Transition Metals and Coordination Chemistry Questions and Answers

Answer: C

The statement that is not a limitation is It explains the colour exhibited by coordination compounds.

Valence bond theory in fact does not explain the colour of coordination compounds; that is one of its well-known limitations, and crystal field theory explains colour through d-d transitions. A statement that VBT explains colour cannot be listed as a limitation.

The other four are its recognised limitations: no quantitative account of thermodynamic stability or magnetic data, no distinction between weak and strong ligands, and no exact prediction of tetrahedral or square planar structures for 4-coordinate complexes.

Answer: A

The name is Pentaamminenitritocobalt (III) chloride.

The ligand written ONO is nitrite bound through oxygen, named nitrito (nitrito-O). With five ammonia ligands (pentaammine), the complex ion carries a 2+ charge to balance two chloride ions. Ammonia is neutral and nitrite is -1, so cobalt is x - 1 = +2, x = +3. Ligands are named in alphabetical order, ammine before nitrito.

Cobalt (II) gets the oxidation state wrong. 'Nitro' is the name for NO₂ bound through nitrogen, the linkage isomer. Putting nitrito first breaks the alphabetical order, and 'mononitrito' is not used for a single ligand.

Answer: D

The IUPAC name is Dichloridobis(ethane-1,2-diamine)cobalt(III)chloride.

Find the oxidation state of cobalt: the complex cation is [CoCl₂(en)₂]⁺, en is neutral and each chloride is -1, so x - 2 = +1 and x = +3. Ligands are named in alphabetical order, ignoring prefixes: chlorido (c) before ethane-1,2-diamine (e). Since the ligand name ethane-1,2-diamine already contains a number, 'bis' is used for two of them.

The options with cobalt(II) have the wrong oxidation state, those with 'ethylenediamine' use the common rather than the IUPAC ligand name, and 'Bis(ethane-1,2-diamine)dichlorido...' lists the ligands out of alphabetical order.

Answer: D

The correct order is \(\mathrm{SCN^{-} \lt S^{2-} \lt OH^{-} \lt NCS^{-} \lt CN^{-}}\).

The spectrochemical series lists ligands from weak field to strong field. Its standard order runs: iodide, bromide, SCN^- (thiocyanate bonded through S), chloride, S^2-, fluoride, OH^-, oxalate, water, NCS^- (bonded through N), edta, ammonia, en, CN^-, CO. Picking out the five given ligands in that order gives the answer.

Thiocyanate bonded through sulphur is a weak-field ligand, while bonded through nitrogen it is much stronger. Cyanide is one of the strongest, so any order that does not end with CN^- is wrong, and option B even lists NCS^- twice.

Answer: B

The mixed oxide is Mn₃O₄.

Mn₃O₄ contains manganese in two oxidation states, +2 and +3, and can be written as MnO·Mn₂O₃; the average oxidation state of \(\frac{8}{3}\) is not a whole number. Such a compound is called a mixed oxide, like Fe₃O₄.

The others each contain manganese in a single oxidation state: MnO (+2), Mn₂O₃ (+3), MnO₂ (+4) and Mn₂O₇ (+7).

Answer: B

The formula is [Co(NH₃)₅(CO₃)]Cl.

The complex ion contains five neutral ammine ligands and one carbonato ligand, CO₃^2-, around Co(III). Its charge is \(+3 - 2 = +1\), so one chloride ion balances it.

Two or three chlorides would need a complex ion of charge +2 or +3, which is impossible with a carbonate ligand on Co(III). The (CO) options use carbonyl, not carbonato.

Answer: D

The increasing order of wavelength of absorption is (iv) \(\lt\) (iii) \(\lt\) (ii) \(\lt\) (i).

The wavelength absorbed depends on the crystal field splitting \(\Delta_{o}\): a larger splitting means higher-energy, shorter-wavelength absorption. By the spectrochemical series, CN^- \(\gt\) NH₃ \(\gt\) H₂O \(\gt\) Cl^- in field strength.

[Co(CN)₆]^3- has the largest splitting and absorbs the shortest wavelength; [Co(NH₃)₆]³⁺ comes next; replacing one NH₃ by H₂O and then by Cl^- lowers the splitting, so those absorb at longer wavelengths.

Option A is the reverse order, and the others misplace the cyanide or chloro complex.

Answer: A

The isomerism is ionisation isomerism.

The two compounds have the same composition but exchange the ion inside the coordination sphere with the counter-ion outside it: in one, sulphate is a ligand and bromide is free; in the other, bromide is the ligand and sulphate is free. In water they give different ions (Br^- or SO₄^2-), which is ionisation isomerism. (The paper prints the first formula as [Co(NH₃)(SO₄)]Br; the pair is the standard [Co(NH₃)₅SO₄]Br and [Co(NH₃)₅Br]SO₄.)

Linkage isomerism involves one ambidentate ligand binding through different atoms, and coordination isomerism needs both cation and anion to be complexes. Geometrical and optical isomerism are stereoisomerism.

Answer: B

The ligand is EDTA.

EDTA is a hexadentate chelating ligand that binds lead strongly in a soluble complex, which the body then excretes. Its calcium complex is used in the treatment of lead poisoning.

The textbook links D-penicillamine and desferrioxime B to removing excess copper and iron, and cis-platin is an anticancer drug. CO is a ligand that is itself toxic.

Answer: A

The diamagnetic ion is Zn²⁺.

An ion is diamagnetic when all its electrons are paired. Zn²⁺ is 3d¹⁰: a full d subshell with no unpaired electrons.

The others all have unpaired d electrons: Ni²⁺ (3d⁸) has 2, Co²⁺ (3d⁷) has 3, Cu²⁺ (3d⁹) has 1, and Mn²⁺ (3d⁵) has 5, so all four are paramagnetic.