Coordination Compounds - Practice Questions with Answers

Q1.

A coordination compound consists of a central metal atom surrounded by:

• A Ligands (ions or molecules donating electron pairs)
• B Mainly covalent bonds with little donor-acceptor character
• C Mainly ionic bonds formed by complete electron transfer
• D No surrounding atoms or groups whatsoever as generally observed

Q2.

What is a ligand?

• A An ion or molecule that donates a lone pair to the metal
• B The central metal atom around which the structure is built
• C The overall charged species formed once the structure is assembled
• D A neutral complex with no net charge on the whole assembly

Q3.

The coordination number of the metal in [Co(NH₃)₆]3+ is:

• A 6
• B 3
• C 4
• D 2

Q4.

What is the name of [Cu(NH₃)4]2+?

• A Tetraamminecopper(II) ion
• B Copper tetraamine as widely reported
• C Diammine copper in standard practice
• D Cupric amide under most conditions encountered

Q5.

EDTA is an example of which type of ligand?

• A Hexadentate (6 donor atoms)
• B Monodentate, donating through only a single atom
• C Bidentate, donating through exactly two atoms
• D Tridentate, donating through exactly three atoms

Q6.

A bidentate ligand donates how many electron pairs?

• A Two (from two donor atoms)
• B One, from a single donor atom on the ligand
• C Three, from three separate donor atoms on the ligand
• D Six, from six separate donor atoms on the ligand

Q7.

What is the oxidation state of cobalt in [Co(NH₃)5Cl]2+?

• A +3
• B +2
• C +1
• D 0

Q8.

What is the IUPAC name for [Fe(CN)₆]4-?

• A Hexacyanoferrate(II) ion
• B Hexacyanoferrate(III) ion
• C Iron hexacyanide
• D Ferrocyanide

Q9.

The shape of [Ni(CN)4]2- is:

• A Square planar
• B Tetrahedral
• C Octahedral
• D Linear

Q10.

The chelate effect refers to:

• A Extra stability of complexes formed with polydentate ligands due to increased entropy
• B The particular colour displayed by a coordination complex once it forms
• C A general magnetic property uniformly shown by every chelated complex
• D Simply the total count of separate ligand molecules attached to the central metal

Q11.

What does the term 'Werner complex' refer to?

• A Classic coordination compounds proposed by Alfred Werner (Nobel 1913)
• B A purely covalent organic compound unrelated to coordination chemistry
• C A simple ionic solid lacking any coordinate bonds
• D An organic polymer formed by repeating monomer units

Q12.

CN- is a strong field ligand because:

• A It is a good sigma and pi acceptor (back-bonding with metal d orbitals causes large Delta)
• B It generally carries a negative charge like many weak field ligands also do as frequently observed in practice
• C It has an unusually small ionic size compared to other ligands in many documented cases
• D It happens to contain a nitrogen atom in its structure according to conventional understanding

Q13.

Which of the following is NOT a ligand?

• A Na+
• B CN-
• C NH₃
• D Cl-

Q14.

The formula for potassium hexacyanoferrate(III) is:

• A K3[Fe(CN)₆]
• B K₄[Fe(CN)₆]
• C K[Fe(CN)₆]
• D K2[Fe(CN)₆]

Q15.

Ethylenediamine (en) is a bidentate ligand because it has:

• A Two NH₂ donor atoms
• B One NH₂ group
• C A carboxyl group
• D A carbon-carbon double bond

Q16.

The primary valence in Werner's theory corresponds to:

• A Oxidation state (ionisable valence)
• B The coordination number, which Werner instead termed secondary valence
• C A separate secondary valence distinct from the primary valence
• D Simply the total number of ligands bound to the metal centre

Q17.

Which complex is used as an anticancer drug?

• A Cisplatin [cis-Pt(NH₃)₂Cl₂]
• B [Co(NH₃)₆]Cl3 in routine practice
• C [Fe(CN)₆]4- overall in most cases
• D [Cu(NH₃)4]2+ under typical conditions

Q18.

Haemoglobin is a coordination complex in which the metal is:

• A Fe²⁺ (iron II)
• B Mg²⁺ according to standard textbooks
• C Cu²⁺ in general practice
• D Co3+ as frequently described

Q19.

An ambidentate ligand is one that can coordinate through:

• A Two different donor atoms (e.g., SCN- via S or N)
• B Two identical donor atoms positioned on opposite ends of the ligand
• C Multiple chelate rings formed simultaneously with one metal centre
• D Oxygen exclusively, regardless of any other potential donor atoms

Q20.

