IChO 2025 · Theory problem 8

Carbon monoxide: deadly poison or promising therapeutic agent?

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7.5% of the total

8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 Total
4 1 6 3 2 2 4 8 5 35

Carbon monoxide is a toxic gas because it binds to haemoglobin and blocks oxygen binding. A major source of \(\ce{CO}\) is incomplete combustion of engine fuel. Therefore, many exhausts are fitted with catalytic converters which remove \(\ce{CO}\) and other hazardous compounds including unburnt hydrocarbons and nitrogen oxides (\(\ce{NO_x}\)), forming mainly water vapour, carbon dioxide, and nitrogen.

8.1 Write the equations for the reactions of the following in a converter. Tick the box to indicate whether the carbon or nitrogen atom(s) have been oxidised by oxygen, "Ox", been reduced "Red", or stayed at the same oxidation state "Same". Please ignore any reactions between the exhaust gases.

(a) \(\ce{CO}\)
(b) Unburnt hydrocarbon \(\ce{C_xH_y}\)
(c) Nitrogen oxides \(\ce{NO_x}\)

Catalytic converters contain nanoparticles of noble metals which act as catalysts, immobilised on a cheap high surface area material such as \(\ce{Al2O3}\).

In one experiment, \(m(\mathrm{cat.})=0.10\ \mathrm{g}\) of catalyst containing \(w(\ce{Pd})=10\ \mathrm{wt\%}\) was used in a tube reactor. Gases were passed over the catalyst under continuous flow. The flow rate of \(\ce{CO}\) gas was \(v(\ce{CO})=10\ \mathrm{mL\,min^{-1}}\), which was mixed with a second flow of air (assume 20% \(\ce{O2}\), 80% \(\ce{N2}\)), at \(T=273.15\ \mathrm{K}\) and \(p=1.00\ \mathrm{atm}\), before entering the reactor.

8.2 Calculate the flow rate \(v\) (in \(\mathrm{mL\,min^{-1}}\)) of air required for the reactants to be present in a stoichiometric ratio.

Assume 10% of the \(\ce{Pd}\) atoms in the nanoparticles are exposed on the surface. Assume the conversion of \(\ce{CO}\) to \(\ce{CO2}\) was \(\eta=70\%\).

8.3 Calculate the number of \(\ce{CO2}\) molecules, \(N\), formed after \(t=1\ \mathrm{h}\) per surface \(\ce{Pd}\) atom.

Ruthenium nanoparticles are efficient catalysts for \(\ce{CO}\) combustion. The bonding interactions between \(\ce{CO}\) molecules and the surface \(\ce{Ru}\) atoms are of two types:

  1. \(\sigma\)-bonding through a lone pair on the carbon atom;
  2. \(\pi\)-interaction through an overlap between the \(\ce{Ru}\) valence d-orbitals and \(\ce{CO}\) molecular orbitals.

8.4 Complete the MO diagram for \(\ce{CO}\) with electrons and the following table with the type \((\sigma/\pi)\) of overlap for forming each MO.

Figure 1
MO 1 2 3 4 5 6 7 8
Type of overlap

8.5 What is the effect of the \(\ce{Ru-CO}\) \(\pi\)-interaction on the \(\ce{Ru-CO}\) and the \(\ce{C-O}\) bond strengths? Tick the correct answer.

(i) \(\ce{Ru-CO}\) bond strength:

☐ increases "in"
☐ decreases "dec"
☐ no change "nc"

(ii) \(\ce{C-O}\) bond strength:

☐ increases "in"
☐ decreases "dec"
☐ no change "nc"

Adsorbed \(\ce{CO}\) molecules have strong IR absorptions due to \(\ce{C-O}\) stretching vibrations. Three binding modes for \(\ce{CO}\) adsorbed on a metal surface are shown, where \(\ce{CO}\) can interact with one or more metal atoms. IR peaks in the spectra of free \(\ce{CO}\) and the three binding modes of adsorbed \(\ce{CO}\) were observed at values of 1850, 1930, 2100, and \(2143\ \mathrm{cm^{-1}}\).

