Coordination Chemistry4 Molecular Properties

1
Coordination Chemistry(4)Molecular Properties
Fred J. Grieman
Explanation of Coordination Compound Properties
2
Properties to be explained
A. Colors Complexes tend to absorb in visible
transmitting complementary
color electronic transition of d
electrons Why different colors?
B. Reaction Rates (Kinetics to be studied
next) Slow reaction Co(NH3)63 6H
? Co(H2O)63 6NH4 Even though Keq 3 x 1021
!!!!! Why?
C. Magnetic Properties via d electrons CoF63-
paramagnetic but Co(NH3)63 is diamagnetic !!!!
Why?
3
D. Structures Can we understand why certain
structures are favored? Can we explain
structure changes that occur with change in
oxidation number? Pt2 sq. planar Pt4
octahedral
Coordination 4 4 6
tetrahedral
E. d orbital occupation stability d0, d5 are
stable, but so are d3, d6, d8 where as d4 is
unstable Why?
Square planar
octahedral
4

• Explanation of Coordination Compound Properties
• Considerations
• C.N. 6 favored over C.N. 4 (bond formation
  lowers energy)
• ?oct vs. ?tet vs. ?sqp can affect energy (d
  orbital occupation)
• Ligand Field Strength can affect ?s (M.O.
  interaction)
• Structure
• Octahedral favored (6 bonds)
• d-orbital e-s can alter favored structure
  because of high energy
• oct sq.
  planar tetra

d orbitals
d9
d7
Tetrahedral never predicted Very rare Ni is
example of exception
d1 thru d6 favors octahedral
d7, d8 favors square planar
d9, d10 favors octahedral
Example Pt2 vs. Pt4 ?
d8 d6
sq. pl. octa v
Results Predictions are pretty good
5
Stable d orbitals configuration d0,d1,d2,d3
no e- repulsion d4 x
e- in ? orbital
e- - e- repulsion
d5 Weak Field
d6 Strong Field (? - bonding)
d7, d8 sq. planar
6

• Color due to d orbital e- electronic transition
• Color changes due to ligand
• ? varies due to M.O. interaction
• (extent and type of ?-bonding)
• 2) Color changes due to change in charge on metal
  
• ? varies due to increased ?-bonding
• with increased charge
• (increased overlap)
• 3) Octahedral - weak transition (light colored
  compounds)
• center of symmetry makes transition
  forbidden
• weak if ligands all the same
• Tetrahedral strong transition (dark colored
  compounds)
• no center of symmetry makes
  transition allowed
• Spin Forbidden transitions weak e.g. d5
  weak field

h?
Spin must flip!!! Very difficult weak transition
7

• Magnetism ? vs. EPR changes d orbital
  occupation
• Fe(III) d5
• Fe F63-
  Fe(CN)63-

?mag FeF63- ? 2 x ? mag Fe(CN)63-
2) Co(II) vs. Co(III) Co(NH3)62
Co(NH3)63
Co3 draws NH3 closer giving more orbital
interaction giving higher energy ?d (eg) and
bigger d-orbital splitting
d6
d7
paramagnetic diamagnetic
8
Measurement of Magnetic Moment Magnetic
Susceptibility Lab manual Molar ?m N2
?2/3RT Na
Paramagnetic diamagnetic (ignore) Term
term
NAvogadros RGas Constant Tabsolute
temperature
? magnetic moment (Bohr Magnetons) ? 2.824
T ?m ½ B.M. (combining
constants) ? n(n2) ½ B.M.
n of unpaired e-s !!!! (assuming ?
is only due to unpaired electrons) Measure
magnetic moment ? of unpaired electrons
You will do this in the laboratory!!
9
Magnetic Susceptibility Measurement Gouy
Method No magnetic field Take measurement Turn
on magnetic field paramagnetic big increase
in mass diamagnetic slight decrease in
mass Evans Method (In laboratory) Permanent
suspended magnets Measure position with no
sample Measure position with sample Use
Standard HgCo(SCN)4
10
Kinetics Inert vs. Labile Complexes
(reacts slowly) (reacts quickly) e.g. Co3
is very inert Co(H2O)63(aq) 6NH3(aq) ?
Co(NH3)6(aq) 6H2O Keq 2.3 x 1034 !!! But
very, very slow - Why?? Co3 d6
e- from reactant must go into eg (?d) high
energy orbital
OH2 H2O OH2 ?Co?
NH3 H2O ?OH2 OH2

transition state Co(H2O)63 6NH3

Co(NH3)6(aq) 6H2O
?
e- must go into eg orbital
E
EA, activation energy the higher, the slower the
reaction
Reaction Coordinate
Next Chemical Kinetics