ENVIRONMENTAL CLASSIFICATION OF METALS

1. ENVIRONMENTAL CLASSIFICATION OF METALS 2--CLASSIFICATION OF METALS 2--CLASSIFICATION OF METALS

2. Metals are elements that have alustrous appearance, are good conductors of electricity, and that act as cation in water or when reacting with other chemicals. Brown…………….alkali and alkaline earth metals Red…………….....other metals Blue…………….....metalloids yellow fill………….most toxic metals Blackand green…..non-metals

3. 1 - TRADITIONAL CHEMICAL CLASSIFICATION OF METALS Metals and metalloids comprise all elements in the periodic table except: Noble gases, H, B, C, N, O, F, P, S, Cl, Br, I, and At. METALLOIDS ARE SEMI METALS: Si, Ge, As, Se, Sb, and Te

4. 1.1. CHEMICAL CLASSIFICATION BASED ON METAL-LIGAND COMPLEX STABILITY CONSTANT DATA • Proposed by Ahrlard et al., 1958 • Divides metals into: • TYPE A metals, • TYPE B metals, and • Intermediate or BORDERLINE metals • This classification is governed by the number of electron in the outer shell

5. 1.1.1. Type A Metal Cations (hard metals) • Li+, Na+, K+, Be2+, Mg2+, Ca2+, Sr2+, Al3+, Sc3+, La3+, Si4+, Ti4+, Zr4+, Th4+ • Inert gas (do) electronic configuration  • Spherical symmetry (hard ions) • Low polarizability • Preference of oxygen (O) and fluorine (F) containing ligands over S and higher halides

6. Type A metal cations cont’d • These metal ions may form insoluble complexes with OH-, CO2-3, and PO3-4(e.g. CaCO3; AlPO4) • Complexes with OH- are more stable than those with HS- or S2-because OH-displaces HS- and S2- in aqueous solutions. • Complexes with Cl-, Br-, and I- tend to be weak • Complexes with H2O are more stable than those with NH3 or CN-

7. Type A metal cations cont’d • An electrostatic model approximately explains the stability of type A metal-ligand complexes—That is, the stability of type A metal-ligand complexes is positively correlated with theionic index (Z2/r) • Z = ionic charge • r = radius • Ionic Index is the ion’s propensity to form ionic bonds

8. 1.1.2. Type B Metal Cations(SOFT METALS) • Cu+, Ag+, Au+, Tl+, Ga+, Zn2+, Cd2+, Hg2+, Pb2+, Sn2+, Tl3+, Au3+, In3+,Bi3+ They are metal cations with the following electronic configuration: • nd10 and nd10(n+1)S2 • Examples: • Zn2+: Ar (3d10) • Pb2+: Xe (4f145d106S2)

9. Type B metal cations (cont’d) • They exhibit high polarizability Unlike type A metal cations, here covalent bonding plays a role in complex formation, and therefore, an electrostatic model alone is unable to explain stability relations. • The stability of their complexes is affected by their tendency to accept electrons (high en) from ligands • Example: Zn (1.6)<Cd(1.7)<Hg(1.9) And the stability complex with these metals are generally in the same order as their en

10. Type B metal cations (cont’d) • The electronegativity (en) of ligand donor atoms and the stability (n) of formed complexes with type B metal ions vary in the following order: • Low en High en S I Br Cl N O F • High n Low n

11. Type B cations exhibit the following complexation properties: • Form more stable complexes than do type A • Complex stability with halides decrease in the orderI- > Br- > Cl- > F­ (reverse of type A) • Complexes with N-containing ligands are favored over O-containing ligands • Examples: NH3 over H2O and CN- over OH- • Complexes with S2- and HS- or organosulfides are stable, and frequently, insoluble • Form stable organometallic compounds in water

12. 1.1.3. Intermediate or Borderline Metal Cations • Fall in this group, metal cations with -electronic configuration ndX, with 0<x<10 (i.e. they have 1 to 9 outer shell electrons). • Examples are: • V2+, Cr2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Ti3+, V3+, Cr3+, Mn3+, Fe3+, Co3+ • They form complexes with all types of donor ligands • The 2nd transition row in the periodic table shows more type-B character than do those in the 1st row

13. Intermediate or borderline metal cations (cont’d) • Within borderline metals, type B character tend to increase somewhat as one moves from left to right in the periodic table • Electrostatic factors play a role in the stability of borderline metals • The increase in stability comes from a stronger electrostatic effect b/c of high Z2/r associated with the decrease in ionic radius across the series

14. 2. Environmental classification of metals Based on Traditional Chemical Classification • Proposed by Nieboer and Richardson (1980) • This classification is based on both • The IONIC INDEX(Z2/r), and • The COVALENT INDEX(X2m.r) • X2m.r is defined as the ability to accept e- from donor ligands, and where • Xm= metal ion electronegativity (en) • r = ionic radius

15. Environmental classification of metals (cont’d) ·X2m.ris used as a parameter, which differentiates between type A, bordeline, and type B. Ag+ Pb2+ Hg2+ 4 Type B metals 3 Cu2+ Co2+ Covalent Index X2m.r Sn4+ Fe2+ Ni2+ Overall trend of stability of Formed complexes 2 Mn2+ Zn2+ Borderline metals Mg2+ Al3+ K+ 1 Ca2+ Na+ Type A Metals 4 24 Ionic Index, Z2/r

16. From the graph • Type B are characterized by large X2m.r • Type A have the smallest X2m.r • Highly charged species will tend to be on the right end of the diagram and these are species which tend to behave as Bronsted acids (proton-donors). Macronutrients (K+ and Ca2+) are type A and associated with O-electron donors • Micronutrients (Mn, Cu, Zn) are intermediate and favored ligands are O-N-S-containing electron-donors • Type B = mostly toxic metals • Overall, the toxicity increases in the order type A< Intermediate<Type B. 

17. In spite of limitations, concepts discussed above do help understand some of the features of natural aqueous ion solution chemistry.