TRANSITION ELEMENTS
1st row of transition elements:

typical transition element- element which forms at least one aqueous ion w/ a partially filled d-orbital
typical transition element also exhibit
-variable oxidation state
-coloured aqueous ions
-catalytic activity
-formation of complex ions
Sc & Zn: 3d-elements, not typical transition elements (colourless aqueous ions, Sc- empty 3d-orbitals, Zn- completely filled 3d-orbitals)

Physical properties of 3d-elements
Metallic & ionic radii
-decreases very slightly from Ti to Cu
-due to electrons added to penultimate (2nd last) 3d-orbital which effectively shield outer 4s electrons from positive nuclear charge attraction (nuclear charge attractn does not increase significantly)
Melting & boiling pts

all above 1000°C (except Zn)
-strong metallic bonding (due to bonding involving 3d & 4s electrons which have small energy diff)
[strength of metallic bonding roughly dpds on # of unpaired electrons in atom (not only factor)]
-Sc to Cr: mp increase (due to increase in unpaired electrons)
-rel low mp of Mn due to extra stability of ½ filled 3d orbital which results in less availability of electron for delocalisatn to form metallic bond
-Mn to Zn: mp decrease (decrease in unpaired electrons)
Density

-denser than group I & II metals due to rel smaller metallic bonds & closely packed solid structure
-irregular increase in density from Sc to Cu due to small, irregular decrease in metallic radii & increase in RAM
Ionisation energy

-generally, all IE increases from Ti to Cu
1st, 2nd IE: small variation from Ti to Cu (3d electrons effectively shield 4s electrons from nucleus > positive nuclear attraction minimal)
irregularities: 2^nd IE of Cr, 3^rd IE of Mn, 4^th IE of Fe (due to half-filled 3d orbital)

Chemical properties of typical transition metals
-variable oxidtn states (element / cpd)
-catalytic effect (element / cpd)
-coloured cpds / ions
-form complex ions
Variable oxidation state
-due to electrons in 3d & 4s orbitals having almost equal energy (electrons lost from either orbital)
max / no of oxidtn state increases then decreases from Sc to Zn
-due to # of unpaired electrons

Sc	Ti	V	Cr	Mn	Fe	Co	Ni	Cu	Zn
	+1	+1	+1	+1	+1	+1	+1	+1
	+2	+2	+2	+2	+2	+2	+2	+2	+2
+3	+3	+3	+3	+3	+3	+3	+3	+3
	+4	+4	+4	+4	+4	+4	+4
		+5	+5	+5	+5	+5
			+6	+6	+6
				+7	[in bold: important oxid state]

Sc > Mn: all electrons in 3d, 4s orbital available for bonding
decrease in oxid # after Mn due to rel stability of electrons in 3d orbital
stability of higher oxid states decreases from left to right
+2: increased stability from left to right (2 4s electrons more easily lost than 3d electrons which are more tightly held)
(Mn²⁺, Fe³⁺ more stable than Mn³⁺, Fe²⁺ due to half filled 3d orbital)
intermediate oxid states: rel unstable > disproportionates
lower oxid states shown in ionic cpds
higher oxid states shown in covalent cpds (electron sharing)

Catalytic property
due to -availability of empty 3d orbitals (which form weak bonds w/ gas molecules by surface adsortptn), -variable oxid state
Fe: Haber process, V₂O₅: contact process, Pt / Rh: productn of nitric acid (4NH₃ + 5O₂ <> 4NO + H₂O), Ni: hydrogenatn, Cu: oxidatn of propan-2-ol (to propanone)
Homogenous catalyst
catalyst same phase as reactants: liq catalyst (aq sol)
catalyst changes oxid state & regenerates at end
mechanism requires less activatn energy
Heterogenous catalyst
catalyst diff phase form reactants: solid catalyst, gaseous reactants
solid catalyst forms weak bonds w/ gaseous reactants using empty 3d orbitals
empty orbitals accept electrons of gaseous reactants > dative covalent bond > gas adsorped on solid
adsorption increases rate of reaction as it
-lowers activatn energy
-weakens reactant molecule bonds
-increases conc of reactant on surface
-provides right orientation for reaction to occur

mechanism requires less activatn energy & increases surface conc of gaseous reactants

Formation of colorued cpds & aq complex ions
isolated transitn metal atom: 5 3d orbitals of same energy
in presence of electric field due to ligands, 5 orbitals split into 2 groups of different energy
colour formed when electrons absorb light & transfer from lower E level to higher one
due to small E diff light absorbed in visible spectrum > visible colour
factors affecting colour:
-shape / stereochemistry of complex ions
-# of 3d electrons surrounding metal ion
-nature of ligands (diff ligand, diff E difference)

