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)

1TM I-Intro to Complexes

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)

2TM I-Intro to Complexes

Examples of Metal Complexes

Ni(H2O)62+ Co(NH3)4Cl2+

Six H2O ligands bound to a Ni2+ cation

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

http://www.3dchem.com/3dinorgmolecule.asp?ID=490 http://www.3dchem.com/3dinorgmolecule.asp?ID=196

3TM I-Intro to Complexes

More Examples

Ni(H2O)4SO4

Four H2O ligands and one SO42-

ligand bound to a Ni2+ cationhttp://www.3dchem.com/3dinorgmolecule.asp?ID=440

Ni(CN)42-

Four CN- ligands bound to a Ni2+ cation

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

4TM I-Intro to Complexes

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

5TM I-Intro to Complexes

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)6

2+ and two Cl- ions:– [Ni(NH3)6]Cl2 Ni(NH3)6

2+ + 2 Cl-

• e.g., (NH4)4[Fe(CN)6] ; complex an anion; NH4+’s, counterions

– (NH4)4[Fe(CN)6] 4 NH4+ + Fe(CN)6

4-

Charge of complex is +2 + 2(0) + 2(-1) = 0

6TM I-Intro to Complexes

**The species inside the brackets is always a complex ion (metal cation + ligands); the part(s) outside of brackets are counterions.**

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-)

• Note! 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

7TM I-Intro to Complexes

Common C.N.’s and their Geometries

C.N.: 2 4 4 6

8TM I-Intro to Complexes

Example of a tetrahedral metal complex

Ni(CO)42+

Four CO ligands bound to a Ni2+ cation

http://www.3dchem.com/3dinorgmolecule.asp?ID=489 9TM I-Intro to Complexes

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

10TM I-Intro to Complexes

Monodentate (donor atom circled)

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

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)

11

NOTE: Some ligands are…

neutral

And some ligands are…

negatively charged

12

Ligand Types (Table 20.13, McMurry & Fay; Similar to Tro, 24.2)

13TM I-Intro to Complexes

Ligand Types (Tables 20.13 (McMurry) and 24.2 (Tro), continued)

14TM I-Intro to Complexes

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

15TM I-Intro to Complexes