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Metal Complexes. metal cation is attached to a group of surrounding molecules or ions ( ligands ) by coordinate covalent bonds coordinate => ligand donates both electrons each ligand makes at least one coordinate covalent bond to the metal cation

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metal complexes
Metal Complexes
  • metal cation is attached to a group of surrounding molecules or ions (ligands) by coordinate covalent bonds
    • coordinate => ligand donates both electrons
  • each ligand makes at least one coordinate covalent bond to the metal cation
    • any atom (from a ligand) that is directly bound to the metal cation is called a donor atom
  • coordination number (C.N.) = the total number of donor atoms surrounding a metal cation (= total number of coordinate covalent bonds)

TM I-Intro to Complexes

lewis definition of acids and bases
Lewis definition of acids and bases
  • Base: e- - pair donor
  • Acid: e- - pair acceptor
  • In metal complexes, the ligand is always a Lewis base (makes a coordinate covalent bond) and the metal cation is a Lewis acid (accepts the lone pair from ligand).
  • Note: definition applies to things other than metal complexes (e.g., NH3-BF3)

TM I-Intro to Complexes

examples of metal complexes
Examples of Metal Complexes

Ni(H2O)62+

Co(NH3)4Cl2+

Four NH3 ligands and two Cl- ligands bound to a Co3+ cation

Six H2O ligands bound to a Ni2+ cation

http://www.3dchem.com/3dinorgmolecule.asp?ID=490

http://www.3dchem.com/3dinorgmolecule.asp?ID=196

TM I-Intro to Complexes

more examples
More Examples

Ni(H2O)4SO4

Ni(CN)42-

Four H2O ligands and one SO42- ligand bound to a Ni2+ cation

Four CN- ligands bound to a Ni2+ cation

http://www.3dchem.com/3dinorgmolecule.asp?ID=440

http://www.3dchem.com/3dinorgmolecule.asp?ID=500

TM I-Intro to Complexes

charge on complex vs charge on cation
Charge on complex vs. charge on cation
  • charge on complex = sum of the charges of the metal cation (oxidation #) plus charges on all ligands bound
    • don’t mix up charge on cation with charge on the (whole) complex! **charge on complex appears as right superscript**
  • Examples
    • Co(NH3)4Cl2+
      • Charge on Co is +3
      • Charge of each NH3 is 0
      • Charge of each Cl- is -1
  • Charge of complex is
  • +3 + 4(0) + 2(-1) = +1
  • Ni(CN)42-
    • Charge on Ni is +2
    • Charge of each CN- is -1
  • Charge of complex is
  • +2 + 4(-1) = -2

TM I-Intro to Complexes

coordination compounds are neutral
Coordination Compounds are Neutral
  • if charge on complex is
    • neutral, the complex itself is a coordination compound
      • e.g., Pt(NH3)2Cl2
        • Charge on Pt = +2
        • Charge on NH3 = 0
        • Charge on Cl- = -1
    • not neutral, complex plus counterions can form a neutral salt, also called a coordination compound
      • e.g.,[Ni(NH3)6]Cl2 ; complex is a cation; Cl-’s are counterions
        • In water, it dissociates into Ni(NH3)62+ and two Cl- ions:
        • [Ni(NH3)6]Cl2 Ni(NH3)62+ + 2 Cl-
      • e.g., (NH4)4[Fe(CN)6] ; complex an anion; NH4+’s, counterions
        • (NH4)4[Fe(CN)6]  4 NH4+ + Fe(CN)64-
  • Charge of complex is
  • +2 + 2(0) + 2(-1) = 0

TM I-Intro to Complexes

example
Example
  • e.g., [Co(NH3)4Cl2]Cl
    • inside brackets = metal complex
    • outside= counter ion(s)
    • # ligands = 6 (four neutral NH3’s; two Cl-’s)
    • # counterions = 1 (Cl-)
      • N.B., anions can sometimes be ligands and sometimes be counterions!
      • neutral ligands can never be counterions!
    • # donor atoms = 6 (four N’s from NH3’s; two Cl’s from Cl-’s)  CN = 6

TM I-Intro to Complexes

common c n s and their geometries
Common C.N.’s and their Geometries

C.N.: 2 4 4 6

TM I-Intro to Complexes

example of a tetrahedral metal complex
Example of a tetrahedral metal complex

Ni(CO)42+

Four CO ligands bound to a Ni2+ cation

TM I-Intro to Complexes

http://www.3dchem.com/3dinorgmolecule.asp?ID=489

ligand types see table 24 3 in tro next slide
Ligand Types (See Table 24.3 in Tro [next slide])
  • Ligands that bind to a metal cation via:
    • one donor atom are called monodentate ligands
      • Cl-, NH3, CN-, H2O (donor atoms are, respectively, Cl, N, C (or N!), O
    • more than one donor atom are called polydentate ligands
      • two atoms per ligand – bidentate (en, ox2-, bpy)
      • three atoms, tridentate
      • etc

TM I-Intro to Complexes

slide11

NOTE: All donor atoms must have at least one ____________, which it uses to make a bond to the metal cation.

Monodentate

(donor atom circled)

lone pair

Monodentate

(but two possible donor atoms [only one can bond at a time])

Bidentate

(two donor atoms; both bond to the same metal cation at the same time)

Learn the abbreviations for these two (ox and en)

slide12

NOTE: Some ligands are…

neutral

And some ligands are…

negatively charged

example ethylenediamine en
Example – ethylenediamine (en)
  • en = NH2CH2CH2NH2
    • each N in one en ligand can

bind to a metal cation!

  • Co(en)2Cl2+
    • # ligands = 4 (two en’s, two Cl-’s)
    • C.N. = 6 (not 4!) because each en ligand makes two coordinate covalent bonds to the Co3+ using two different N atoms per ligand

TM I-Intro to Complexes

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