The coordination number of Pt in [PtCl₄]2- is:

• A 4
• B 2
• C 6
• D 8

Q21.

Ionisation isomers differ in:

• A Ions inside and outside the coordination sphere
• B Arrangement of ligands around the metal in typical laboratory settings
• C The metal's oxidation state under usual circumstances
• D The number of ligands according to most researchers

Q22.

[Co(NH₃)5Cl]SO4 and [Co(NH₃)5SO4]Cl are examples of:

• A Ionisation isomers
• B Geometrical isomers
• C Optical isomers
• D Linkage isomers

Q23.

Geometrical isomerism (cis-trans) is possible in square planar complexes with:

• A MA2B2 type (e.g., [Pt(NH₃)₂Cl₂])
• B MA4 type complexes, which show no geometric isomerism in square planar geometry
• C MA6 type complexes, which are not square planar at all
• D MA2 type complexes, which have too few ligands to show cis-trans isomerism

Q24.

In an octahedral complex MA3B3, the two geometrical isomers are:

• A Facial (fac) and meridional (mer)
• B Cis and trans
• C Clockwise and anticlockwise
• D Alpha and beta

Q25.

Optical isomers in coordination chemistry (enantiomers) are non-superimposable mirror images. A classic example is:

• A [Co(en)3]3+ (tris-chelate)
• B [Co(NH₃)₆]3+ in the majority of cases studied
• C [Ni(CN)4]2- as widely reported
• D [CrCl6]3- in standard practice

Q26.

What does VBT (Valence Bond Theory) explain about coordination compounds?

• A Hybridisation of metal orbitals and geometry; doesn't explain colour or magnetism well
• B Every property of the complex, including its colour and magnetism in full detail
• C Mainly the magnetic behaviour of the complex and nothing else as frequently observed in practice
• D Mainly the kinetics and reaction rates of the complex's ligand exchange in many documented cases

Q27.

In VBT, inner orbital complexes use d orbitals from:

• A The inner (n-1)d subshell (e.g., 3d with 4s, 4p for period 4 metals)
• B The outer nd subshell instead of the inner (n-1)d subshell under most conditions encountered
• C 4f orbitals borrowed from the lanthanide series as frequently observed in practice
• D 5d orbitals regardless of which period the metal belongs to in many documented cases

Q28.

In CFT, the crystal field stabilisation energy (CFSE) for d⁶ high spin in octahedral field is:

• A -0.4 Delta_o (4 in t_2g × -0.4 + 2 in eg × +0.6 = -1.6 + 1.2 = -0.4 Delta_o)
• B Zero, since the high-spin d⁶ arrangement gives no net stabilisation
• C -1.2 Delta_o, the value corresponding instead to a d³ configuration
• D -2.4 Delta_o, the value corresponding instead to a low-spin d⁶ configuration

Q29.

Linkage isomers differ in:

• A The donor atom through which an ambidentate ligand coordinates (e.g., NO₂- via N or O)
• B Their overall molecular formula, which differs between the two isomers according to conventional understanding
• C The oxidation state assigned to the central metal atom in routine practice overall
• D The total number of electrons present in the complex in most cases under typical conditions

Q30.

Prussian blue is:

• A Fe4[Fe(CN)₆]3 (mixed valence Fe²⁺/Fe³⁺ complex with CN- bridges)
• B A simple aqueous CuSO₄ solution with little iron present according to conventional understanding
• C Plain FeCl₃ dissolved in water with little cyanide bridges
• D K3[Fe(CN)₆], the separate potassium ferricyanide salt in routine practice

Q31.

The spectrochemical series for common ligands in order of increasing field strength:

• A I- < Br- < Cl- < F- < OH- < H₂O < NH₃ < en < CN- < CO
• B CO < CN- < NH₃ < F- < Cl-, listed in the reverse field-strength order
• C All ligands shown to have identical field strength toward any metal
• D The field strength ranking depending entirely on which metal is used

Q32.

How does the EDTA complex [EDTA-Ca]2- help in water softening?

• A It sequesters Ca²⁺ and Mg²⁺ by forming very stable (high K) chelates, preventing them from causing hardness
• B It precipitates the calcium and magnesium ions out of solution as insoluble solid salts according to standard textbooks
• C It promotes additional ionisation of the surrounding bulk water molecules in general practice as frequently described
• D It generally raises the overall pH of the water without complexing any of the ions in most textbook accounts

Q33.

