2.5.1 - 2.5.3 properties, substitution reactions, shapes of complexes

d-block element

highest energy electron in a d sublevel

when d-block atoms form ions, which sublevel is filled and emptied first?

4s fills and empties before 3d

which period 4 d-block elements do not follow the trend for electronic configuration?

why?

• Cr is [Ar] 4s¹ 3d⁵

• Cu is [Ar] 4s¹ 3d¹⁰

• they have only one electron in their 4s sublevel before electrons fill up the 3d sublevel.

• this allows them to have a hall-full or completely full 3d sublevel, which is more stable

what is a transition metal?

which elements are transition metals?

• an element that has an incomplete d sublevel in its atoms or ions

• elements Ti - Cu

why isn’t Sc a transition metal?

• Sc electronic configuration: [Ar] 3d¹ 4s²

• its only stable oxidation state, Sc³⁺, results in an empty 3d orbital, 3d⁰

• doesn’t form stable ions with an incomplete d sublevel so doesn’t meet the criteria

why isn’t Zn a transition metal?

• Zn electronic configuration: [Ar] 3d¹⁰ 4s²

• its only stable oxidation state, Zn²⁺, results in an empty 4s orbital, 4s⁰

• doesn’t form stable ions with an incomplete d sublevel so doesn’t meet the criteria

characteristic properties of transition metals

• formation of complexes

• formation of coloured ions

• variable oxidation states

• catalytic activity

how do the characteristic properties of transition metals arise?

arise due to the incomplete d sub level

Sc and Zn form what colour compounds and what colour solutions?

• white compounds

• colourless solutions

ligand

a molecule or ion with that forms a co-ordinate bond with a transition metal by donating a lone pair of electrons

a complex

a cental metal atom or ion surrounded by ligands

co-ordination number

the number of co-ordinate bonds to the central metal atom or ion

Lewis acid vs Brønsted–Lowry acid

• Lewis: electron pair acceptor (electrophile)

• Brønsted–Lowry: proton donor

Lewis base vs Brønsted–Lowry base

• Lewis: electron pair donor (nucleophile)

• Brønsted–Lowry: proton acceptor

Lewis acid in a complex

Lewis base in a complex

• acid → electron pair acceptor → metal

• base → electron pair donor → ligand

monodenate ligands

examples

ligands which form one co-ordinate bond to a metal ion

• H₂O

• NH₃

• Cl^-

bidentate ligands

examples

ligands which form two co-ordinate bonds to a metal ion

• 1,2-diaminoethane: NH₂CH₂CH₂NH₂ or en

• ethanedioate ion: C₂O₄^2-

mulitdentate ligands

examples

example of the formula of a complex

• ligands which form many co-ordinate bonds to a metal ion

• EDTA^4–, forms 6 co-ordinate bonds

• [CuEDTA]^2-

how does the size of a ligand affect the shape of a complex?

• smaller ligand: higher co-ordination number, octahedral complexes.

• larger ligand: smaller co-ordination number, tetrahedral, square planar and linear complexes

compare and contrast ligands H₂O, NH₃ and Cl^-

• all are monodentate, as their geometry prevents multiple lps from bonding to the same metal atom/ion

• H₂O and NH₃ are small, similar in size and neutral

• Cl^- is larger and charged

counter ions

ions that bond to the complex ion

• e.g. NO₃^- can bond to [Fe(H₂O)₆]²⁺

  • this forms [Fe(H₂O)₆](NO₃)₂

what shape complex do Ag⁺ ions typically form?

what shape complex do Pt²⁺ ions typically form?

give reasons why

• Ag⁺ forms linear complexes

• Pt²⁺ forms square planar complexes

  • shape more stable due to electron configuration of Ag⁺ / Pt²⁺ ions

  • electron pairs repel as far apart as possible

• formula of Tollens’ reagent

• shape of complex

• conditions for aldehyde test

• what is oxidised, what is reduced

• observation for positive result

• [Ag(NH₃)₂]⁺

• linear

• test: [Ag(NH₃)₂]⁺ complex is in alkaline conditions, then warm water bath with aldehyde

• aldehyde gets oxidised to carboxylate ion, [Ag(NH₃)₂]⁺ gets reduced to metallic silver

• colourless to silver mirror

equation of positive result for Tollens’ reagent test

RCHO + 2 [Ag(NH₃)₂]⁺ + 3 OH^- →

RCOO^- + 2 Ag + 4 NH₃ + 2 H₂O

reaction when a silver halide dissolves in aqueous NH₃

Tollens’ is produced

e.g. AgBr _(s) + 2 NH_{3 (aq)} → [Ag(NH₃)₂]⁺ _(aq) + Br^- _(aq)

• AgCl dissolves in dilute and conc NH₃

• AgBr dissolves in conc NH₃ only

• AgI doesn’t dissolve in NH₃

types of stereoisomerism in transition metal complexes

• cis-trans isomerism

• optical isomerism

what type of isomerism is cis-trans isomerism?

when does it occur in complexes?

