Part 2 - Coordination Complexes Flashcards
discovery of Blomstrand
- used titration to obtain precipitate of silver chloride upon addition of Ag+
- found that not all the chloride ions will be be precipitated (i.e. equivalents of the chloride ions precipitated varies for different complexes) upon addition of Ag+
- proposed that there must be two different types of chloride groups. chloride ions that are attached to NH3 will dissociate, while those attached to metals will not
- proposed that these two compounds have 2 types of valences: primary valence/ionizable valence and secondary valence/non-ionizable valence
- his proposal was based on measured equivalents
- structures he proposed were incorrect (did not match with Werner’s conductivity measurements)
discovery of Alfred Werner
- measured conductivity to determine the number of ions released
- molar electric conductivity of platinum (IV) complexes
- proposed a different interpretation than Blomstrand
- proposed that these complexes have an octahedral shape
- this proposal matched with the experimental data: precipitation of the correct amount of AgCl upon addition of Ag+, conductivity measurements, number of isomers (cis and trans of [CoCl2(NH3)4]Cl)
- realizing that the cis-isomer of the complex would be chiral, he was able to use its optical activity properly to show that these species were octahedral
- replaced NH3 with ethylenediamine
- won Nobel Prize for Chemistry
- created the first optically active complex that did not contain carbon
primary valence
ionizable valence
secondary valence
non-ionizable valence
complex
a chemical entity consisting of a central metal atom or ion surrounded by a set of ligands
ligand
an ion or a molecule that binds to a metal
coordination complex
- a complex that does not contain metal-carbon bonds
- also known as Werner complex
organometallic complex
a complex with metal-carbon bonds
coordination sphere
set of ligands around the central atom/ion
coordination number (CN)
number of atoms directly attached to the central atom/ion
coordination geometry
arrangement in space of the atoms linked to the metal
chelating ligand
ligand capable of making several bonds to one metal
monodentate ligand
ligand has one attachment point
bidentate ligand
ligand has two attachment points
tridentate ligand
ligand has three attachment points
multidentate ligand
ligand has more than one attachment point
bonding between ligand and metal
- a ligand binds to the metal center using its lone pair
- if the ligand has more than one lone pair, it will make more than one coordination bond
- covalent bond
- metal acts as a Lewis acid, ligand acts as a Lewis base
factors that strongly influence coordination numbers and geometries
1) size of a metal
- larger metal leads to higher coordination number possible
2) size of ligands
- larger ligands leads to smaller coordination number
3) electronic effects
- d-electron configuration of a metal
- the type of metal: early vs late, light vs. heavy
- the type of ligand
CN = 2
- observed almost exclusively in d10 complexes
- common for Ag(1+), Au(1+), Hg(2+)
- less common for Cu(1+), Zn(2+), Cd(2+)
- linear geometry (D∞h)
CN = 3
- very rare
- trigonal planar geometry (expected), usually for d10 metals or ligands with stringent steric demands
- trigonal pyramidal for d0 metals and ligands with stringent steric demands
- D3h
CN = 4
- very common
- tetrahedral (very common)
- square planar for d8 metals Rh(1+), Ir(1+), Pd(2+), Pt(3+), Au(3+)
- exception: Cu(2+) is the only non-d8 metal (its d9) that adopts a square planar complex
- tetrahedral (Td), square planar (D4h)
CN = 5
- common
- trigonal bipyramidal (D3h), square pyramidal (C4v)
- trigonal bipyramidal geometry is slightly more common than square pyramidal geometry, but the energy difference is very small
CN = 6
- very common
- octahedral (very common) (Oh)
- trigonal prismatic (rare) (D3h): mostly found for d0 metals with certain ligands
- octahedral geometry is sometimes distorted towards square planar (stretched/squashed) or trigonal prismatic
- distortions can be explained by Jahn-Teller theorem
- distortions observed for d4 high-spin, d7 low-spin, and d9
CN = 7
- relatively rare
- observed with very small ligands or with early 2nd or 3rd row TM
- more common for lanthanides and actinides
- capped-octahedral (C3v), capped trigonal-prismatic (C2v), pentagonal-bipyramidal (D5h)
CN = 9
- relatively rare
- for d0 TM, [TcH9]2- and [ReH9]2-
- more common for Sc, Y, f-elements
CN = 10-12
- complexes of f-elements
- complexes containing [BH4]- or related ligands
CN = 8
square antiprismatic, dodecahedral
types of structural isomerism
ionization isomers, hydration isomers, coordination isomers, linkage isomers
types of stereoisomerism
diastereoisomers, enantiomers
ionization isomers
- these result from the interchange of an anionic ligand within the first coordination sphere with an anion outside the coordination sphere
- Ex: [Co(NH3)5Br][SO4] and [Co(NH3)5(SO4)]Br
- can be distinguished through chemical means, IR spectra, X-ray diffraction
hydration isomers
result from the interchange of H2O and another ligand in the coordination sphere
coordination isomers
- result from the interchange of ligands between the two metal centres
- possible only for salts in which both cation and anion are complex ions
linkage isomers
- arise when one or more of the ligands can coordinate to the metal ion in more than one way
- these ligands are ambidentate
enantiomers
- optical isomers
- two molecular species which are non-superposable mirror images of each other
diastereoisomers
- two molecular species which are not mirror images of each other
- mer- and fac-isomers for octahedral complexes
fac-isomers
ligands of same type occupy the same face
mer-isomers
ligands of same type do not occupy the same face
Kf
- formation constant of a complex
- expresses the coordinating strength of a ligand relative to the strength of the solvent molecules (usually water)
- [products]/[reactants]
Kd
[reactants]/[products]
stepwise constant
- Kfn = [MLn]/[M][L]^n for M+nL ≤≥ MLn
- typically lie in the order Kn > Kn+1
overall formation constant
ßn = [MLn]/[M][L]^n = Kf1Kf2Kfn for M+nL ≤≥ MLn
chelate effect
- refers to the greater stability of a complex containing a coordinated polydentate ligand compared with a complex containing an equivalent number of analogous monodentate ligands
- mainly an entropic effect (i.e. reason why chelate effect is associated with greater stability is because of the increase in entropy that it provides)
macrocycle effect
- refers to the greater stability of a complex containing a macrocyclic ligand compared with a complex containing a comparable acyclic (open-chain) ligand
- mainly an entropic effect
Irving-Williams series
- relative stability of the M2+ complexes for the 3d metals
- Ba(2+) < Sr(2+) < Ca(2+) < Mg(2+) < Mn(2+) < Fe(2+) < Co(2+) < Ni(2+) < Cu(2+) > Zn(2+)
- order is insensitive to the choice of ligands
- correlates with ionic radius
carbonyl (mond) process for Ni purification
1) Ni is reacted with Syngas at 200˚C to remove oxygen, leaving impure Ni. Impurities include Fe and Co. NiO(s) + H2(g) –> Ni(s) + H2O(g)
2) the impure nickel is reacted with excess CO at 50-60˚C to form Ni(CO)4. Ni(s) + 4CO(g) –> Ni(CO)4(g)
3) the mixture of excess carbon monoxide and nickel carbonyl is heated to 220-250˚C. On heating, nickel tetracarbonyl decomposes to give nickel: Ni(CO)4(g) –> Ni(s) + 4CO(g)