Coordination compounds with the same formula and donor atoms but different arrangements are called:

• A Stereoisomers (including geometric and optical isomers)
• B Constitutional (structural) isomers, which differ in connectivity instead
• C Resonance structures of a single fixed arrangement of atoms
• D Tautomers related by a simple proton-shift equilibrium

Q34.

The effective atomic number (EAN) rule by Sidgwick states:

• A Metal in a complex acquires electrons from ligands to achieve the electron count of the next noble gas
• B The metal instead loses electrons to its surrounding ligands in the complex during normal conditions
• C Ligands serve mainly to fix the oxidation state with little electron-count rule involved as generally observed
• D The coordination number is usually numerically equal to the oxidation state in typical laboratory settings

Q35.

Which of the following is an outer orbital complex (uses nd orbitals, sp³d²)?

• A [CoF6]3- (F- is weak field, uses 4d orbitals, sp³d², high spin)
• B [Co(CN)6]3-, an inner orbital low-spin complex using 3d orbitals
• C [Co(NH₃)₆]3-, an inner orbital low-spin complex using 3d orbitals
• D [Fe(CO)₅], a zero-valent carbonyl complex with trigonal bipyramidal geometry

Q36.

The stability constant (formation constant) Kf of a complex indicates:

• A The extent to which the complex forms in solution (higher Kf = more stable)
• B The specific colour the complex displays once largely formed under usual circumstances
• C The melting point of the solid complex once isolated according to most researchers
• D The overall molecular weight of the formed complex in the majority of cases studied

Q37.

Which complex shows the trans effect most prominently?

• A Square planar Pt(II) complexes
• B Octahedral Co(III) as widely reported
• C Tetrahedral Ni(II) in standard practice
• D Linear Au(I) under most conditions encountered

Q38.

Coordination isomers differ by:

• A Exchange of ligands between two complex centres in a compound with both + and - complex ions
• B The oxidation state formally assigned to each central metal ion present in the salt
• C The total number of ligands individually attached to each of the two metal centres
• D Geometry alone, while keeping an identical ligand distribution shared between centres

Q39.

The role of EDTA in analytical chemistry (complexometric titration) is:

• A It forms 1:1 stable complexes with metal ions at controlled pH, allowing accurate quantification
• B It functions generally as a strong acid that lowers the solution's pH as frequently observed in practice
• C It precipitates the target metal ions as an insoluble solid in many documented cases according to conventional understanding
• D It acts mainly as a visual indicator that changes colour at the endpoint in routine practice

Q40.

Which coordination compound was historically important in proving Werner's theory?

• A [Co(NH₃)₆]Cl3 and its series of Co-NH₃-Cl complexes
• B NaCl, a simple ionic salt with no coordinate covalent bonding
• C [Pt(CN)4]2-, a square planar complex unrelated to Werner's original series
• D K₄[Fe(CN)₆], the separate potassium ferrocyanide salt

Q41.

Using CFSE, predict whether [Co(CN)6]4- (Co2+, d7) is high or low spin:

• A Low spin: CN- is strong field, large Delta causes maximum pairing (t_2g⁶ eg1, 1 unpaired)
• B High spin with three unpaired electrons distributed across t_2g and eg in the majority of cases studied
• C Largely diamagnetic with zero unpaired electrons in this d7 ion as widely reported
• D High spin with five unpaired electrons, the maximum possible for d7 in standard practice

Q42.

The stability of chelate complexes is explained by:

• A Chelate effect = combination of enhanced entropy (more particles released on complex formation) and entropic contribution to Gibbs energy
• B Each individual metal-donor bond within the chelate ring being inherently far stronger than usual under most conditions encountered
• C The chelate generally containing a much larger total number of covalent bonds overall as frequently observed in practice in many documented cases
• D The chelate possessing a far greater degree of ionic character than ordinary monodentate complexes according to conventional understanding

Q43.

The trans effect series in Pt(II) chemistry ranks ligands by their ability to labilise the trans position. The order includes:

• A CN-, CO, NO+ > H- > CH₃^- > PR₃ > SC(NH₂)₂ > NH₃ > NR₃ > Cl- > Br- > I- > H₂O
• B H₂O > Cl- > CN-, listed in the reverse order of true trans-labilising strength
• C All ligands shown to labilise the trans position with identical strength
• D Only the halogen ligands shown to exhibit any trans effect at all

Q44.

