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Chelate effect Typical Ligands

Inorganic chemistry

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Page 1: Inorganic chemistry

Chelate effect

Typical Ligands

Page 2: Inorganic chemistry

Chelate Complex

• EDTA occupies 6 coordination sites, for example [CoEDTA]- is an octahedral Co3+ complex.

• Both N atoms (blue) and O atoms (red) coordinate to the metal.

Page 3: Inorganic chemistry

Equilibrium log β ΔG ΔH /kJ mol−1 −TΔS /kJ mol−1

Cd2+ + 4 MeNH2 = Cd(MeNH2)42+ 6.55 -37.4 - 57.3 19.9

Cd2+ + 2 en = Cd(en)22+ 10.62 -60.67 - 56.48 - 4.19

The enthalpy term should be approximately the same for the two reactions. The difference between the two stability constants is due to the entropy term. In equation (1) there are two particles on the left and one on the right, whereas in

equation (2) there are three particles on the left and one on the right. This means that less entropy of disorder is lost when the chelate complex is

formed than when the complex with monodentate ligands is formed.

These data show that the standard enthalpy changes are indeed approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term.It is clear that the chelate effect is predominantly an effect of entropy.

Cu2+ + en [Cu(en)]2+ (1)

Cu2+ + 2 MeNH2 [Cu(MeNH2)2]2+ (2)

ΔG = −RT ln K = ΔH – TΔS

Page 4: Inorganic chemistry

CoCl3 . 6 NH3 ORANGE-YELLOWCoCl3.5NH3.H2O REDCoCl3.5NH3 PURPLECoCl3.4NH3 GREEN

Werner’s coordination theory

Measurements of the conductivity of aqueous solutions of the above complexes suggest:

CoCl3.6NH3 and CoCl3.5NH3.H2O complexes dissociate in water to give a total of four ions.

CoCl3.5NH3 dissociates to give three ions

CoCl3.4NH3 dissociates to give only two ions.

Werner explained these observations by suggesting that transition-metal ions such as the Co3+ ion have a primary valence and a secondary valence.

Page 5: Inorganic chemistry

The primary valence is the number of negative ions needed to satisfy the charge on the metal ion. In each of the cobalt(III) complexes previously described, three Cl- ions are needed to satisfy the primary valence of the Co3+ ion.

The secondary valence is the number of ions of molecules that are coordinated to the metal ion. Werner assumed that the secondary valence of the transition metal in these cobalt(III) complexes is six.

The formulas of these compounds can therefore be written as follows.

[Co(NH3)63+][Cl-]3 orange-yellow

[Co(NH3)5(H2O)3+][Cl-]3 red

[Co(NH3)5Cl2+][Cl-]2 purple

[Co(NH3)4Cl2+][Cl-] green

The cobalt ion is coordinated to a total of six ligands in each complex, which satisfies the secondary valence of this ion.

Each complex also has a total of three chloride ions that satisfy the primary valence.

Some of the Cl- ions are free to dissociate when the complex dissolves in water. Others are bound to the Co3+ ion and does not dissociate.

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Carbonyl complexes

The carbonyl ligand forms a huge number of complexes with metal ions, most commonly in low oxidation states, where it binds to the metal through its C-donor, as in the complexes below, where all the metal ions are zero-valent:

[Ni(CO)4] [Fe(CO)5] [Cr(CO)6] Td TBP (D3h) Oh

Page 16: Inorganic chemistry

18 Electron "Rule"Organic compounds, of course, follow the 8 electron rule: there can only be a maximum of 8 valence electrons around a carbon center. The vast majority of stable diamagnetic organometallic compounds have 16 or 18 valence electrons due to the presence of the five d orbitals which can hold 10 more electrons relative to C, O, N, etc. Electron counting is the process of determining the number of valence electrons about a metal center in a given transition metal complex. To determine the electron count for a metal complex:1) Determine the oxidation state of the transition metal center(s) and the metal centers resulting d-electron count. To do this one must:

a) note any overall charge on the metal complexb) know the charges of the ligands bound to the metal

center (ionic ligand method)c) know the number of electrons being donated to the metal

center from each ligand (ionic ligand method)2) Add up the electron counts for the metal center and ligandsComplexes with 18 e- counts are referred to as saturated, because there are no empty low-lying orbitals to which another incoming ligand can coordinate. Complexes with counts lower than 18e- are called unsaturated and can electronically bind additional ligands.

Page 17: Inorganic chemistry

One might wonder why in the above complexes Ni(0) has four C≡O groups attached to it, Fe(0) five C≡O, and Cr(0) six C≡O. A very simple rule allows us to predict the numbers of donor groups attached to metal ions in organometallic complexes, called the eighteen electron rule. The latter rule states that the sum of the d-electrons possessed by the metal plus those donated by the ligands (2 per C≡O) must total eighteen:

[Ni(CO)4] [Fe(CO)5] [Cr(CO)6]

Ni(0) = d10 Fe(0) = d8 Cr(0) = d6

4 x CO = 8 5 x CO 10 6 x CO = 12

18 e 18e 18e

Carbonyl complexes and the 18-electron rule

Formal oxidation states are all zero.

Page 18: Inorganic chemistry

To obey the 18-electron rule, many carbonyl complexes are anions or cations, as in:

[V(CO)6]- [Mn(CO)6]

+ [Fe(CO)4]2-

V(0) = d5 Mn(0) = d7 Fe(0) = d8

6 CO = 12e 6 CO = 12e 4 CO = 8e 1- = +1e 1+ = -1e 2- = 2e

= 18e = 18 e = 18e

Carbonyl complexes and the 18-electron rule

Formal oxidation Formal oxidation Formal oxidationstate = V(-I) state = Mn(I) state = Fe(-II)

[NOTE: In applying the 18-electron rule, metal ions are always considered to be zero-valent, not the formal oxidn. state]

Page 19: Inorganic chemistry

Metal Carbonyl compounds

Metal carbonyls form one of the oldest (and important) classes of organometallic complexes. Most metal carbonyls are toxic!

Page 20: Inorganic chemistry

Metal ions in biological system

Page 21: Inorganic chemistry

Mg

• Photosynthesis,

• ATP hydrolysis, • Phosphate group transfer reactions (i.e kinase reactions),

• Structure formation, stabilizing DNA and RNA, construction of cell membranes,• DNA polymerase enzyme catalyzing the transcription of DNA.

Page 22: Inorganic chemistry

• Enzymes like cytochrome c oxidase (also fe), amine oxidase, ascorbic acid oxidase, tyrosinase • Electron transport proteins like plastocyanin, azurin, stellacyanin • Oxygen transport protein hemocyanin (in lower forms of life)• Storage protein ceruloplasmin.

Cu

Page 23: Inorganic chemistry

Fe

• O2 uptake proteins (i.e hemoglobin, myoglobin, hemerythrin);

• Oxygenase enzymes; • Catalase, peroxidase, cytochrome P-450; • Aconitase, in cytochrome c oxidase (also cu); • Nitrogenase (also mo), in hydrogenase; • Electron transport proteins like fe-s protein, cytochromes; • In storage protein ferritin; about 70 fe-proteins are well known