10.13a. 2nd row of d block series (4d) Y to Cd including Transition Elements Zr to Ag

3rd row of d block series (5d) La/Lu to Hg including Transition Elements Hf to Au

• Yttrium, Y, Z=39, [Kr]4d¹5s², like cadmium [Kr]4d¹5s², with a filled inner d shell, is not a true transition metal and is more like scandium Z=21 forming the colourless Y³⁺_(aq) ion.

• Rhodium, Rh, Z=45, [Kr]4d⁸5s¹ : A metal used as catalyst in car exhaust systems (see platinum below for more details).

• Silver, Ag, Z=47, [Kr]4d¹⁰5s¹ 

  • Silver is a white lustrous metal that is soft, so very malleable and ductile. and has the highest electrical and thermal conductivity of any element of the periodic table.

  • Silver is used extensively in the electronics industry, jewellery, silverware and in alloys for use as coinage.

  • Silver is used as a catalyst in the chemical industry e.g. the production of alkene oxides.

  • Silver compounds are used in photography (AgCl, AgBr, AgI), antiseptics and for producing 'artificial' rain by seeding clouds with silver salts.

  • The silver(I) ion forms linear complexes with several ligands.

    • The bond angle is 180^o and co–ordination number 2 e.g.

    • The water molecule ligands in the aqueous silver ion [Ag(H₂O)₂]⁺_(aq) can be replaced e.g.

    • (i) with ammonia, NH₃, (neutral ligand) giving a cationic complex, [H₂O→Ag←OH₂]⁺

    • (ii) with the negative cyanide ion CN^– giving an anionic complex, [NC→Ag←CN]^-

    • (iii) the negative thiosulfate ion S₂O₃^2– forming an anionic complex:

    [Ag(NH₃)₂]⁺_(aq) solution is used as Tollen's reagent (ammoniacal silver nitrate) in organic chemistry.

    • The diamminesilver(I) ion is readily reduced by aldehydes (NOT ketones) to form a 'silver mirror' on the side of the test tube on warming the mixture to 60^oC.

    • 2[Ag(NH₃)₂]⁺_(aq), + 2R–CHO_(aq) + 2OH^–_(aq) ===> 2Ag_(s) + 2RCOOH_(aq) + 4NH_3(aq)

    • So this reaction forms the basis of a simple test to distinguish aldehydes from ketones.

    • This redox reaction involves reduction of the silver ion, Ag(+1) to elemental silver Ag(0).

    • The aldehyde is oxidised to a carboxylic acid.

    • [Ag(CN)₂]^–_(aq) solution is used in the electrolyte in silver electroplating. The object to be coated in silver is made the negative cathode electrode.

    • [Ag(H₂O)₂]⁺_(aq) + 2CN^–_(aq) [Ag(CN)₂]^–_(aq) + 2H₂O_(l)

    • The dicyanosilver(I) ion undergoes a ligand exchange reaction (water for cyanide ion) without change in co-ordination number (2), or shape (linear) or silver's oxidation state (+1).

    • However, although not a redox reaction, the overall charge on the silver complex changes from plus (+) to minus (-).

    • The equilibrium is well over to the right but a very low concentration of silver ions gives a good even and strongly adhering surface deposit of silver metal on the conducting negative cathode electrode.

    • At the cathode (-) reduction: [Ag(H₂O)₂]⁺_(aq) + e^– ===> Ag_(s) + 2H₂O_(l)

    • The silver is replenished using a silver anode

    • At the anode (+) oxidation: Ag_(s) + 2CN^–_(aq) – e^– ===> [Ag(CN)₂]^–_(aq)

  • [Ag(S₂O₃)₂]^3–_(aq) is formed when sodium thiosulfate is used to remove unreacted silver bromide (AgBr) or silver iodide (AgI) crystals in developing photographic films.

    • AgBr(s) + 2S₂O₃^2–_(aq) [Ag(S₂O₃)₂]^3–_(aq) + Br^–_(aq)

    • This is NOT a redox reaction, Ag is +1 and Br is –1 throughout the reaction. The thiosulfate ion is here acting as a ligand and not a reducing agent e.g. like with iodine.

    • This an example of using a complex forming reaction to dissolve an insoluble silver salt.

    • The overall charge of the silver-thiosulfate complex is 3- (1+ -4)

  • Use of silver nitrate and ammonia for the halide test is described in the Chemical Tests Notes for anions

  • Non-redox titration questions including the use of standard silver nitrate solution

TOP OF PAGE and sub-indexes

10.13b. 3rd d block row - the 5d block elements La, Hf to Hg including the transition metals Hf to Pt and Au

• Platinum, Pt, Z=78, [Xe]5d⁹6s¹ 

  • Platinum is a bluish–white lustrous metal that is very malleable and ductile.

  • Platinum is relatively unreactive towards oxygen, water, and most acids, but it will dissolve in oxidising acids like nitric acid.

  • Platinum is used in the manufacture of jewellery because of its attractive appearance and anti–corrosion properties.

  • Platinum is used for thermocouples when alloyed with iridium Ir and Rhodium Rh.

  • Platinum is used for electrical contacts because of its excellent electrical conductivity and resistance to corrosion.

  • Platinum and platinum compounds are used as catalysts in the chemical industry e.g. in the manufacture of ammonia, nitric acid, sulfuric acid and methanal.