Which of the following octahedral complexes shows optical isomerism?

• A [Co(en)₂Cl₂]+ (cis form is chiral)
• B [Co(NH₃)₆]3+, a perfectly symmetric homoleptic complex with a plane of symmetry
• C [CoCl₆]3-, a perfectly symmetric homoleptic complex with a plane of symmetry
• D trans-[Co(en)₂Cl₂]+, which possesses an internal mirror plane and is achiral

Q45.

The pi-backbonding (back-donation) in metal carbonyl complexes: CO donates sigma electrons to the metal, and the metal:

• A Donates electron density from filled d orbitals back into the CO pi* orbital (backbonding), strengthening M-C and weakening C-O
• B Instead generally accepts additional pi electron density directly back from the filled CO pi orbital in routine practice overall
• C Transfers most of its electrons to the CO ligand mainly through an ionic electron-transfer mechanism in most cases under typical conditions
• D Forms highly no pi-type interaction with CO, relying mainly on simple sigma bonding according to standard textbooks in general practice

Q46.

Why is [PtCl₄]2- square planar while [NiCl₄]2- is tetrahedral?

• A Pt²⁺ (5d⁸) has larger CFSE favouring square planar; Ni²⁺ (3d⁸) has smaller Delta and the pairing energy penalty for square planar is not compensated
• B Platinum's much greater atomic mass relative to nickel is what forces the square planar shape on its own as frequently described in most textbook accounts
• C The chloride ligand bound to nickel is somehow physically larger than the same chloride ligand bound to platinum during normal conditions as generally observed
• D Nickel is supplied with extra coordinating chloride ligands in solution that platinum is largely denied access to in typical laboratory settings

Q47.

The spectrochemical series order is explained by sigma and pi donation/acceptance. Why does H₂O cause smaller splitting than NH₃?

• A H₂O is a pi-donor (lone pairs on O donate into empty metal d orbitals), decreasing effective Delta; NH₃ has no pi-donor capacity
• B Water is actually claimed to be a far stronger sigma donor than ammonia toward the metal centre under usual circumstances according to most researchers
• C Ammonia is instead claimed to accept pi electron density back from the filled metal d orbitals in the majority of cases studied
• D Water's oxygen donor atom is generally claimed to be much larger than ammonia's nitrogen donor atom as widely reported in standard practice

Q48.

Prussian blue reaction: K₄[Fe(CN)₆] (potassium ferrocyanide) added to FeCl₃ gives:

• A Prussian blue (KFe[Fe(CN)₆]) intensely blue precipitate
• B A pale yellow precipitate forming instead of any blue colour
• C A deep red colouration with no precipitate forming at all
• D No visible reaction occurring between the two reagents

Q49.

The Irving-Williams series describes:

• A The stability order of divalent transition metal complexes: Mn²⁺ < Fe²⁺ < Co2+ < Ni²⁺ < Cu²⁺ > Zn²⁺
• B The spectrochemical series ranking ligands by field strength instead under most conditions encountered
• C The trans effect series ranking ligands by their labilising power in Pt(II) complexes as frequently observed in practice
• D The general stability ranking of different oxidation states for one metal in many documented cases

Q50.

Which of the following has the most CFSE stabilisation in an octahedral field?

• A d³ (t_2g³, CFSE = -1.2 Delta_o) and d⁶ low spin (t_2g⁶, CFSE = -2.4 Delta_o)
• B d⁰, which by definition has zero d electrons and therefore zero CFSE
• C d¹⁰, which has all orbitals fully occupied giving zero net CFSE
• D d⁵ high spin, which has one electron in every orbital giving zero net CFSE

Q51.

The reaction [PtCl₄]2- + 2NH₃ → cis-[Pt(NH₃)₂Cl₂] uses the trans effect of Cl- to control the product. The mechanism:

• A First NH₃ goes to any position; second NH₃ goes cis to the first Cl- (Cl- labilises trans site, so second substitution is trans to Cl-)
• B Both ammonia ligands are said to substitute in a largely random, loosely controlled order overall in most cases under typical conditions
• C The second NH₃ ligand is said to substitute specifically trans to the first NH₃ ligand instead according to standard textbooks in general practice
• D Chloride is said to act generally as a passive spectator ion with little influence on the order as frequently described in most textbook accounts

Q52.