Figure 2

8.6 Tick which wavenumber corresponds to each adsorption mode (A, B, and C) and to free \(\ce{CO}\).

Dispersion of a metal catalyst refers to the percentage of its atoms exposed on the surface. Dispersion can be determined by pulse chemisorption of \(\ce{CO}\) since it adsorbs selectively onto surface atoms. Equal pulses of known amounts of \(\ce{CO}\) are injected and a certain amount is adsorbed from each pulse. The amount remaining is measured by a detector and a peak, the area under which correlates with the amount of unadsorbed \(\ce{CO}\) from each pulse, is plotted as shown below. Injections continue until no more \(\ce{CO}\) adsorbs.

One experiment used \(50\ \mathrm{\mu L}\) pulses consisting of 10% vol \(\ce{CO}\) in \(\ce{He}\), at \(25\ ^\circ\mathrm{C}\) and \(100\ \mathrm{kPa}\). \(15\ \mathrm{mg}\) catalyst was used, composed of 10 wt% \(\ce{Ni}\) on an \(\ce{Al2O3}\) support. Assume one \(\ce{CO}\) molecule adsorbs on each surface \(\ce{Ni}\) atom.

Figure 3
Pulse # (x axis) 1 2 3 4 5 6 7
% of pulse unadsorbed (y axis) 4 10 20 50 80 100 100

8.7 Calculate the percentage of \(\ce{Ni}\) atoms exposed on the surface, \(\%_{\ce{Ni}}\).

Despite being toxic at higher concentrations, at a concentration of 10-250 ppm, \(\ce{CO}\) shows anti-inflammatory properties. Prodrugs known as \(\ce{CO}\) releasing molecules (CORMs) target the delivery of effective dosages of \(\ce{CO}\) to tissues in response to a trigger. Mn(I) carbonyl complexes have shown promise as PhotoCORMs as they can release \(\ce{CO}\) when exposed to light.

Compounds A-D are all neutral Mn(I) carbonyl complexes.

B, C, and D do not contain two \(\ce{CO}\) ligands in a trans arrangement.

The IR spectrum of C contains a band at \(2042\ \mathrm{cm^{-1}}\) in addition to the \(\ce{CO}\) stretching bands. The table with the number of \(\ce{CO}\) stretching bands in the IR spectra of metal carbonyl complexes is given in the data sheet section at the beginning of the exam.

Compound %Mn by mass Number of CO stretching bands
A 19.98 3
B 12.15 3
C 13.26 3
D 12.95 2
Figure 4

For simplicity, the ligand can be drawn as:

Figure 5

8.8 Draw the structures of compounds A-D, ensuring the three-dimensional shape is clear. If the compound is chiral, only one enantiomer needs to be drawn.

Compound C reacts with alkynes and this can be used to tag proteins. This is known as an Inorganic Click (iClick) reaction.

Figure 6

During the conversion of C to E, the \(2042\ \mathrm{cm^{-1}}\) band in the IR spectrum of C disappears. The IR spectrum of E has two new \(\ce{C=O}\) bands in the range of 1735-1725 \(\mathrm{cm^{-1}}\). After a further 6 h, compound E converts to compound F which only has one \(\ce{C=O}\) band in the range of 1735-1725 \(\mathrm{cm^{-1}}\). Compound G does not contain manganese and has two planes of symmetry.

8.9 Draw the structures of compounds E, F, and G, ensuring the three-dimensional shape is clear. If the compound is chiral, only one enantiomer needs to be drawn.

