Coordination Compound flash cards

1. 1.What is a coordination compound?

   A compound in which a central metal atom or ion is bonded to a fixed number of ions or molecules (ligands) by coordinate bonds, and which retains its identity even when dissolved. Example: $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.

   Hint: Central metal + ligands held by coordinate bonds.

2. 2.Distinguish a double salt from a coordination (complex) compound.

   A double salt (e.g. carnallite, Mohr's salt) dissociates completely into its constituent ions in water and loses its identity. A complex compound (e.g. $[\text{Fe}(\text{CN})_6]^{4-}$) retains its identity and the complex ion does not give simple metal ion tests.

   Hint: Both are addition compounds; one keeps identity in solution.

3. 3.State the postulates of Werner's theory of coordination compounds.

   (1) Metals show two valencies — primary (ionisable, satisfied by anions) and secondary (non-ionisable, satisfied by ligands). (2) Primary valency = oxidation number; secondary valency = coordination number, which is fixed. (3) Secondary valencies are directed in space giving definite geometry.

   Hint: Two valencies: primary (ionisable) and secondary (fixed, directional).

4. 4.In Werner's terms, what do primary and secondary valencies correspond to today?

   Primary valency = oxidation state of the metal (ionisable, satisfied by negative ions). Secondary valency = coordination number (non-ionisable, satisfied by ligands and directed in space).

   Hint: Oxidation number vs coordination number.

5. 5.For $\text{CoCl}_3\cdot 6\text{NH}_3$, how many chloride ions are precipitated by $\text{AgNO}_3$ and why?

   All 3 chlorides are precipitated. The complex is $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$: the three $\text{Cl}^-$ are ionisable (primary valency), while all 6 $\text{NH}_3$ occupy the secondary valencies.

   Hint: All Cl are outside the coordination sphere.

6. 6.For $\text{CoCl}_3\cdot 4\text{NH}_3$, how many ions form in solution and how many $\text{Cl}^-$ are precipitated?

   It is $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]\text{Cl}$, giving 2 ions in solution; only 1 $\text{Cl}^-$ (ionisable) is precipitated by $\text{AgNO}_3$. Two chlorides are coordinated inside the sphere.

   Hint: Two Cl are ligands, one is ionisable.

7. 7.Define coordination sphere and how it is written.

   The central metal atom/ion together with the ligands directly attached to it, enclosed in square brackets. Its net charge is balanced by the counter ions written outside the brackets. Example: $[\text{Co}(\text{NH}_3)_6]^{3+}$ is the coordination sphere in $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.

   Hint: Everything inside the square brackets.

8. 8.What is a ligand?

   An ion or molecule bound to the central metal atom/ion in a coordination compound through a coordinate (dative) bond. Ligands are Lewis bases — they donate at least one lone pair of electrons to the metal (Lewis acid).

   Hint: Electron-pair donor attached to metal.

9. 9.What is the donor atom of a ligand?

   The atom of the ligand that is directly bonded to the central metal and donates the electron pair. E.g., in $\text{NH}_3$ the donor atom is N; in $\text{H}_2\text{O}$ it is O; in $\text{CN}^-$ it is C.

   Hint: The atom carrying the lone pair that bonds.

10. 10.Define denticity of a ligand.

    The number of donor atoms of a single ligand that are directly bonded to the central metal atom. Monodentate = 1, bidentate = 2, polydentate = many.

    Hint: How many 'teeth' the ligand bites with.

11. 11.Give examples of monodentate ligands.

    Ligands with one donor atom: $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{Cl}^-$, $\text{CN}^-$, $\text{CO}$, $\text{OH}^-$, $\text{NO}_2^-$.

    Hint: One donor atom only.

12. 12.Give examples of bidentate ligands.

    Ligands with two donor atoms: ethane-1,2-diamine (ethylenediamine, en, donors = 2 N) and oxalate ion $\text{C}_2\text{O}_4^{2-}$ (ox, donors = 2 O).

    Hint: 'en' and oxalate.

13. 13.What is a hexadentate ligand? Give the standard example.

    A ligand with six donor atoms. The classic example is EDTA (ethylenediaminetetraacetate ion), which binds through 2 N and 4 O donor atoms, wrapping fully around the metal.

    Hint: EDTA: 2 N + 4 O = 6.

14. 14.Define a chelate ligand and the chelate effect.

    A di- or polydentate ligand that binds a metal through two or more donor atoms forming a ring is a chelate ligand. The extra stability such ring formation gives to the complex is the chelate effect; five- and six-membered rings are most stable.

    Hint: Ring-forming ligand gives extra stability.

15. 15.What is an ambidentate ligand? Give examples.

    A monodentate ligand that can coordinate through either of two different donor atoms. Examples: $\text{NO}_2^-$ (through N = nitrito-N/nitro, or through O = nitrito-O) and $\text{SCN}^-$ (through S = thiocyanato, or through N = isothiocyanato).

