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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

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Metal Complexes

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  1. 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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Common C.N.’s and their Geometries C.N.: 2 4 4 6 TM I-Intro to Complexes

  9. 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

  10. 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

  11. 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)

  12. NOTE: Some ligands are… neutral And some ligands are… negatively charged

  13. Ligand Types (Table 20.13, McMurry & Fay; Similar to Tro, 24.2) TM I-Intro to Complexes

  14. Ligand Types (Tables 20.13 (McMurry) and 24.2 (Tro), continued) TM I-Intro to Complexes

  15. 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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