Periodic table and data sheet

Physical constants and formulae

Avogadro constant \(N_A\) \(6.022\times10^{23}\ \mathrm{mol^{-1}}\)
molar gas constant \(R\) \(8.314\ \mathrm{J\,K^{-1}\,mol^{-1}}\)
Faraday constant \(F\) \(96485\ \mathrm{C\,mol^{-1}}\)
Planck constant \(h\) \(6.626\times10^{-34}\ \mathrm{J\,s}\)
speed of light in vacuum \(c\) \(2.998\times10^8\ \mathrm{m\,s^{-1}}\)
mass of electron \(m_e\) \(9.109\times10^{-31}\ \mathrm{kg}\)
charge of electron \(e\) \(1.602\times10^{-19}\ \mathrm{C}\)
atmospheric pressure \(p_{\mathrm{atm}}\) \(101325\ \mathrm{Pa}\)
ionic product of water at 298 K \(K_w\) \(10^{-14}\)
\(1\ \mathrm{nm}=1\times10^{-9}\ \mathrm{m}\)
\(1\ \text{Å}=1\times10^{-10}\ \mathrm{m}\)
\(1\ \mathrm{pm}=1\times10^{-12}\ \mathrm{m}\)
\(0\ ^\circ\mathrm{C}=273.15\ \mathrm{K}\)
\(1\ \mathrm{kWh}=3600\ \mathrm{kJ}\)
\(1\ \mathrm{W}=1\ \mathrm{J\,s^{-1}}\)
\(1\ \mathrm{eV}=1.602\times10^{-19}\ \mathrm{J}\)
\(1\ \mathrm{bar}=10^5\ \mathrm{Pa}\)
area of triangle \(A=\frac12 ab\cdot\sin C\)
volume of cube \(V=a^3\)
volume of sphere \(V=\frac43\pi r^3\)
surface area of sphere \(S=4\pi r^2\)
ideal gas equation \(pV=nRT\)
frequency of light \(\nu=\frac{c}{\lambda}\)
energy of a photon \(E=\frac{hc}{\lambda}\)
Beer-Lambert law \(A=\log\left(\frac{I_0}{I}\right)=\varepsilon cd\)
spin-only magnetic moment \(\mu=\sqrt{n(n+2)}\ \mathrm{BM}\)
Henderson–Hasselbalch equation \(\mathrm{pH}=\mathrm{p}K_a+\log\left(\frac{[\ce{A-}]}{[\ce{HA}]}\right)\)
\(0^{\mathrm{th}}\) order integrated rate law \([\ce{A}]=[\ce{A}]_0-kt\)
\(1^{\mathrm{st}}\) order integrated rate law \(\ln[\ce{A}]=\ln[\ce{A}]_0-kt\)
Arrhenius equation \(k=A\cdot e^{-E_a/(RT)}\)
enthalpy \(H=U+pV\)
Gibbs energy \(G=H-TS\)
standard Gibbs energy change \(\Delta_rG^\circ=-RT\ln K=-nFE^\circ_{\mathrm{cell}}\)
Clausius-Clapeyron equation \(\ln\left(\frac{P_1}{P_2}\right)=\frac{\Delta H_{\mathrm{vap}}}{R}\left(\frac1{T_2}-\frac1{T_1}\right)\)
specific heat \(Q=mC_p\Delta T\)
boiling point elevation \(\Delta T=iK_bm_i\)
freezing point depression \(\Delta T=iK_fm_i\)

Consider all gases ideal unless told otherwise.

In equilibrium constant calculations, all concentrations are referenced to a concentration of \(1\ \mathrm{mol\,dm^{-3}}\).

IR absorption data

Bond stretching frequencies

Bond type Wavenumber \((\mathrm{cm^{-1}})\)
\(\ce{C-H}\) Alkane 3000-2850
Alkene 3100-3000
Aromatic 3150-3050
Alkyne \(\sim3300\)
Aldehyde 2900-2700
\(\ce{C=C}\) Alkene 1680-1600
Aromatic 1600-1400
\(\ce{C#C}\) Alkyne 2250-2100
\(\ce{C=O}\) Aldehyde 1740-1720
Ketone 1725-1705
Carboxylic acid 1725-1700
Ester 1750-1720
Amide 1670-1640
Anhydride 1810, 1760
\(\ce{C-O}\) Alcohol, ether, ester, carboxylic acid, anhydride 1300-800
\(\ce{O-H}\) Alcohol, phenol, free 3600-3200
Hydrogen bonded 3500-3200
Carboxylic acid 3400-2500
\(\ce{N-H}\) Primary and secondary amine and amide 3500-3100
\(\ce{C-N}\) Amine 1350-1000
\(\ce{C=N}\) Imine and oxime 1690-1640
\(\ce{C#N}\) Nitrile 2260-2240
\(\ce{N=O}\) Nitro (\(\ce{R-NO2}\)) 1600-1500, 1400-1300
\(\ce{S=O}\) Sulfoxide 1050
Sulfate, sulfonamide 1200-1140

Characteristic bond stretching frequencies of anionic species

Anion Wavenumber \((\mathrm{cm^{-1}})\)
\(\ce{SO4^2-}\) \(1100\text{-}1200\;(\nu_{\mathrm{as}})\)
\(\ce{ClO4-}\) \(1050\text{-}1170\;(\nu_{\mathrm{as}})\)
\(\ce{NO3-}\) \(1370\;(\nu_{\mathrm{as}})\)
\(\ce{CN-}\) 2089
\(\ce{SCN-}\) \(2053\;(\nu_{\ce{C-N}}),\ 748\;(\nu_{\ce{C-S}})\)
\(\ce{N3-}\) \(2042\;(\nu_{\mathrm{as}}),\ 1343\;(\nu_{\mathrm{s}})\)
\(\ce{CNO-}\) \(2052\;(\nu_{\ce{C-N}}),\ 1057\;(\nu_{\ce{N-O}})\)

