topic 7- intro to transition metal complexes

what is a transition metal?

• any element possessing a partially filled d-subshell in its common oxidation state(s)

• eg. Sc is not a TM bc SC3+ has an empty d-subshell and Zn isn’t either bc Zn2+ has a full d-subshell

• group 11 - Cu, Ag - bourderline bc it can form complexes with both filled and partially filled d subshells

what are the different transition series?

• 1st transition series - 3d - period 4

• 2nd transition series - 4d - period 5

• 3rd transition series - 5d - period 6

rules of electron configurations in the first row

1. in the transition ELEMENTS fill 4s before 3d (except copper and chromium) then rewrite in order of increasing value of n

2. in transition COMPOUNDS or COMPLEXES fill 3d before 4s bc they’re lower in energy in complexes

d^n configuration

• the number of outer electrons present

• number of outer s + d electrons

• eg. Ni2+ is a “d^8 metal”

physical properties of transition metals

• hard

• ductile

• malleable

• high electrical conductivity

• high thermal conductivity

(form coloured complexes)

solid state structure of TMs

• all TMs except Mn, Zn, Cd + Hg adopt one of three typical metal structures

• hexagonal close packed

• face centred cubic

• body centred cubic (not a close packed arrangement)

ligands

the atoms to molecules directly bonded to a central metal ion

coordination number

the number of direct points of attachment to the metal ion or bonds

• teh number of donor atoms bound to a transition metal centre

source of colour in transition metal complexes

1. ‘d-d’ transitions - electrons moving between d orbitals

2. L-M charge transfer - electrons originally from the ligand get excited to the metal - normally gives rise to absorbance in the visible region

3. M-L charge transfer - electrons originally from metal are excited to orbitals on ligand

which type occurs depends on the properties of the ligands and the metals themselves

diamagnetism

• substances that contain only paired electrons

• diamagnetic substances are repelled from a magnetic field

• show a decrease in weight when placed into an applied magnetic field (using a guoy balance)

paramagnetism

• substances that contain unpaired electrons

• paramagnetic substances are attracted into a magnetic field

• paramagnetic compounds show an increase in weight when placed into an applied magnetic field (using a guoy balance)

which effects are weaker? diamagnetic or paramagnetic?

• diamagnetic effects re much weaker than paramagnetic effects

• if the compound has unpaired electrons, paramagnetic effects dominate and the compound is attracted into the magnetic field

predicting effective magnetic moment

• spin only formula

• mu effective = square root n(n+2)

• where n=the number of unpaired electrons in d orbitals

are high oxidation state complexes oxidising or reducing agents?

oxidising

are low oxidation state complexes oxidising or reducing agents?

reductants

why are high oxidation states on the left and low oxidation states on teh right?

• ionisation potential gets bigger from left to right due to nuclear charge increasing so it is harder to remove an electron

would you expect CrO3 to be an oxidising or reducing agent?

+6 is chromiums maximum oxidation state so it can only be reduced therefore it’s and oxidising agent

paulings electroneutrality principle

• the charge of a single atom can never be greater than one - if it is it needs to be spread about a wider molecule

• the net charge of a molecule is close to neutrality - each atom in a stable molecule has a charge close to zero

examples of neutral two electron donors

• NH3, CO, H2O, PR3, SR3

• usually coordinate with lower oxidation state mental ions

examples of anionic two electron donors

• CN-, Me-, Cl-, I-, H-

• common with high oxidation states metal ions

examples of cationic two electron donors

• NO+

• very rare

• will only coordinate to low oxidation states

“inner sphere” vs “outer sphere”

inner sphere - bound directly to metal centre i.e. L in [ML_n]X_y

outer sphere - ligands associated with the ‘inner sphere’ complex i.e. X in [ML_n]X_y

water of crystilisation

• in the solid state water molecules that are not directly coordinated to the transition metal, which are equivalent to outer sphere ligand molecules are called ‘water crystillization’

unidentate/mono-dentate

ligands which occupy only one coordination site

ambidentate ligands

• have two teeth but only one can bond at a time due to their angles

• more than one potential donor atoms that could coordinate

bidentate ligands

• ligands with two donor atoms that are able to bond to the same metal at the same time - tehy occupy two coordination sites

tridentate ligands

• ligands possessing three donor sites - occupy three coordination sites

possible geometries for complexes with coordination number of 4

• tetrahedral - majority adopt this structure

• square planar - seen for d^8 transition metal centres eg. some Ni²⁺, Pd²⁺, Au³⁺, Rh⁺

geometric isomers in square planar complexes

• cis - like ligands are 90 degrees from each other

• trans - like ligands are 180 degrees from each other

• if bidentate ligand is used the the structure must be cis

possible structures of coordination number 5

• trigonal bi pyramid

• square pyramid

geometric isomers of octahedral complexes

• cis/trans

• mer/fac

cis/trans geometric isomerism in octahedral complexes

• trans - like ligands are 180 degrees apart

• cis - like ligands are 90 degreee apart

mer/fac geometric isomerism in octahedral complexes

linkage isomerism

• with ambidentate ligands

• two different sites on the ligand that can each form a covalent bond

optical isomerism in octahedral complexes