Coordination Compounds — Questions & Answers

Sample questions from Coordination Compounds

1. Q1. The complex [Ti(H2O)6]3+ is coloured. What is the physical origin of this colour?

   • A.Absorption of visible light causing d-d electronic transition from t2g to eg✓
   • B.Emission of light from the metal nucleus
   • C.Reflection of all visible wavelengths
   • D.Ligand-only pi to pi* transitions
   Solution

   Ti3+ is d1; the single electron in t2g absorbs light of a specific wavelength (~500 nm, green-yellow region) to be promoted to eg, and the complementary colour (violet/purple) is observed.

2. Q2. Using CFSE (Crystal Field Stabilization Energy, ignoring pairing energy) alone, which of the following octahedral configurations gains the LARGEST magnitude of stabilization: high-spin d4, low-spin d4, high-spin d7, or low-spin d7?

   • A.High-spin d7 (CFSE = -1.8 Delta-o + 1.2 Delta-o, giving -0.6 Delta-o)
   • B.High-spin d4 (CFSE = -0.6 Delta-o)
   • C.Low-spin d4 (CFSE = -1.6 Delta-o)
   • D.Low-spin d7 (CFSE = -2.4 Delta-o + 0.6 Delta-o = -1.8 Delta-o)✓
   Solution

   Calculating each: high-spin d4 (t2g^3 eg^1): CFSE = 3(-0.4)+1(0.6) = -1.2+0.6 = -0.6 Delta-o -- low-spin d4 (t2g^4): CFSE = 4(-0.4) = -1.6 Delta-o -- high-spin d7 (t2g^5 eg^2): CFSE = 5(-0.4)+2(0.6) = -2.0+1.2 = -0.8 Delta-o -- low-spin d7 (t2g^6 eg^1): CFSE = 6(-0.4)+1(0.6) = -2.4+0.6 = -1.8 Delta-o -- comparing magnitudes: -0.6, -1.6, -0.8, -1.8 -- the LARGEST magnitude (most negative, most stabilizing) is LOW-SPIN d7, at -1.8 Delta-o.

3. Q3. Which of the following complexes is used in the metallurgical extraction of silver from its ore via the cyanide process?

   • A.[Ag(CN)2]-✓
   • B.[Ag(NH3)2]+
   • C.[AgCl2]-
   • D.[Ag(H2O)2]+
   Solution

   Silver ore is treated with dilute NaCN solution in the presence of air; silver dissolves as the soluble dicyanidoargentate(I) complex [Ag(CN)2]-, from which metallic silver is later displaced by zinc.

4. Q4. The complex Na3[Co(NO2)6] contains the nitrite ion bound through nitrogen. Which IUPAC name with the correct kappa descriptor is appropriate?

   • A.Sodium hexanitrito-kappa-O-cobaltate(III)
   • B.Sodium hexanitrito-kappa-N-cobaltate(III)✓
   • C.Sodium hexanitritocobalt(III)
   • D.Sodium hexanitrito-kappa-N-cobaltate(II)
   Solution

   NO2- is ambidentate; binding through the nitrogen atom is denoted nitrito-kappa-N (older name nitro), while binding through oxygen is nitrito-kappa-O (older nitrito). The complex is anionic so cobalt takes the -ate form cobaltate, and the charge balance 3(+1) + x + 6(-1) = 0 gives x = +3, hence cobaltate(III).

5. Q5. What is the coordination number and oxidation state of platinum in K2[PtCl6]?

   • A.Coordination number 6, oxidation state +4✓
   • B.Coordination number 4, oxidation state +2
   • C.Coordination number 6, oxidation state +2
   • D.Coordination number 4, oxidation state +4
   Solution

   In K2[PtCl6], 2 K+ balance the charge of [PtCl6]2-; with 6 Cl- (each -1) totaling -6, Pt must be +4 for overall charge -2; coordination number is 6 (six chlorido ligands).

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