Rationale

Question 1

Aqueous solutions of [M(H₂O)₆]²⁺ are extremely pale pink whereas those of
[Ni(H₂O)₆]²⁺ are a bright green.

Answer 1

Mn2+

• high spin, d5, Oh
• no spin-or Laporte-allowed absorption
• therefore, weak forbidden transitions and a pale colour

Ni2+,

• d8, Oh
• no laporte-allowed but three spin-allowed transitions
• therefore, stronger bands, more colour

Question 2

The major band in the electronic spectrum of the pink [Co(H₂O)₆]²⁺ ion has a
maximum at 513 nm (ε = 0.5 L mol-1 cm-1). On the addition of concentrated
hydrochloric acid the solution turns blue and is more intensely coloured with
absorption maxima at 625, 670, and 700 nm (ε = 35-60 L mol-1 cm-1).

Answer 2

For octahedral [MoCl6]3- with pi-donor Cl- ligands, the pertinent portion of its MO diagram is: see image

For the Mo2+ complex, both Cl- => Mo sigma and pi-bonding is relatively diminished. Hence, its eg* and t2g* levels are lower in energy, evidently by approximately the same amount since the splitting energy is about the same for both complexes

Question 3

Somewhat surprisingly, the Δ_o values for both [MoCl₆]^3- and [MoCl₆]^4- are
approximately 19,000 cm^-1.

Answer 3

The microstate table for p⁴ leads to the same free-ion terms as for ^p2 (done in class), namely

¹D, ³P and ¹S

The ground state term is ³P

Question 4

The infrared spectrum of [(η⁵-C₅H₅)Mo(CO)₃]₂ in CCl₄ exhibits three strong bands at
1961, 1917, and 1909 cm^-1.

Answer 4

Spectrum reflects the complex having the molecular structure (see image) with all terminal carbonyl ligands

Question 5

Ligand substitution reactions at four-coordinate palladium(II) centres generally have
ΔS^‡ and ΔV^‡ less than zero, whereas for four-coordinate palladium(0) complexes,
ΔS^‡ and ΔV^‡ are generally greater than zero.

Answer 5

Four-coordinate Pd(II) d8 complexes are square-planar and react associatively whereas four-coordinate Pd(0) d10 complexes are tetrahedral and react dissociatively

Question 6

The rate of oxidation of [Cr(OH₂)₆]²⁺ by [CoCl(NH₃)₅]²⁺ is high (kobs = 6.0 x 105 M^-1
s-1) compared to that of its oxidation by [Co(NH₃)₆]³⁺ (kobs = 1.0 x 10^-3 M^-1 s^-1).
There is, however, little difference in analogous reactivity between [V(OH₂)₆]²⁺ and
[RuCl(NH₃)₅]²⁺ or [Ru(NH₃)₆]³⁺ as oxidants

Answer 6

Reaction between [Cr(OH₂)₆]²⁺ (high-spin d⁴) and [Co(NH₃)₅]²⁺ occurs via an inner-sphere mechanism, but wiht [Co(NH3)6]3+ via an outer-sphere mechanism.

The analogous V(II) complex (d³) is substituationally inert, and so its reactions with the Ru(III) complexes both occur via an outer-sphere mechanism

Question 7

Cobaltocene, a very air-sensitive, black solid, generally behaves as a reducing agent.

Answer 7

Cp2Co is a 19 C complex that readily loses the extra electron to form 18 electrons [Cp₂Co]⁺ which is isoelectronic with ferrocene

Question 8

Square-planar substitution reactions frequently show two-term rate laws of the form:
Rate = k₁[Complex] + k₂[Complex][Y]
where Y is the incoming ligand.

Answer 8

Both pathways (both terms in the rate law) are considered to be associative in spite of the difference in order.

The k₂ term easily fits an associative mechanism. The accepted explanation for the k₁ term is a solvent-assisted reaction, with solvent replacing the leaving ligand in the first step.

Question 9

In general, ethyl complexes of transition metals are less thermally stable than the analogous methyl
complexes.

Answer 9

Ethyl complexes can readily undergo beta-H elimination reactions. Methyl complexes cannot.

Question 10

The compound trans-Fe(o-phen)₂(NCS)₂ has a magnetic moment of 0.65 Bohr magnetons at 80 K,
increasing with temperature to 5.2 Bohr magnetons at 300 K.

Answer 10

The compound trans-Fe(o-phen)₂(NCS)₂ contains Fe²⁺ (d⁶). At 80 K it has 0 unpaired electrons (assumimng a spin-only magnetic moment), and at 300 K it has 4 unpaired electrons.

The increase in magnetic moment wih temperature can be explained by the spin crossover.

t_2g⁶ (80K) => t_2g⁴_eg² (300K)

Question 11

Which should be a stronger-field ligand in a transition-metal amide complex (e.g.
MNMe2), an amide ligand with an M-N-C angle of 120°, or an amide ligand with an MN-
C angle of 109°? Why?

Answer 11

An Amide ligand with an M-N-C angle of 120 degree is both a sigma and a pi donor

An amide ligand with an M-N-C angle of 109 degree is only a sigma donor-hance the stronger field ligand

Question 12

The substitutionally inert complex, Re(S2CNR2)3, cannot be resolved into
enantiomers

Answer 12

Either the complex is octahedral and racemizes rapidly or it is trigonal prismatic and not optically active.

Question 13

The Δ_o values for both [WCl₆]^3- and [WCl₆]^4- are approximately 20,000 cm^-1.

Answer 13

For the W²⁺ complex, both Cl^- => W sigma and pi bonding is realtively diminished. Hence, its E_g* and T_2g* levels are lower in energy, evidently by approximately the same amount since Δ_o is the same for both tungsten complexes.

