Transition-Metal Chemistry 201A

-a,b single degeneracy, e for two and t for three

-a is symmetric with respect to Cn, b is antisymmetric

-1 and 2 mean symmetric and antisymmetric respectively, with respect to a C2 axis perpendicular to the principle axis

-g and u are symmetric and antisymmetric with respect to an inversion centre (g is symmetric)

more electronegative => less σ- interaction => eg∗ is lower ∴t2g -> eg∗ gap is less

π-donor< weak π-donor

∆S = 0

partially allowed by spin-orbit coupling

∆l = ±1(g↔u, p-d)

partially allowed if no centre of symmetry (tetrahedron, vibrations in an octahedron) or if there is orbital mixing

µ = √4S(S+1)

since S = 1/2n where n is the number of unpaired electrons:

µ = √n(n+2)

predicted value of magnetic moment

µ = √4S(S+1) + L(L+1)

-orbital angular momentum is usually quenched

µ = √4S(S+1)

M + L -> ML => K₁ = [ML] / [M][L]

ML + L -> ML₂ => K₂ = [ML₂]/[ML][L]

etc

(metals are rarely free ion so it is likely that the ligand is displacing solvent molecules that are surrounding the metal but that is usually ommitted)

M + 4L -> ML₄ => β₄ = [ML₄]/[M][L]⁴

β₄ = K₁.K₂.K₃.K₄

They decrease as due to

-increasing number of the previously displaced ligand being in the solution (statistical factor)

-sterics and electronic factors

Statistical factors - successive equilibium constants decrease by at least a half as more ligands are added because of the increasing likelihood of displacing a ligand.

Charge - increasing charge on metal increases logK

Steric effects- bulkier groups decrease logk

Hardness = (I - A)/ 2 = (HOMO - LUMO)/2

I = ionisation potential, A = electron affinity

Ca<<V<Cr>Mn<Fe<Co<Ni<Cu>Zn

All 2+

-increasing charge/radius ration across the period

-effects arising from CFSE (note dips at d⁵ and d¹⁰ corresponding to zero CFSE)

-Stabilisation from Jahn-Teller stabilisation of Cu(II) which inceases the binding of the first four ligands/atoms at the expense of teh remaining two

The most stable complexes of the transition metals are those in which the metal centre accepts enough electrons from the ligands to attain 18 electrons.

Applies particularly to low oxidation state and covalent complexes

18 electron rule not obeyed.

The

∆₀ is relatively small for 3d metals and for ligands at the lower end of the spectrochemical series t2g is non bonding (0-6 electrons)

eg is weakly antibonding (0-4 electrons)

∴12-22 valance electrons

due to small value of ∆tet tetrahedral complexses fall into this category also

18 electron rule is never exceeded

-∆o is larger for 4d and 5d metals and for

σ ligands relatively high in the spectrochemical series

-t2g is still essentially non-bonding (0-6 electrons)

-eg is antibonding and is no longer available for occupancy

-similar for 3d metals with ligands with very high ligand field strength (like CN-)

obeys 18 electron rule

-∆o is largest for ligands at the uppermost end of the spectrochemical series

-t2g becomes bonding due to interactions with

π∗ orbitas and occupied by 6 electrons

-eg is strongly antibonding and is empty

-18 is obeyed unless steric factors prevent it

-most low oxidation state organometallic compounds are of class III

tendency towards metal-metal bonding increases descending a group as higher members tend to repel one another

∆G⁰ = -nFE⁰

where F is faradays constant

-Increases for the first 5 transition metals to Mn then decreases after (due to pairing of electrons)

-increases for the first 6 transitoin metals for second and third row then decreasing after

decrease from Sc to Zn

left of the dblock - become more electropositive down the group (decreasing electrode potential)

middle - little difference down the group

right - become more noble (increasing electrode potential)

More thermodynamically stable states will be found lower in the diagram.

A species on a convex curve will tend to disproportionate

A species on a concave curve will not disproportionate

A species which is high and on the right will tend to be strongly oxidising

A species which is high and on the left will tend to be strongly reducing

disproportionation occurs for convex curve - Mn³⁺ - >Mn²⁺ + MnO₂

Comproportionation is the opposite - V²⁺ + VO²⁺ -> V³⁺

-Few examples of Cr(IV) and Cr(V), however Mo and W have a very large range of compounds with many stereochemistries through their entire oxidation state range

-Mo most stable is +4 and W is +6, Cr is +3

Inert - reacts slowly if at all

labile - reacts quickly

Stable -> hard to react

Unstable -> easily reacts

Chelate effects

size

charge

d-electrons

what row of the transition metal you are in

Jahn Teller effects

small atomic radii form stronger bonds than ions with larger radii therefore making it less labile

larger ion have higher coordination number making them more labile

-electrons in eg orbitals are antibonding weakening bonds to ligands increasing lability

-electrons in t2g orbitals are mostly non-bonding but not having empty orbitals complicates the donation of the substituting ligands electrons decreasing lability

-no d electrons at all increases lability as there are empty d-orbitals for ligands to donate electrons into

The higher the FCAE the more inert the compound is

low spin are more inert than high spin

Associative, Dissociative and 2 x Interchange

A, D, Ia, Id

intermediate has an increased coordination number

LnMX + Y -> LnMXY -> LnMY + X

Rate constant has small dependence on leaving group and a large dependence on entering group

intermediate has a decreased coordination number

LnMX + Y -> LnM + X + Y -> LnMY + X

Rate constant has a large dependence on leaving group and no dependence on entering group

there is no detectable intermediate only a transition state

LnMX + Y -> X----LnM----Y -> LnMY + X

Two types

Id: The rate is not sensitive to entering group but is sensitive to leaving group. Implying that the groups are weakly bound to the transition state

-3d metals tend to undergo Id substitutions

-larger metals (4d, 5d) lean towards Ia

-low d electron density encourages partly Ia characteristics

kinetic data

-rate law gives us teh composition of the transition state species i.e. teh stoichiometric mechanism

-variation of rate constant with structure and substituent gives the intimate mechanism

Electrostatic - compound T-Pt-X, T induces a dipole in X as such that the electrons are closer to X and X can leave more easily

π-Component - if T is better at π-bonding than the rest of the ligands then it is more likely to take up an equatorial position in the trigonal bipyramidal 5 co-ordinate intermediate

Inner sphere mechanims

outer sphere mechanism

high activation energy related to the reorganisation energy can decrease the rate i.e. t2g⁶ + t2g⁵eg² -> t2g⁵eg2 + t2g⁶

gives a theoretical estimate of the rate constant for the outer sphere electron exchange between two different metal ions when the rate constants for self-exchange are known

kab = √kaakbbKabF

Where Kab is the equilibrium constant and F is a correction term (often 1) for the difference in free energies of the two reactants

∆G = -nFE⁰

= -RTlnK

= ∆H - T∆S

transfer of two inner sphere electrons

-complementary reactions - oxidised and reduced forms of both ions differ by two units

-non-complementary reactions - one pair of species has an oxidation state differenc of 2 but the second has a difference of one requiring a multistep mechanism with a unstable intermediate