    Hint: Two possible donor atoms, uses one at a time.

16. 16.Define coordination number (CN) of a central metal ion.

    The number of ligand donor atoms directly bonded to the central metal ion (the number of sigma bonds between ligands and metal). For $[\text{PtCl}_6]^{2-}$, CN = 6; for $[\text{Ni}(\text{en})_3]^{2+}$, CN = 6 (3 bidentate ligands).

    Hint: Count donor atoms, not ligand molecules.

17. 17.What is the coordination number of the metal in $[\text{Co}(\text{en})_3]^{3+}$?

    6. 'en' is bidentate, so 3 ligands × 2 donor atoms = 6 donor atoms bonded to Co.

    Hint: 3 bidentate ligands.

18. 18.Define oxidation number of the central metal in a complex.

    The charge the central atom would carry if all ligands were removed along with the electron pairs shared with the metal. Found from the overall charge and the charges of the ligands. It is written in Roman numerals in the name.

    Hint: Hypothetical charge after removing ligands with their electrons.

19. 19.What geometry is associated with coordination number 6, and with 4?

    CN 6 → almost always octahedral. CN 4 → either tetrahedral or square planar. CN 2 → linear.

    Hint: 6 = octahedral; 4 = two possibilities.

20. 20.What is the charge on a coordination sphere and how is it found?

    It is the algebraic sum of the oxidation number of the central metal and the charges of all the ligands. Example: in $[\text{Fe}(\text{CN})_6]^{4-}$, $+2 + 6(-1) = -4$.

    Hint: Metal charge + sum of ligand charges.

21. 21.What are homoleptic and heteroleptic complexes?

    Homoleptic: metal bound to only one kind of ligand, e.g. $[\text{Co}(\text{NH}_3)_6]^{3+}$. Heteroleptic: metal bound to more than one kind of ligand, e.g. $[\text{Co}(\text{NH}_3)_4\text{Cl}_2]^{+}$.

    Hint: One ligand type vs many.

22. 22.IUPAC rule: in what order are ligands named in a complex?

    Ligands are named in alphabetical order before the metal, regardless of their charge. The alphabetising uses the ligand name (ignoring multiplying prefixes like di, tri).

    Hint: Alphabetical, prefixes ignored for ordering.

23. 23.IUPAC rule: how are anionic ligand names formed?

    Anionic ligands ending in '-ide' become '-o' (chloride → chlorido, cyanide → cyanido); '-ite' → '-ito' (nitrite → nitrito); '-ate' → '-ato' (sulfate → sulfato, oxalate → oxalato).

    Hint: -ide→-o, -ite→-ito, -ate→-ato.

24. 24.IUPAC rule: what special names are used for the neutral ligands $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{CO}$, $\text{NO}$?

    $\text{H}_2\text{O}$ = aqua, $\text{NH}_3$ = ammine (note double m), $\text{CO}$ = carbonyl, $\text{NO}$ = nitrosyl.

    Hint: aqua, ammine, carbonyl, nitrosyl.

25. 25.IUPAC rule: when are multiplying prefixes di, tri vs bis, tris used?

    Use di, tri, tetra for simple ligands (dichlorido). Use bis, tris, tetrakis for ligands whose names already contain a prefix or are complex (e.g. bis(ethane-1,2-diamine)), with the ligand name in parentheses.

    Hint: bis/tris for complicated ligand names.

26. 26.IUPAC rule: how is the metal named when the complex ion is a cation vs an anion?

    If the complex ion is a cation or neutral, the metal keeps its ordinary name (e.g. cobalt). If the complex ion is an anion, the metal name ends in -ate (e.g. cobaltate, ferrate). Oxidation state follows in Roman numerals in parentheses.

    Hint: Anionic complex → metal + 'ate'.

27. 27.IUPAC rule: which Latin metal names are used with the '-ate' suffix for anionic complexes?

    Iron → ferrate, copper → cuprate, lead → plumbate, silver → argentate, tin → stannate, gold → aurate.

    Hint: ferrate, cuprate, argentate, aurate.

28. 28.How is the formula of a coordination compound written (order inside brackets)?

    Central metal symbol first, then anionic ligands, then neutral ligands; within each type ligands are listed alphabetically by donor-atom symbol/name. The whole coordination entity is enclosed in square brackets; overall no spaces.

    Hint: Metal, then anionic ligands, then neutral.

29. 29.Write the IUPAC name of $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$.

    Hexaamminecobalt(III) chloride.

    Hint: 6 ammine, Co is +3, cationic complex.

30. 30.Write the IUPAC name of $\text{K}_4[\text{Fe}(\text{CN})_6]$.

    Potassium hexacyanidoferrate(II).

    Hint: Anionic complex → ferrate; Fe is +2.

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