Formation of complex ions
complex ion: ion in which central metal ion w/ empty d orbitals is bonded to surrounding anions / molecules (ie ligands) by coordinate / dative bonding (ligands donate lone pair of electrons to empty orbital)

ligand: neutral molecule / anion w/ one or more lone pair of electrons which can be donated to empty d orbitals of central metal ion to form dative bond
ligands: monodentate, bidentate, polydentate
monodentate- one site of coordination, ie forms 1 dative bond
water, ammonia, OH^-, Cl^-, CN^-
didentate- 2 sites of coordination (forms 2 dative bond)
polydentate- more than 2 sites of coordination
transitn metals form complex ions due to
-empty 3d, 4s, 4p orbitals which can form dative bonds by accepting lone pair of electrons
-small size & high charge density (> attracts electron pairs)
Nomenclature of complex ions
(prefix)(ligand)(metal)(oxid state of metal) ion
prefix; # of ligands

water: aqua-	chloride: chloro-	oxide: oxo-	carbon monoxide, CO: carbonyl-
ammonia: ammine-	bromide: bromo-	cyanide, CN^-: cyano-	sulphate, SO₄^2-: sulphato
hydroxide: hydroxo-	iodide: iodo-	nitrite, NO₂^-: nitro-	C₂O₄^2-: oxalato- / ethanedioate-
edta^4-: edta-	fluoride: fluoro-	ethane-1,2-diamine, en: ethane-1,2-diamine

metal	metal as cation	metal as anion	metal	metal as cation	metal as anion
copper	copper	cuprate	vanadium	vanadium	vanadate
iron	iron	ferrate	titanium	titanium	titanate
zinc	zinc	zincate	manganese	manganese	manganate
cobalt	cobalt	cobalate	nickel	nickel	nickelate
chromium	chromium	chromate

Shapes of complex ions
shape depends on: -# of bond pairs around central metal ion, -type of orbital hybridisatn of metal ion
linear: 2 bond pairs
tetrahedral: 4 bond pairs
square planar: 4 bond pairs (PtCl₂(NH₃)₂,[Ni(CN)₄]^2-)
octahedral: 6 bond pairs

Isomerism of complex ions
conformational isomerism: take diff shape
coordination isomerism: salt of cation & anion complexes > metal ions form ligands w/ other metal ion

Structural isomerism
-diff structures
X: ligand coordinated to metal ion / free ion
for salts:
eg [M(H₂O)_a]X_n
[M(H₂O)_a-1X]X_n-1.H₂O
[M(H₂O)_a-2X₂]X_n-2.2H₂O
-specifically called ionisation isomerism

Stereoisomers
-same structures diff arrangement
Geometrical isomers / cis-trans isomers
(cis: same side, trans: diff side => usually results in no resultant dipole)

found in complexes w/ shape
-square planar of type MA₂B₂

-octahedral complex of type MA₄B₂ / M(L-L)₂B₂[(L-L): bidentate ligand]

[consider the planar atoms/molecules]

Optical isomers
-non-superimposable mirror images of each other
found in: M(L-L)₃, M(L-L)₂B₂[(L-L): bidentate ligand]

Stoichiometry of complex ions
found by -complexiometric titration / -colorimetric method
complexiometric titration
-involves a complex and titration
-titration used to form coloured complex or decolourise a coloured complex
colorimetric method
-measures colour intensity of complex ion formed

Relative stability of complex ions
-measured by stability const, K_stab, a measure of stability of complex ion w/ respect to dissociation to ions at given temp
M^m+(aq) + xL^-(aq) <> [ML^x]^(m-x)
K_stab = [[ML^x]^(m-x)] / [M^m+][L^-]^x
higher stab const > more stable complex ion
Competitive ligand displacement (competitive complexing reaction)
-more powerful ligand displaces weaker ligand from a central metal ion to form more stable complex
complexing strength: H₂O < Cl^- < NH₃ < Br^- < S₂O₃^2- < I^- < CN^- < S^2- < en < edta^4-
(polydentate > bidentate > monodentate ligand)

Competitive ligand displacement in blood:
Fe ← H₂O, Fe ← O₂, Fe ← CO₂: weak dative bonds > reversible
Fe ← CO, Fe ← CN: strong dative bonds > irreversible

Magnetic properties of transition metal complexes
diamagnetic / paramagnetic - dpds on electrons paired / unpaired
diamagnetic: no unpaired electrons (all paired)- v.weakly repelled from magnetic field
paramagnetic: unpaired electrons present (odd # of electrons)- v.weakly attracted from magnetic field (electrons align themselves so that attracted)
ferromagnetic: Fe, Co-Ni alloy-very paramagnetic that attraction to mag field can be observed w/ eye (dia/para-magnetic- cannot see w/ eye, use balance > weight increase/decrease)
relative paramagnetism: measure of paramagnetism based on # of upaired electrons
ligands in complex metal ions cause 3d orbital to split into 2 groups w/ diff energy levels
-if energy diff is small: orbitals filled w/ parallel spin 1^st before pairing up (high spin complex) (weaker ligands: H₂O) ([Fe(H₂O)₆]²⁺, [Fe(H₂O)₆]³⁺)
-if energy diff is large: orbitals w/ lower energy completely filled (parallel 1^st then paired), leaving orbitals w/ higher energy empty, before the filling the higher energy orbitals (low spin comples) (stronger ligands: CN^-) ([Fe(CN)₆]^4-, [Fe(CN)₆]^3-)

Back to 'A' level notes index

Back to notes index