Number of CO stretching bands in the IR spectra of metal carbonyl complexes

Complex Number of bands
\(\ce{M(CO)6}\) 1
\(\ce{M(CO)5X}\) 3
\(cis\text{-}\ce{M(CO)4X2}\) 4
\(trans\text{-}\ce{M(CO)4X2}\) 1
\(fac\text{-}\ce{M(CO)3X3}\) 2
\(mer\text{-}\ce{M(CO)3X3}\) 3
\(fac\text{-}\ce{M(CO)3X2Y}\) 3
\(cis\text{-}\ce{M(CO)2X4}\) 2
\(trans\text{-}\ce{M(CO)2X4}\) 1

Periodic table

Figure 7

Official English text of the problem.

Answers

  • 8.1 — 4 points

(a) \(\ce{2CO + O2 -> 2CO2}\): Ox.

(b) \(\ce{C_xH_y + (x+y/4)O2 -> xCO2 + (y/2)H2O}\): Ox.

(c) \(\ce{2NO_x -> N2 + xO2}\): Red.

The molecular species are carbon monoxide [C-]#[O+], oxygen O=O, carbon dioxide O=C=O, water O, and nitrogen N#N. The variable formulas \(\ce{C_xH_y}\) and \(\ce{NO_x}\) specify families, so neither has a unique SMILES.

  • 8.2 — 1 point

\[ \boxed{v(\mathrm{air})=25\ \mathrm{mL\,min^{-1}}} \]

  • 8.3 — 6 points

\[ \boxed{N=1.994\times10^3\approx2.0\times10^3 \text{ molecules of }\ce{CO2}\text{ per surface Pd atom}} \]

  • 8.4 — 3 points
MO 1 2 3 4 5 6 7 8
Electrons \(\uparrow\downarrow\) \(\uparrow\downarrow\) \(\uparrow\downarrow\) \(\uparrow\downarrow\) \(\uparrow\downarrow\) Empty Empty Empty
Type of overlap \(\sigma\) \(\sigma\) \(\pi\) \(\pi\) \(\sigma\) \(\pi\) \(\pi\) \(\sigma\)
  • 8.5 — 2 points

(i) \(\ce{Ru-CO}\) bond strength: increases, “in”.
(ii) \(\ce{C-O}\) bond strength: decreases, “dec”.

  • 8.6 — 2 points
Species or adsorption mode Wavenumber \((\mathrm{cm^{-1}})\) SMILES or schematic coordination SMILES
A: terminal carbon monoxide 2100 [C-](->*)#[O+]
B: doubly bridging carbon monoxide 1930 [C-](->*)(->*)#[O+]
C: triply bridging carbon monoxide 1850 [C-](->*)(->*)(->*)#[O+]
Free carbon monoxide 2143 [C-]#[O+]

Here * denotes an unspecified surface metal atom. These fragments describe connectivity; they do not represent the metal surface or its \(\pi\)-backbonding electronically.

  • 8.7 — 4 points

\[ \boxed{\%_{\ce{Ni}}=2.65\%\quad\text{(approximately }2.7\%\text{)}} \]

  • 8.8 — 8 points

Write tpy for 2,2′:6′,2″-terpyridine, n1c(-c2ccccn2)cccc1-c3ccccn3.

All four complexes are octahedral. The coordination SMILES use donor-to-metal arrows; the @OH descriptors specify the octahedral arrangement. Explicit spatial descriptions are supplied in the Solution.