Which of the following statements about EDTA (ethylenediaminetetraacetate) is correct?

• A It is hexadentate (2 N + 4 O donor atoms) and forms thermodynamically very stable 1:1 chelates with most metal ions
• B It is generally bidentate, donating through mainly two of its six potential donor atoms according to conventional understanding
• C It is said to bind mainly to transition metal ions and not to any s-block cations in routine practice overall
• D It is said to become notably unstable once the surrounding solution pH rises above 7 in most cases under typical conditions

Q53.

In coordination chemistry, the 'symbiosis principle' states:

• A Hard (sigma-donor) ligands stabilise hard metal centres; soft (pi-acceptor) ligands stabilise soft centres (HSAB principle applied to complexes)
• B Every ligand type is said to behave in an largely equivalent way regardless of hard or soft donor character according to standard textbooks
• C Mainly strong field ligands such as CN- are said to ever be capable of forming any reasonably stable complex in general practice as frequently described
• D The coordination number on its own is said to be the single factor that largely determines complex stability in most textbook accounts during normal conditions

Q54.

The fluxional behaviour of [Fe(CO)₅] (trigonal bipyramidal) refers to:

• A Rapid exchange of axial and equatorial CO positions at room temperature via Berry pseudorotation
• B A largely fixed, rigid structure with little exchange between CO positions as generally observed
• C Permanent loss of one CO ligand followed by its slow recoordination in typical laboratory settings
• D Spontaneous ring formation among the five coordinated CO ligands under usual circumstances

Q55.

For a d⁸ metal ion in square planar field, the dx2-y2 orbital is highest in energy because:

• A It points directly at the four ligands (sigma interaction); the other d orbitals are less directly aligned with ligands
• B It generally happens to contain more electrons than the other four d orbitals combined according to most researchers
• C The d⁸ electron configuration is said to fill this particular orbital last by convention in the majority of cases studied
• D The four surrounding ligands are positioned specifically so as to avoid this orbital largely as widely reported in standard practice

Q56.

Cisplatin's anticancer activity is due to:

• A Cross-linking DNA by displacing Cl- with purine N-donors (N7 of guanine), distorting DNA helix
• B Generally releasing free toxic platinum ions that poison the entire cell under most conditions encountered
• C Directly inhibiting a specific metabolic enzyme without touching DNA as frequently observed in practice
• D Binding to and disrupting the lipid bilayer of the cell membrane in many documented cases

Q57.

The total number of isomers (including geometric and optical) for [Co(en)(NH₃)₂Cl₂]+ is:

• A Five (cis and trans geometric isomers, with the cis form giving three: two cis diastereomers of en orientation and optical isomers)
• B Just two isomers in total, counting mainly a single cis form and a single trans form according to conventional understanding in routine practice
• C Exactly three isomers in total, largely ignoring any optical activity within the cis form overall in most cases under typical conditions
• D Mainly one single possible isomer, since the en ligand is said to fix a unique overall geometry according to standard textbooks

Q58.

In MO theory treatment of octahedral complexes, which MO is responsible for the colour (d-d transition)?

• A t_2g to eg* transition (technically t_2g to eg MO; the antibonding eg* in MO theory corresponds to eg in CFT)
• B A simple sigma bonding to sigma antibonding transition occurring within the ligand framework in general practice
• C A pi bonding to pi antibonding transition localised largely on the surrounding ligand atoms as frequently described
• D A deep core 1s to 2s electronic transition occurring within the central metal atom itself in most textbook accounts

Q59.

The spectrochemical series and the nephelauxetic series differ in that:

• A Spectrochemical series ranks field strength (Delta); nephelauxetic series ranks the ability to expand d orbital size (reduce electron repulsion, measured by beta)
• B The two series are claimed to be in fact largely identical in highly everything that each one measures during normal conditions as generally observed in typical laboratory settings
• C The spectrochemical series is claimed to exist mainly to classify complex colour and to serve no other purpose under usual circumstances according to most researchers
• D The nephelauxetic series is claimed to instead classify coordination complexes mainly by their geometric isomer type in the majority of cases studied as widely reported

Q60.

In the complex [Co(NH₃)₆]³⁺, cobalt(III) with the strong-field NH₃ ligand uses the hybridisation:

• A sp³ tetrahedral
• B dsp² square planar
• C sp³d² outer orbital
• D d²sp³ inner orbital