More Coordinate Covalent Compounds

web date: September 25, 2002

editorial note: I hope to add mlecular structures (drawings and diagrams) to these pages; ceck again in a few days.

remember:

• these compounds involve covalent bonds with a central metal ion
• the species that bonds is called a ligand
• the ligand supplies both electrons for the bond
  • hence, called coordinate covalent bond
  • generally involves an oxygen, nitrogen or halide ion
• some ligands bind quite tightly
  • other are weaker bonds, easily undone
• some ligands have more than one binding site
  • bidentate ligands "bite" twice
  • some can bind at three or four different sites
  • as a rule, the more binding sites the stronger the overall binding
  • EDTA is an example of a ligand that binds six times

• We began by deliberately synthesizing a compound where oxalate ions bind to a center iron ion.
  • K₃Fe (C₂O₄)₃ 3 H₂O
  • This solid is a brilliant green species.
    • Yet we know that the oxalate ion is colorless
    • the Fe(III) ion is at best weakly colored
    • and of course the K+ ions and the water are colorless
  • The color comes from the covalent bonds between oxalate and Fe
    • both electrons come from the oxygen on oxalate
      • this is what characterizes a coordinate covalent bond
  • The oxalate is bound fairly tightly and the compound seems fairly stable

• This past week some of you titrated Ca⁺² with EDTA
  • the EDTA binds to the Ca⁺² ion
    • it actually forms six separate bonds
    • this is a very strong interaction-- EDTA holds Ca tightly
      • each bond gets both electrons from the EDTA
  • in this case both the EDTA and the EDTA-Ca complex are colorless
• We used another powerful ligand to see the endpoint
  • called Eriochrome black T
    • selected for three features
      • 1. it is strongly colored (blue)
      • 2. it binds to Ca and Mg to form a compound of another color (red)
      • 3. it binds less powerfully than EDTA so after the endpoint, EDTA steals the ligand and we see the color changes

• The next experiment will take advantage of the color of a complex
  • We will allow Fe²⁺ ion to react with a ligand called 1, 10 phenanthroline
  • 
    • this is another bidentate ligand
    • two nitrogen atoms that can bond to the same Fe ion
  • Fe²⁺ is normally present in solution as Fe(H₂O)₆2+
    • a nearly colorless ion
    • faintly blue-green in most solids
    • blue green in very concentrated solutions
  • 1,10 phenanthroline
    • is slightly soluble in water
    • is colorless
    • is a strong bidentate ligand
  • The two react to form an intensely colored compound
    • a 3:1 complex
    • bright red/orange
    • strongly absorbs green light (510 nm)
    • we use a spectrophotometer to measure Absorbance and evaluate Fe concentration

• A very important example is hemoglobin and the transport of oxygen.
  • take a look at your textbook (McMurray and Fay, Chemistry), page 877
    • another Web site with Hemoglobin information
    • still another hemoglobin site to browse
  • heme is a large planar molecule (flat)
    • it has four nitrogen atoms arranged around a central "hole"
    • the hole can accommodate an Iron ion
    • the four electron pairs can bind the iron tightly
  • the globin is a protein molecule, also bound by a N to the iron
    • the globin wraps around the heme structure
  • that still leave one binding site (iron can take on six electron pairs)
    • the candidate is oxygen, O₂
      • hemoglobin accepts oxygen
      • this is a moderately weak binding
      • the oxygen is easily released in oxygen poor regions (like muscle tissue)
      • so blood can transport oxygen from the lungs to where oxygen is needed

• This is an explanation for the severe toxicity of Carbon Monoxide
• CO binds to hemoglobin at the same site used by oxygen
  • it binds better than oxygen and is not released, thus making the hemoglobin unavailable for oxygen transport
• this also explains some colors
  • the color of the oxygen-less form of hemoglobin is blue
  • fully oxygenated blood is red
    • hospitals often monitor a patient's oxygen level with a little device that fits on the end of the finger. It has a red light that is absorbed by oxygen depleted blood.
  • when CO binds, the color is an even brighter red
    • one of the telltale signs of CO poisoning is a very reddish skin color -- yes, you can look too healthy.

• Another very important complex ion is chlorophyll
  • try this Web site for more chlorophyll information
  • the metal ion is Mg²⁺
    • it is surrounded by a large molecular structure called a porphorin
    • like hemoglobin this molecule provides core of 4 nitrogen atoms
    • the Mg²⁺ ion sits in a "hole" surrounded by the four nitrogen atoms
• Many vitamins need a "cofactor" to be effective
  • Vitamin B12 needs to form a complex with Co⁺² ions

Before the semester is over, you'll probably find that we use another 6-10 compound ions of this sort.