A: Bromidopentacarbonylmanganese(I), \(\ce{[MnBr(CO)5]}\).
SMILES: [Br-]->[Mn+](<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])<-[C-]#[O+]

B: fac-Bromidotricarbonyl(κ²-terpyridine)manganese(I), \(fac\text{-}\ce{[MnBr(CO)3(tpy)]}\). The central pyridyl nitrogen and one terminal pyridyl nitrogen bind; the other terminal nitrogen remains unbound.
SMILES: [O+]#[C-]->[Mn@OH3+]1(<-[C-]#[O+])(<-[C-]#[O+])(<-[Br-])<-[n]2c(-c3cccc[n]->13)cccc2-c1ccccn1

C: fac-Azidotricarbonyl(κ²-terpyridine)manganese(I), \(fac\text{-}\ce{[Mn(N3)(CO)3(tpy)]}\). Azide binds through a terminal nitrogen.
SMILES: [O+]#[C-]->[Mn@OH3+]1(<-[C-]#[O+])(<-[C-]#[O+])(<-[N-]=[N+]=[N-])<-[n]2c(-c3cccc[n]->13)cccc2-c1ccccn1

D: cis-Bromidodicarbonyl(κ³-terpyridine)manganese(I), \(cis\text{-}\ce{[MnBr(CO)2(tpy)]}\). All three terpyridine nitrogens bind in a meridional arrangement.
SMILES: [O+]#[C-]->[Mn@OH9+]12(<-[C-]#[O+])(<-[Br-])<-[n]3c(-c4cccc[n]->14)cccc3-c1cccc[n]->21

  • 8.9 — 5 points

E: fac-Tricarbonyl(κ²-terpyridine)(4,5-bis(methoxycarbonyl)-1,2,3-triazolato-κN¹)manganese(I). The triazolate binds through an end nitrogen of its three-nitrogen sequence.
SMILES: [O+]#[C-]->[Mn@OH3+]1(<-[C-]#[O+])(<-[C-]#[O+])(<-[n-]2nnc(C(=O)OC)c2C(=O)OC)<-[n]2c(-c3cccc[n]->13)cccc2-c1ccccn1

F: fac-Tricarbonyl(κ²-terpyridine)(4,5-bis(methoxycarbonyl)-1,2,3-triazolato-κN²)manganese(I). The triazolate now binds through the middle nitrogen of its three-nitrogen sequence.
SMILES: [O+]#[C-]->[Mn@OH3+]1(<-[C-]#[O+])(<-[C-]#[O+])(<-[n-]2nc(C(=O)OC)c(C(=O)OC)n2)<-[n]2c(-c3cccc[n]->13)cccc2-c1ccccn1

G: Dimethyl 2H-1,2,3-triazole-4,5-dicarboxylate. The hydrogen is on the middle nitrogen, N2.
SMILES: COC(=O)c1n[nH]nc1C(=O)OC

Solution

8.1

Carbon monoxide is oxidised to carbon dioxide:

\[ \boxed{\ce{2CO + O2 -> 2CO2}} \]

The oxidation state of carbon increases from \(+2\) to \(+4\), so the correct box is Ox.

Complete combustion of an unburnt hydrocarbon gives carbon dioxide and water:

\[ \boxed{\ce{C_xH_y + (x+y/4)O2 -> xCO2 + (y/2)H2O}} \]

The products contain \(x\) carbon atoms, \(y\) hydrogen atoms, and \(2x+y/2\) oxygen atoms. Thus the oxygen coefficient is \(x+y/4\). The average carbon oxidation state rises from \(-y/x\) to \(+4\), so the correct box is Ox.

With reactions between exhaust gases excluded, the requested formal conversion of nitrogen oxides is:

\[ \boxed{\ce{2NO_x -> N2 + xO2}} \]

Nitrogen changes from an average oxidation state \(+2x\) to zero, so the correct box is Red. For example:

\[ \ce{2NO -> N2 + O2} \]

\[ \ce{2NO2 -> N2 + 2O2} \]

Here nitrogen monoxide is [N]=O, and nitrogen dioxide is O=[N+][O-]. These equations follow the problem’s instruction to exclude reduction by other exhaust constituents.

8.2

The combustion equation requires one mole of oxygen for every two moles of carbon monoxide. At the same temperature and pressure, gas volume flow rates have the same ratio:

\[ v(\ce{O2})=\frac12v(\ce{CO}) =\frac12(10)=5.0\ \mathrm{mL\,min^{-1}}. \]

Air contains 20% oxygen by volume, so

\[ 0.20\,v(\mathrm{air})=5.0\ \mathrm{mL\,min^{-1}}, \]

giving

\[ \boxed{v(\mathrm{air})=25\ \mathrm{mL\,min^{-1}}}. \]

8.3

In one hour, the inlet volume of carbon monoxide is

\[ V(\ce{CO})=(10\ \mathrm{mL\,min^{-1}})(60\ \mathrm{min}) =600\ \mathrm{mL}=6.00\times10^{-4}\ \mathrm{m^3}. \]

