligand susbtitution reaction.

delta S^‡ is the entropy of activation for a chemicaly transformation. It can be determined from the intercept of an Eyring plot of ln(k/T) vs. 1/T. delta S^‡ is postive for a dissociative ligand substitution reaction and negative for an associative ligand substitution reaction.

Question 29

alpha-H elimination and alpha-H abstraction

Answer 29

alpha-H elimination describes the intramolecular transfer of a alpha-H atom from a hydrocarbyl ligand to the metal centre, e.g. conversion of an alkyl complex into a hydrido alkyidene complex.

L_nM-CH2-R => L_nM(H)(=CHR)

alpha-H abstraction describes the inter- or intra-molecular removel of an alpha-H atom from a hydrocarbyl ligand, e.g. conversion of an alkyl complex into an alkylidene complex:

[Cp₂TaMe₂]+ + Me₃P=CH₂ => Cp₂Ta(Me)(=CH₂) + [Me₄P]+

Question 30

CFSE and CFAE

Answer 30

The Crystal Field Stabilization Energy (CFSE) is the energy difference betwen the actual distribution of electrons in a particular crystal field and that for all electrons being in a uniform field. For instance, the CFSE for a low spin d⁶ octahedral complex is -2.4Δ_oct + 2P where P is the pairing energy.

The Crystal Field Activation Energy (CFAE) is the difference in the Crystal Field Stabilization Energies (CFSE’s) between the reactant transition-metal complex and the intermediate transition-metal complex in a ligand-substitution reaction. The CFAE is considered to be a contribution to the activation energy for the reaction, and the larger is the CFAE, the slower is the reaction. A particular striking example is provided by d³ system which have positive CFAE’s for both dissociative and associative mechanisms and are thus substitutionally inert

Question 31

ISET and OSET

Answer 31

Inner-sphere electron transfer (ISET) reactions proceed in three steps: (1) a substitution reaction that leaves the oxidant and reductant linked by a bridging ligand, (2) the actual transfer of the electron (frquiently accompanied by transfer of the ligand), and (3) the separation of the products. Any step can be rate-determing, and ISET reactions are the fastest for σ* => σ* electron transfers in O_h complexes.

OSET (outer-sphere electron transfer) reactions , where the ligands in the coordination sphere do not change, the primary change on electron transfer is a change in bond distance. For example, the changes in bond distance are larger for octahedral complexes when eg (i.e. σ*) electrson are involved, Consequinetly, OSET reactions are fastes for pi*=>pi* electron transfers in Oh complexes.

Question 32

ΔG^‡ and ΔG.

Answer 32

ΔG^‡ is the free energy of activation for a chemical transformation. It can be calculated from the relationship ΔG^‡ = ΔH^‡ - TΔS^‡ for which ΔH^‡ and TΔS^‡ are determined from an Eyring plot of ln(k/T) vs 1/T. For a dissociative ligand substitution reaction ΔG^‡ is the free-energy difference between the reactants MX + Y and the transition state of {M + X + Y} and is expected to be positive.
ΔG is the free energy change accompanying a chemical transformation. It can be calculated from the relationship ΔG = ΔH - TΔS for which ΔH and TΔS must be determined independently. For a dissociative ligand substitution reaction ΔG is the free-energy difference between the reactants MX + Y and the final products MY + X and is expected to be negative

Question 33

A gerade and an ungerade molecular orbital

Answer 33

A gerade molecular orbital is a centrosymmetric, one-electron wave function that extends over the whole molecule, e.g. the A_1g molecular orbital in an octahedral transition-metal complex.
An ungerade molecular orbital is a non-centrosymmetric, one-electron wave function that extends over the whole molecule, e.g. a T_1u molecular orbital in an octahedral transition-metal complex.

Question 34

A precatalyst and a catalyst

Answer 34

Some so-called catalysts are really precatalysts. Precatalysts convert to catalysts for the reaction. For example, Wilkinson’s catalyst RhCl(PPh3)3 loses one triphenylphosphine ligand before entering the true catalytic cycle (see question 6). Precatalysts are easier to store, but can be readily activated in situ. Because of this preactivation step, many catalytic reactions involve an induction period.

Catalysis is the increase in the rate of a chemical reaction of two or more reactants due to the participation of an additional substance called a catalyst. Unlike other reagents in the chemical reaction, a catalyst is not consumed by the reaction. With a catalyst, less free energy is required to reach the transition state, but the total free energy from reactants to products does not change. The catalyst for hydrogenation reactions initiated by Wilkinson’s compound, RhCl(PPh3)3, is RhCl(PPh3)2 which may or may not be solvated (see question 6).

Question 35

An inner-sphere electron-transfer process.

Answer 35

ISET proceeds in three steps:

(i) a substitution reaction that leaves the oxidant and reductant linked by the bridging ligand,
(ii) the actual transfer of the electron (often accompanied by transfer of the ligand)
(iii) separation of the products.
e. g. the reaction of [Co(NH₃)₅Cl]²⁺ with [Cr(H₂O)₆]²⁺

Question 36

Jahn-Teller distorted complex

Answer 36

is any non-linear molecule that would otherwise be in a degenerate electronic state

e.g. O_h complexes of Cu(II) such as [Cu(H₂O)₆]²⁺

Question 37

Eyring plot

Answer 37

Eyring plot involves a plot of ln(k/T) vs. I/T

Δ S^‡; may be computed from the intercept

Δ H^‡; may be computed from the slope

ln(K/T) = -ΔH^‡/(RT) + ln(K¹/h) + ΔS^‡/R