c24 - transition elements

transition metal

metal that contains an incomplete d subshell in at least 1 ion

which metals are in the transition metal block but aren’t transition metals

Zn/Zn2+, Sc/Sc3+

properties of transition metals (found in d-block)

all have variable oxidation states, form coloured compounds, used as catalysts e.g. ni in hydrogenation of alkenes

colour of fe ions

fe2+ green, fe3+ yellow

colour of cr ions

cr3+ green, cr6+ orange

colour of cu2+

blue

colour of mn ions

mn7+ purple, gets lighter as oxidation no. decreases

complex ions

formed when one or more molecules/anions (ligands) datively bond to a central metal ion

ligand

molecule or ion that donates an electron pair to a central metal ion to form a coordinate/dative covalent bond

coordination number

indicates number of coordinate bonds bonded to the central metal ion, not necessarily the number of ligands

monodentate ligand

ligand that donates 1 electron pair to a central metal ion

bidentate ligand

ligand that donates 2 electron pairs to a central metal ion, forming 2 coordinate bonds e.g. 1,2-diaminoethane (h2n(ch2)2nh2), ethanedioate ion (c2o4 2-)

linear complex ions

coordination number of 2, 180 bond angle around central metal ion, occurs in Ag+ complexes

tetrahedral complex ions

coordination number of 4, 109.5 bond angle around central metal ion, occurs when there are large ligands (cl-)

square planar complex ions

coordination number of 4, 90 bond angle around central metal ion, occurs in pt2+ complexes

octahedral complex ions

coordination number of 6, 90 bond angle around central metal ion, commonest complex

cis-trans isomerism in square planar complexes

cis = 2 identical groups adjacent to one another, trans = 2 identical groups opposite one another

cis-trans isomerism in monodentate ligand octahedral complexes

cis = 2 identical groups have 90 angle between them/adjacent, trans = 2 identical groups have 180 bond angle between them/opposite

precipitation reaction for transition metals

reaction in which an aqueous transition metal (complex ion) reacts with aqueous naoh or nh3 to form an insoluble ionic solid (precipitate)

colour of [cu(h2o)6]2+ metal aqua ion

blue

colour of [fe(h2o)6]2+ metal aqua ion

green

colour of [mn(h2o)6]2+ metal aqua ion

pale pink

colour of [fe(h2o)6]3+ metal aqua ion

yellow

colour of [cr(h2o)6]3+ metal aqua ion

violet/ green

observations of reaction of cu2+ with naoh

blue solution reacts to form blue ppt of cu(oh)2, insoluble in excess naoh, cu2+(aq) + 2oh-(aq) → cu(oh)2(s)

observations of reaction of fe2+ with naoh

pale green solution reacts to form green ppt of fe(oh)2, insoluble in excess naoh but turns brown on surface when exposed to air as fe2+ oxidised to fe3+, fe2+(aq) + 2oh-(aq) → fe(oh)2(s) then to fe(oh)3

observations of reaction of fe3+ with naoh

pale yellow solution reacts to form orange-brown ppt of fe(oh)3, insoluble in excess naoh, fe3+(aq) + 3oh-(aq) → fe(oh)3(s)

observations of reaction of mn2+ with naoh

pale pink solution reacts to form light brown ppt of mn(oh)2, insoluble in excess naoh, darkens on standing in air, mn2+(aq) + 2oh-(aq) → mn(oh)2(s)

observations of reaction of cr3+ with naoh

violet solution reacts to form grey-green ppt of cr(oh)3, soluble in excess naoh forming dark green solution, cr3+(aq) + 3oh-(aq) → cr(oh)3(s) then cr(oh)3(s) + 3oh-(aq) → [cr(oh)6]3-(aq)

ligand substitution reaction of [cr(h2o)6]3+ with excess nh3(aq)

cr(h2o)6]3+(aq) + 6nh3(aq) → [cr(nh3)6]3+(aq), violet solution initially forms green-grey ppt of [cr(oh)3(h2o)3], ppt then dissolves in excess nh3 to form [cr(nh3)6]3+ which is purple

ligand substitution reaction of [cu(h2o)6]2+ with nh3(aq) (not in excess)

[cu(h2o)6]2+(aq) + 4nh3(aq) → [cu(nh3)4(h2o)2]2+ + 4h2o, pale blue solution initially forms blue ppt of cu(oh)2, ppt then dissolves in excess nh3 to form deep blue solution

ligand substitution reaction of [cu(h2o)6]2+ with cl-(aq) ions from hcl (conc)

[cu(h2o)6]2+(aq) + 4cl-(aq) → [cucl4]2-(aq) + 6h2o(l), pale blue solution turns green initially due to equilibrium, after more conc hcl added solution turns yellow

why does coordination number change in the ligand substitution reaction of [cu(h2o)6]2+ with cl-(aq) ions from hcl (conc)

[cu(h2o)6]2+ has coordination number of 6 whereas [cucl4]2- has coordination number of 4, coordination number reduces by 2 because new cl- ligands are larger than h2o ligands that have been substituted so fewer fit around the central cu ion

ligand substitution in haemoglobin

in o2 rich environments, o2 bonds to fe2+ reversibly and fills 6th coordination site to form oxyhaemoglobin, o2 released when oxyhaemoglobin reaches o2 deprived environments and replaced by co2 to be exhaled

carbon monoxide ligand substitution in haemoglobin

co bonds strongly and irreversibly with fe2+ to form carboxyhaemoglobin, if co conc is too high then o2 transport is prevented which can lead to death by asphyxiation