Each converted carbon monoxide molecule produces one carbon dioxide molecule. Therefore,

\[ n(\ce{CO2}) =\eta\frac{pV(\ce{CO})}{RT} =\frac{0.70(101325)(6.00\times10^{-4})} {(8.314)(273.15)} =1.87394\times10^{-2}\ \mathrm{mol}. \]

The total palladium mass is

\[ m(\ce{Pd})=(0.10\ \mathrm{g})(0.10)=0.010\ \mathrm{g}. \]

Since 10% of the palladium atoms are surface atoms,

\[ n(\ce{Pd})_{\mathrm{surface}} =0.10\frac{0.010\ \mathrm{g}}{106.42\ \mathrm{g\,mol^{-1}}} =9.39673\times10^{-6}\ \mathrm{mol}. \]

Avogadro’s constant cancels in the ratio:

\[ N= \frac{n(\ce{CO2})N_A} {n(\ce{Pd})_{\mathrm{surface}}N_A} =\frac{1.87394\times10^{-2}} {9.39673\times10^{-6}} =1994.24. \]

Thus,

\[ \boxed{N\approx2.0\times10^3\ \text{carbon dioxide molecules per surface Pd atom}}. \]

8.4

Carbon contributes four valence electrons and oxygen contributes six, giving ten valence electrons.

Fill the supplied levels from lowest to highest energy. Orbitals 1 and 2 each receive two electrons, the degenerate pair 3 and 4 receives four electrons in total, and orbital 5 receives the remaining two. Orbitals 6, 7, and 8 remain empty.

The valence configuration, using the usual orbital labels, is

\[ (\sigma_{2s})^2(\sigma_{2s}^{*})^2 (\pi_{2p_x})^2(\pi_{2p_y})^2(\sigma_{2p_z})^2. \]

Taking the molecular axis as \(z\), end-on overlap produces \(\sigma\) orbitals and side-on overlap produces \(\pi\) orbitals. Therefore:

MO Occupancy Overlap type
1 2 \(\sigma\)
2 2 \(\sigma\)
3 2 \(\pi\)
4 2 \(\pi\)
5 2 \(\sigma\)
6 0 \(\pi\)
7 0 \(\pi\)
8 0 \(\sigma\)

In the requested drawing, place an opposite-spin electron pair in each of boxes 1–5 and leave boxes 6–8 empty.

8.5

The \(\pi\)-interaction is metal-to-carbonyl back-donation: occupied ruthenium d orbitals donate electron density into the empty carbon monoxide \(\pi^*\) orbitals, represented by levels 6 and 7.

This interaction is bonding between ruthenium and the carbonyl ligand, but antibonding within carbon monoxide. Consequently:

\[ \boxed{\ce{Ru-CO}\text{ strengthens; }\ce{C-O}\text{ weakens}.} \]

8.6

For a stretching vibration,

\[ \widetilde{\nu}=\frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}, \]

where \(k\) is the bond force constant and \(\mu\) is the reduced mass. As the atoms remain carbon and oxygen, the frequency ordering mainly reflects the \(\ce{C-O}\) force constant.

Free carbon monoxide has the strongest internal bond and therefore the highest wavenumber, \(2143\ \mathrm{cm^{-1}}\). Metal back-donation weakens that bond. In the adsorption modes shown, increasing the number of interacting metal atoms gives progressively lower stretching wavenumbers:

\[ \boxed{ \begin{aligned} \text{Free carbon monoxide}:&\quad2143\ \mathrm{cm^{-1}},\\ \text{A, terminal}:&\quad2100\ \mathrm{cm^{-1}},\\ \text{B, two-metal bridge}:&\quad1930\ \mathrm{cm^{-1}},\\ \text{C, three-metal bridge}:&\quad1850\ \mathrm{cm^{-1}}. \end{aligned}} \]

All three adsorption modes bind through the carbon end.