Question 14

[FeF₆]^3- is colourless whereas [CoF₆]^3- is coloured.

Answer 14

Transitions in [FeF6]3- are both Laporte- and spin forbidden, whereas those in [CoF6]3- are only Laporte forbidden.

Question 15

The IR spectrum of Pt(NH₃)₂Cl₂ as a Nujol mull exhibits ν_Pt-Cl at 330 and 323 cm^-1.

Answer 15

The complex must have a cis-square-planar geometry.

Question 16

When CuI is dissolved in PMe₃, the ⁶³Cu NMR spectrum shows a five-line pattern
with relative intensities 1:4:6:4:1. Both ³¹P and ⁶³Cu have I = ½.

Answer 16

The tetrahedral (by VBT) [Cu(PMe₃)₄]⁺ cation must have been formed.

Question 17

Lowest CO-stretching frequency: [Fe(CO)4]2-, [Co(CO)4]-, or Ni(CO)4.

Answer 17

[Fe(CO)4]2-: because it has the must M => CO backbonding

Question 18

Longest N-N bond: N2, (OC)5Cr(η1-N2), or (OC)5Cr(μ;η1,η1-N2)Cr(CO)5.

Answer 18

(OC)5Cr(μ;η1,η1-N2)Cr(CO)5.

Cr-N2 backbonding at both ends

Question 19

The highest upfield 1H NMR signal of Cp*W(NO)(H)(η3-C3H5).

Answer 19

The resonance due to the hydride ligand

Question 20

The most likely to undergo migratory CO insertion:
cis-Fe(CO)4Me2, CpFe(CO)2Me, or Cp*Fe(CO)2Me

Answer 20

cis-Fe(CO)4Me2,

Contains the most electrophilic CO ligands

Question 21

The most likely to undergo phosphine dissociation:
cis-Mo(CO)4(PPh3)2, cis-Mo(CO)4[P(C6F5)3]2, or cis-Mo(CO)4(PH3)2,

Answer 21

cis-Mo(CO)4[P(C6F5)3]2

contains largest and least Lewis-basic phosphine

Question 22

The Nujol-mull IR spectrum of orange Fe2(CO)9 exhibits strong absorptions at 2082, 2019 and 1829
cm-1. However, its 13C NMR spectrum in C6D6 at room temperature is reported to consist of a
single sharp resonance at δ 212.9

Answer 22

Fe₂(CO)₉ has the molecular structure (see image) which is stereochemically non-rigid in solutions

Question 23

[Cr(H₂O)₆]²⁺ is substitutionally labile whereas [Cr(CN)₆]^4- is inert

Answer 23

[Cr(H₂O)₆]²⁺ d⁴ (t_2g³ e_g¹)
electron in the antibonding eg level makes substitution relatively easy.

[Cr(CN)₆]^4- d⁴ (t_2g⁴)

substitution requires activation into eg level, so reactions are slow. [Also strong Cr-CN bonds-pi bonding]

Question 24

The V-Cl distance in [VCl₄]^- is longer than in VCl₄

Answer 24

Population of antibonding orbitals reduces bond order, thereby incraseing bond length. [VCl₄]^- has two such electrons

so longer V-Cl distance

Question 25

Aniline, C₆H₅NH₂, is a stronger base than (η⁶-aniline)Mo(CO)₃.

Answer 25

In (η⁶-aniline)Mo(CO)₃. the organic ligand is funcitoning as a six-electron donor to the molybdenum centre. This drain of electron denisty reduces the avaibility of the lone pair of electorns on the NH₂ group– rendering it a weaker Lewis base

Question 26

The major band in the electronic spectrum of pink [Co(H2O)6]2+ has a maximum at 513 nm (ε = 0.5
L mol-1 cm-1). On the addition of concentrated hydrochloric acid the solution turns blue and is more
intensely coloured with absorption maxima at 625, 670, and 700 nm (ε = 35-60 L mol-1 cm-1).

Answer 26

[Co(H2O)6]2+ is an Oh complex of Co(II) having d7. So it’s expceted to exhibit absorption due to three laporte-forbidden but spin allowed transition

treatment with conncetrated HCl converts [Co(H2O)6]2+ to [CoCl4]2- which is a tetrahedral complex of Co(II) having a d7. This Td complex exhibts absorption due to three laporte and spin-allowed electronic transitions. The abosrption bands are closer together since half of delta_oh is delta_td. Also Td is more intense b/c of lack of symmetry, so obviating the Laporte selection rule

Question 27

The rate of oxidation of [Cr(OH2)6]2+ by [CoCl(NH3)5]2+ is high (kobs = 6.0 x 105 M-1 s-1) compared
to that of its oxidation by [Co(NH3)6]3+ (kobs = 1.0 x 10-3 M-1 s-1). There is, however, little
difference in analogous reactivity between [V(OH2)6]2+ and [RuCl(NH3)5]2+ or [Ru(NH3)6]3+ as
oxidants.

Answer 27

rxn between [Cr(OH2)6]2+ (Cr(II), high spin d4) and [CoCl(NH3)5]2+ (Co(III), low-spin d6) occurs via an inner-sphere mechanism and a favoured sigma*=>sigma* electron transfer. However, with [Co(NH3)6]3+ (Co(III), low-spin d6 it occurts via an outer -spehere mechanism and an unfavoured sigma* => sigma* electron transfer

In contrast, the analogous V(II) complex, [V(OH2)6]2+ (d3) is substitutionally inert, and so its reaction with teh Ru(III), low-spin d5 complexes, [RuCl(NH3)5]2+ or [Ru(NH3)6]3+, both occur via an outer sphere mech and a foavoured pi*=>pi* elecron transfer