Transition metal complexes/ coordinate compounds

what are ligands

An ion or molecule that donates a pair of electrons to a transition metal ion, forming a dative covalent bond (a coordinate bond.)

what is a transition metal complex

• 2,4 or 6 ligands often surround the TM ion

• Altogether, the TM ion and the ligands= transition metal complex

what is coordinate number

• The total number of dative covalent bonds (coordinate bonds) around the metal ion

why do transition metals form complexes

Transition metals form complexes due to their small size- this allows them to attract electron-rich species more strongly

what are monodentate ligands

• If a ligand donates one pair of e- to a TM ion/only forms one dative bond to the central metal ion

• Examples of monodentate ligands are:

  • Water (H₂O) molecules

  • Ammonia (NH₃) molecules

  • Chloride (Cl^–) ions

  • Cyanide (CN^–) ions

what are bidentate ligands

Can donate two lone pairs of electrons (from two seperate atoms) forming two dative bonds to the central metal ion

• Examples of bidentate ligands are:

  • 1,2-diaminoethane (H₂NCH₂CH₂NH₂) which is also written as ‘en’

  • Ethanedioate ion (C₂O₄^2- ) which is sometimes written as ‘ox’

what are multidentate ligands?

These make more than two coordinate bonds with a TM ion-contain more than two atoms with lone pairs of electrons

• An example of a multidentate ligand is EDTA^4-, which is a hexadentate ligand as it forms 6 dative covalent bonds to the central metal ion

why is EDTA4- called a chelating agent

• EDTA4- binds to metal ions reducing their concentration in solutions.

Uses of EDTA4-

• Detergents (remove Ca2+ / Mg2+ from hard water)

• Foodstuffs- as a stabiliser to remove ions that catalyse

• Medicines- to prevent blood clotting or to remove heavy metal ions and prevent poisoning eg Hg/Pb

Shape of complex with 2 ligands

• Linear

• 180 degrees

shape of complex with 2 ligands/coordinate bonds

• Have tetradedral shape

• Complexes with four chloride ions most commonly adopt this geometry

• Chloride ligands are large, so only four will fit around the central metal ion

• The bond angles in tetrahedral complexes are 109.5^o

what shape do compelxes with 6 ligands make

• Octahedral

• This could be six coordinate bonds with six small, monodentate ligands

• • As there are six ligands, these complexes are sometimes described as having six-fold coordination

  • It could be six-coordinate bonds with three bidentate ligands

    • Each bidentate ligand will form two coordinate bonds, meaning six-coordinate bonds in total

    • It could be six-coordinate bonds with one multidentate ligand

  • Bond angle: 90 and 180, usually H2O. NH3 or OH-

how many bonds do chlroide ions usually make

Chlroide ions much greater in size, so fewer surround the central ion. Usually TM-chloride compelxes have 4 Cl-

can transition elements exhibit stereosiomerism?

• Transition element complexes can exhibit stereoisomerism

• Even though transition element complexes do not have a double bond, they can still have geometrical isomers

• Square planar and octahedral complexes with two pairs of different ligands exhibit cis-trans isomerism

what is cis isomer and trans isomer in TM compelxes

if the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer

what are square planar complexes

• Sometimes, complexes with four coordinate bonds may adopt a square planar geometry instead of a tetrahedral one

  • Cyanide ions (CN^-) are the most common ligands to adopt this geometry

  • An example of a square planar complex is cisplatin

• The bond angles in a square planar complex are 90^o

Use of cis and trans platin

• Cis-platin is a square planar molecule that has a geometric isomer with the side groups in different positions

• Used as a cancer treatment drug

• It binds to nitrogen atoms on bases in DNA in fast-growing cancer cells

• It passses through cell membrane and undergoes ligand exchange where chlorine are replaced by water molecules

• The nitrogen is a better ligand than water and forms dative covalent bonds with the cis-platin

• The cis-platin distorts the shape of the DNA and prevents the DNA from replicating

• Trans-platin is less effective and is toxic, so cis-platin is administered as a single isomer

Adverse effects of cis-platin

• Cis-platin binds to healthy cells as well as cancerous cells, but affects cancer cells more as they are replicating faster

• Unfortunately, this means that other healthy cells which replicate quickly, such as hair follicles, are also affected by cis-platin

• This is why hair loss is a side-effect of people undergoing cancer treatment

what type of TM complex can form optical isomers?

• Octahedral complexes with bidentate ligands also have optical isomers

• This means that the two forms are non-superimposable mirror images of each other

  • They have no plane of symmetry, and one image cannot be placed directly on top of the other

• The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions

what is haem group in hameoglobin

• The haem molecule is a complex with iron(II) at its centre

• The haemoglobin complex contains a multidentate ligand made up of four haem groups

  • These consist of mostly carbon and hydrogen atoms

  • Each haem group has a nitrogen atom forming a dative covalent bond to the Fe²⁺ ion in a square planar complex

• There is a fifth dative bond from the protein (globin) to the Fe²⁺ ion

• Oxygen atoms form a dative covalent bond with the iron(II) which enables oxygen molecules to be transported

job of haemoglboin

• Hb is a pigment foudn in rbc. They transport oxygen from the lungs to every cell in body, where O2 is used for respiraiton.

• Oxygen molecules are not very good ligands and bond weakly to the iron(II)

• The weak bonds allows them to break off easily and be transported into cells

why is carbon monoxide toxic?

• Carbon monoxide binds irreversibly with haemoglobin

• This shows that the coordinate/dative bond formed from CO to Hb/ Iron II is more stronger /stable than the Hb-O2 dative covalent bond

• Prevents oxygen being carried to cells and leads to oxygen starvation and death by suffocation

• If oxygen attached to the haemoglobin (oxyhaemoglobin) is replaced by carbon monoxide (carboxyhaemoglobin), a darker red colour is produced in the haem complex

when does anaemia occur

• The condition anaemia occurs when a person does not have enough haemoglobin in their blood due to a loss of blood or deficiency in iron

  • Deficiency in iron can be restored by taking iron sulfate tables in the diet