8.7

Each pulse contains

\[ V_{\ce{CO},\mathrm{pulse}} =(50\ \mathrm{\mu L})(0.10) =5.0\ \mathrm{\mu L} =5.0\times10^{-9}\ \mathrm{m^3}. \]

At \(T=298.15\ \mathrm{K}\) and \(p=1.00\times10^5\ \mathrm{Pa}\),

\[ n_{\ce{CO},\mathrm{pulse}} =\frac{pV}{RT} =\frac{(1.00\times10^5)(5.0\times10^{-9})} {(8.314)(298.15)} =2.01709\times10^{-7}\ \mathrm{mol}. \]

The adsorbed fractions are obtained by subtracting each unadsorbed fraction from one:

Pulse Adsorbed fraction
1 0.96
2 0.90
3 0.80
4 0.50
5 0.20
6 0
7 0

Their sum is

\[ 0.96+0.90+0.80+0.50+0.20=3.36. \]

Thus the total adsorbed amount is

\[ n_{\ce{CO},\mathrm{ads}} =3.36(2.01709\times10^{-7}) =6.77742\times10^{-7}\ \mathrm{mol}. \]

One adsorbed molecule corresponds to one surface nickel atom, so this is also the amount of surface nickel atoms.

The total nickel amount is

\[ n_{\ce{Ni},\mathrm{total}} =\frac{(15\times10^{-3}\ \mathrm{g})(0.10)} {58.69\ \mathrm{g\,mol^{-1}}} =2.55580\times10^{-5}\ \mathrm{mol}. \]

Therefore,

\[ \boxed{ \%_{\ce{Ni}} =100\frac{6.77742\times10^{-7}} {2.55580\times10^{-5}} =2.65178\%\approx2.65\%. } \]

8.8

The terpyridine ligand has formula \(\ce{C15H11N3}\) and molar mass

\[ M(\mathrm{tpy}) =15(12.01)+11(1.008)+3(14.01) =233.268\ \mathrm{g\,mol^{-1}}. \]

Also,

\[ M(\ce{CO})=28.01\ \mathrm{g\,mol^{-1}}. \]

The proposed compositions give:

Compound Composition Calculated molar mass \((\mathrm{g\,mol^{-1}})\) Calculated Mn mass percentage
A \(\ce{[MnBr(CO)5]}\) 274.890 19.986%
B \(\ce{[MnBr(CO)3(tpy)]}\) 452.138 12.151%
C \(\ce{[Mn(N3)(CO)3(tpy)]}\) 414.268 13.262%
D \(\ce{[MnBr(CO)2(tpy)]}\) 424.128 12.954%

These agree with the supplied compositions, apart from the insignificant last-decimal discrepancy for A.

Identification of A. Bromine cleaves the manganese–manganese bond, oxidising both manganese atoms from Mn(0) to Mn(I):

\[ \ce{Mn2(CO)10 + Br2 -> 2[MnBr(CO)5]}. \]

Bromine has SMILES BrBr. The starting material, decacarbonyldimanganese(0), has a manganese–manganese bond and five terminal carbonyls on each metal; its coordination SMILES is:

[Mn](<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])-[Mn](<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])(<-[C-]#[O+])<-[C-]#[O+]

A is an octahedral \(\ce{M(CO)5X}\) complex, consistent with its three carbonyl stretching bands.

Identification of B. Addition of one terpyridine ligand replaces two carbonyls:

\[ \ce{A + tpy -> B + 2CO}. \]

The mass percentage requires three retained carbonyls and one bromide ligand. These occupy four coordination sites, leaving two sites for terpyridine. Hence terpyridine binds through the central nitrogen and one terminal nitrogen; the other terminal pyridyl group remains pendant.

The absence of trans carbonyls requires a facial carbonyl arrangement. Its three carbonyl bands agree with the supplied \(fac\text{-}\ce{M(CO)3X2Y}\) pattern.

Identification of C. Silver removes bromide as silver bromide, and azide replaces it:

\[ \ce{B + AgCF3SO3 + NaN3 -> C + AgBr + NaCF3SO3}. \]

The species in this substitution are silver(I) triflate [Ag+].[O-]S(=O)(=O)C(F)(F)F, sodium azide [Na+].[N-]=[N+]=[N-], silver(I) bromide [Ag+].[Br-], and sodium triflate [Na+].[O-]S(=O)(=O)C(F)(F)F.

The \(2042\ \mathrm{cm^{-1}}\) band identifies the azide ion, [N-]=[N+]=[N-]. Its replacement of bromide also explains the mass change:

\[ 79.90-3(14.01)=37.87\ \mathrm{g\,mol^{-1}}. \]

C retains the facial tricarbonyl group and κ²-terpyridine. The azide binds end-on through a terminal nitrogen.