• E-Z isomerism (geometric isomerism)

• displayed in octahedral and square planar complexes with monodentate ligands, where 2 identical monodentate ligands differ in their positions in space relative to each other

cis-isomer vs trans-isomer

• cis = 2 identical ligands are next to each other (sisters)

• trans = 2 identical ligands are opposite each other

example of cis-trans isomerism in an octahedral complex

solid copper hydroxide, [Cu(H₂O)₄(OH)₂]

example of cis-trans isomerism in an square planar complex

platin, [Pt(NH₃)₂Cl₂]

why can cis-platin be used as an anti-cancer drug but trans-platin can’t?

trans-platin has the wrong spatial arrangement of its Cl ligands

what type of isomerism is optical isomerism?

when does it occur in complexes?

• a form of stereoisomerism

• displayed by octahedral complexes with bidentate ligands

what are optical isomers?

what’s another name for optical isomers?

• non-superimposable mirror images

• enantiomers

example of optical isomerism in a complex

describe the complex

hexaaminecobalt (III) ion, [Co(NH₃)₆]³⁺

octahedral complex with bidentate ligands

ligand substitution

one ligand is replaced by another

ligand substitution with NH₃ and H₂O

• exchange of NH₃ and H₂O occurs without change of co-ordination number as they are similar sizes

• example: hexa aqua ions + conc NH₃

  • complete: [Co(H₂O)₆]²⁺ + 6 NH₃ → [Co(NH₃)₆]²⁺ + 6 H₂O

  • partial: [Cu(H₂O)₆]²⁺ + 4 NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4 H₂O

ligand substitution with Cl^-

• exchange of H₂O by Cl^- can cause a change of co-ordination number as Cl^- is larger, so fewer ligands can fit around the central metal atom

• example: hexa aqua ions + conc HCl

• shape goes from octahedral to tetrahedral, co-ordination number goes from 6 to 4

  • [Cu(H₂O)₆]²⁺ + 4 Cl^- → [CuCl₄]^2- + 6 H₂O

  • blue to yellow-green solution

  • [Co(H₂O)₆]²⁺ + 4 Cl^- → [CoCl₄]^2- + 6 H₂O

  • pink to blue solution

  • [Fe(H₂O)₆]³⁺ + 4 Cl^- → [FeCl₄]^1- + 6 H₂O

  • yellow to yellow solution

the chelate effect

• bidentate and multidentate ligands (readily) replace monodentate ligands from complexes

• coordination number does not change

• so same number of coordinate bonds broken as made

• bonds have similar enthalpy

• so ΔH negligible

• ligand substitution involves increase in moles (n_{products side} > n_{reactant side})

• so increase in disorder, so positive ΔS

• ΔG = ΔH - TΔS

• ΔG = 0 - positive = negative, so substitution is feasible

using free energy, how will we know if we have a stable product?

• forward reaction has negative ΔG so feasible

• therefore reverse reaction has positive ΔG so will not readily occur

• so product will not readily turn back into reactants

oxyhaemoglobin structure

octahedral complex of iron (II)

horizontal plane

• haem group: Fe²⁺ and porphyrin ring

• 4 co-ordinate bonds from N atoms to Fe²⁺

vertical plane

• below: 1 co-ordinate bond from N atom in globin to Fe²⁺

• above: 1 co-ordinate bond from O₂ to Fe²⁺

how does the function of haemoglobin involve ligand substitution?

• haemoglobin forms oxyhaemoglobin when O₂ forms a coordinate bond to Fe²⁺ in haemoglobin

• this enables oxygen to be transported in the blood

• O₂ can be substituted for H₂O in a reversible ligand substitution reaction, forming deoxyhaemoglobin

• this enables O₂ to be released and used for aerobic respiration

what makes CO toxic?

• it replaces O₂ co-ordinately bonded to Fe(II) in haemoglobin

• forms carboxyhaemoglobin, which is a very stable complex

• CO is more strongly bonded, so the ligand substitution is not easily reversible

• prevents O₂ from binding