  • Platinum has/is? been used as a catalyst, with rhodium, in catalytic converters of car exhausts to bring about reactions like

    • C_xH_{y (g)} + (x + ^y/₄)O_{2 (g)} ===> xCO_{2 (g)} + ^y/₂H₂O _(g)

    • [to reduce unburned hydrocarbons]

    • 2CO _(g) + 2NO _(g) ===> N_{2 (g)} + 2CO_{2 (g)}

    • [to reduce oxides of nitrogen and carbon monoxide emissions]

  • Platinum(II) complexes are used to prepare anti–cancer drugs used in chemotherapy.

    • The compound cis–diamminedichloroplatinum(II), [Pt(NH₃)₂Cl₂]⁰, (known as cisplatin) ...

      • is one of the most effective agents against cancers of the ovaries, bladder, and head and neck and helps as co–agent in the treatment of cancers of the cervix, lung and breast.

      • Its biggest success has been in the treatment of testicular cancer, a form of cancer previously resistant to any therapy but now considered to be curable in most cases.

      • However, cisplatin has three drawbacks which limit its usefulness:

      • (i) It is potentially very toxic,

      • (ii) it only affects a few particular types of tumours and it causes the development of resistance in the tumour cell.

      • The 'clinical point' however, is that you are balancing potentially harmful side effects against possible death.

    • The [Pt(NH₃)₂Cl₂]^o complex exhibits a type of isomerism known as E/Z isomerism (once known as cis/trans geometrical isomerism),

    • The platinum stereochemistry is very important with the trans isomer showing no anti–cancer activity but the cis–isomer which is pharmacologically very active in its anti–carcinogenic activity (diagram below).

    • cisplatin (Z-platin) and transplatin (E-platin), neutral complexes.

    • Z (cis) and E (trans) isomers based on the molecular formula

      • [Pt(NH₃)₂Cl₂]

    • cis/trans–platin are neutral, square planer complexes of co–ordination number 4, exhibiting geometric isomerism. There of two of each of the monodentate ligands, chloride ion and ammonia and the blue lines/dots show the 4 dative co-ordinate covalent bonds.

    • The cis/trans isomerism is know called E/Z isomerism. In this case E = trans and Z = cis  (in 'old money'!)

    • If the platinum complex was tetrahedral in shape, [Pt(NH₃)₂Cl₂] could not exhibit isomerism of any type.

    • It is the square planar arrangement of four ligands, of two different ligands, that allows the two different spatial positional arrangements.

    • You could not get isomers of this platinum complex if it had a tetrahedral shape, you couldn't form two non-superimposable structures.

    • See  Complexes and isomerism  and  Isomerism in Transition metal complexes

    • Many organic analogues have, and are, being tested for their anti-cancer properties, by replacing  the ammonia group with an aliphatic amine or more complex amines that can act as an electron pair donating ligand.

      • For more details see E/Z isomerism in organic compound section 2a.9 (or below)

    • In August 2020 I was recently contacted by a friend who discovered in his house a phial containing 1 g of chloroplatinic acid and wondered if I was interested in it - I was definitely curious, but no longer in a laboratory to make use of it!

    • 

    • structure [H₃O]⁺₂[PtCl₆]^2-.xH₂O (x is a variable number of molecules of water of crystallisation)

    • Its a platinum(IV) complex and was used for the gravimetric assaying of potassium.

    • It now finds a use as a catalyst or precatalyst (the latter is converted into the actual catalyst when the reactor is started up!) and is also used in the extraction of platinum from its ores and can be reduced to very pure platinum.

    • Its actually a very valuable compound, costs over £100 per gram or even a lot more!

    • Its now on sale on ebay, where one quote from a Polish source quoted £283 for 1g on ebay!

Organic analogues of the anti-cancer drug cis-platin

Reminder that platinum(II) complexes are used to prepare anti–cancer drugs used in chemotherapy.  One example is the compound cis–diamminedichloroplatinum(II), [Pt(NH₃)₂Cl₂]⁰, (known as cisplatin).  Cisplatin is a much more effective anti-cancer drug than transplatin.

Cis-platin (Z-platin) and trans-platin (E-platin), square planar neutral complexes.  The square planar bond arrangement allows the existence of E/Z isomers, which would not exist if the bonds around the platinum ion where arranged tetrahedrally. For more see my Transition metal notes on platinum

Many organic analogues have, and are, being tested for their anti-cancer properties, by replacing  the ammonia group with an aliphatic amine or more complex amines that can act, like ammonia, as an electron pair donating ligand to give a dative covalent bond between the ligand and the central transition metal platinum(II) ion.

A huge number of Pt(II) complexes have been synthesised, many with the general formula [(RR'HN:)₂PtCl₂], where R and R' are organic groups, R and R' can be the same e.g. the 'simple' E (trans) and Z (cis) complexes with ethylamine and 1,2-diaminoethane (monodentate and bidentate ligand) shown below, but many have much more complicated organic ligands.

In most case the Z (cis) E/Z geometrical isomer is the most effective as an anti-cancer chemotherapeutic agent but the medical situations are complex and chemotherapy has its obvious side-effects like loss of hair.

A bit of biochemistry: These complexes interfere with the DNA repair mechanisms in a cell and the DNA damage causes the cancer cell to undergo apoptosis - a kind of cell death, thus preventing cell division.