Identification of D. Photolysis removes one carbonyl:

\[ \ce{B ->[h\nu] D + CO}. \]

The previously unbound terminal pyridyl nitrogen then occupies the vacant site. Thus D contains κ³-terpyridine, two carbonyls, and bromide.

Terpyridine binds meridionally: its two terminal nitrogen donors are trans to one another. The two carbonyls are cis, as required by the question and supported by the two carbonyl stretching bands.

All four structures are consistent with an 18-electron Mn(I), \(d^6\), centre.

Precise three-dimensional descriptions. Put manganese at the origin and its six donor atoms along the \(\pm x,\pm y,\pm z\) axes. Opposite signs on the same axis are trans; all other site pairs are cis. Let \(N_c\) denote the central terpyridine nitrogen, \(N_t\) the first terminal nitrogen, and \(N_u\) the other terminal nitrogen.

Compound \(+x\) \(-x\) \(+y\) \(-y\) \(+z\) \(-z\)
A CO CO CO CO CO Br
B CO \(N_c\) CO \(N_t\) CO Br
C CO \(N_c\) CO \(N_t\) CO Terminal N of azide
D CO \(N_c\) \(N_u\) \(N_t\) CO Br

Every carbonyl binds through carbon. In B and C, \(N_u\) is uncoordinated. In D it occupies the site vacated by the \(+y\) carbonyl. Connect \(N_c\) to each terminal pyridyl ring through the actual terpyridine skeleton given in the SMILES.

This specifies one allowed stereoisomer of B and C; their mirror images are equally acceptable.

8.9

The alkyne is dimethyl acetylenedicarboxylate, also called dimethyl but-2-ynedioate, with SMILES COC(=O)C#CC(=O)OC.

Formation of E. The coordinated azide undergoes a \([3+2]\) cycloaddition with the alkyne. The three azide nitrogen atoms and the two alkyne carbon atoms become the five atoms of a 1,2,3-triazolate ring:

\[ \ce{C + C6H6O4 -> E}. \]

The original azide functionality is consumed, explaining the disappearance of its \(2042\ \mathrm{cm^{-1}}\) band. The two ester groups remain intact.

Number the ring consecutively N1–N2–N3–C4–C5, with C5 bonded back to N1. Both C4 and C5 carry a methoxycarbonyl group, \(\ce{-C(=O)OCH3}\).

In E, manganese remains bonded to the azide nitrogen that becomes N1. N1 is adjacent to C5, whereas the opposite end nitrogen N3 is adjacent to C4. This binding arrangement distinguishes the two ester-bearing sides of the ring and is consistent with the two new ester carbonyl bands.

Conversion of E to F. The metal attachment migrates from the end nitrogen N1 to the middle nitrogen N2:

\[ \ce{E -> F}. \]

In F, the triazolate’s two ester-substituted sides are related by the symmetry of the N2-bound ligand fragment, consistent with the reported single ester carbonyl band. E and F are therefore N1-bound and N2-bound coordination isomers, respectively; the ester groups have not been removed.

In both E and F, manganese retains its facial tricarbonyl group and κ²-terpyridine. Their octahedral arrangements are the same as C in the table above, with the \(-z\) site occupied by:

  • E: triazolate N1;
  • F: triazolate N2.

The ring is aromatic and approximately planar. Its rotational conformation about the metal–nitrogen bond is not fixed by the SMILES or by the supplied data.

Formation of G. Hydrobromic acid protonates the triazolate ligand and supplies bromide to regenerate B:

\[ \ce{F + HBr -> B + G}. \]

Hydrogen bromide has molecular SMILES Br; its aqueous dissociation can be represented as [H+].[Br-].

The product G is the 2H tautomer: the hydrogen is attached to N2, the middle nitrogen. Its formula is \(\ce{C6H7N3O4}\), and its SMILES is COC(=O)c1n[nH]nc1C(=O)OC.

For the requested drawing, make the triazole ring planar and draw the two identical ester groups in matching, symmetry-related conformations. The two mirror planes are:

  1. The plane of the triazole ring and the planar ester framework.
  2. A plane perpendicular to the ring, passing through N2–H and the midpoint of the C4–C5 bond. This exchanges N1 with N3, C4 with C5, and the two ester groups.

An N1-protonated tautomer would destroy the second of these planes, so the stated symmetry selects the 2H tautomer.

The solution was checked against the official solution.