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Inorganic Chemistry > Main group organometallic chemistry [Mike Hill] > Flashcards

Main group organometallic chemistry [Mike Hill] Flashcards

1
Q

organometallics

A

synthesis, structure and reactivity (i.e. s/p block) of compounds containing a metal to carbon bond

metal (M) includes metalloids such as boron, silicon
ligand (L) includes alkoxides (oxygen), thiolates (sulphur), enolates

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2
Q

electron precise

A

octet configuration at metal centre

leads to molecular compounds with reduced lewis acidity/electrophilicity

saturated centre -> inert

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3
Q

electron rich

A

leads to molecular compounds with reduced lewis acidity/electrophilicity

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4
Q

electron poor

A

too few valence e-

provide pair for every bonded atom in a Lewis acidic/highly electrophilic molecule

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5
Q

how do organometallic compounds in groups 1,2 and 13 use its valence orbitals in bonding?

A

addition donors

formation of multi centre bonds + aggregation

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6
Q

what happens to the polarity as the difference in electronegativity (effective nuclear charge) between metal and carbon increases

A

increases (+ reactivity of M-C bond increases)

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7
Q

effect of metal size on M-C bonding

A

larger metal = weaker M-C bond

due to less efficient orbital overlap

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8
Q

which group are π-bonds restricted to?

A

1

heavier elements = σ-bonds only

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9
Q

kinetically labile

A

low energy decomposition energy

highly reactive

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10
Q

how can kinetic lability be suppressed?

A

[increased thermal stability]

via formation of Lewis base adduct or by use of ligands without hydrogens in the position (no β-hydrogen)

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11
Q

3 factors that govern the kinetic reactivity of main group organometallic with water and air

A
  1. high polarity of M-C bond (kinetic)
  2. availability of low-lying empty orbitals at metal (kinetic)
  3. availability of free electron pairs at metal (kinetic)
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12
Q

group 13 + air/water

A

vacant p orbital

water/air sensitive

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13
Q

group 14 + air/water

A

electron precise

water/air stable

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14
Q

group 15 + air/water

A

lone pair

water/air sensitive

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15
Q

M-C bond formation - direct synthesis from metal and organic halide

A

2M + nR-X -> RnM + MXn or RnMXn

useful for electropositive metas and alkyl magnesium

= OXIDATIVE ADDITION

driven by enthalpy of formation of M-X bond or MXn salt

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16
Q

M-C bond formation - redox transmetallation

A

M + RM’ -> RM + M’

metal of lower electronegativity (M) replaces metal of higher electronegativity (M’)

M = electropositive group 1,2,13 metal
M’ = Hg

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17
Q

M-C bond formation - metallisation (metal-hydrogen exchange)

A

R-M + R’-H -> R’-M + R-H

driving force = relative acidities of the 2 C-H acids involved (RH and R’H)

forward reaction will proceed if pKa of R’H is lower than RH

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18
Q

M-C bond formation - salt metathesis

A

R-M + M’-X -> R-M’ + MX

important for generation of p-block M-C bonds

driving force = formation of salt with high lattice enthalpy

predicted by electronegativity of 2 metals involved
- if M’ = more electronegative than M; reaction = feasible

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19
Q

M-C bond formation - addition reactions

A

[hydrometallation]

common for B, Al, Si
driving force = formation of C-H bond from weaker M-H bond

[carbometallation]

only occurs with v. electropositive metal

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20
Q

lithium + carbon - electronegativity

A

large difference

BUT Li-C bonds = somewhat covalent due to polarising ability of Li+

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21
Q

LiR formation - direct synthesis

A

2Li + RX -> LiR + LiX

high enthalpy of formation of LiX salt = exothermic reaction

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22
Q

LiR formation - transmetallation

A

2Li + HgR2 -> 2LiR + Hg

based upon relative ease of reduction of Hg and Li

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23
Q

LiR formation - metal-halogen exchange

A

[at low temp.]

Li-R + R’-X -> Li-R’ + R-X

if carbanion formed = more stable, equilibrium will be driven to the right

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24
Q

Wurtz coupling process

A

R’Li + R’‘X -> R’R’’ + LiX

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25
LiR formation - metallation of acidic hydrocarbons with organolithiums
[equilibrium reaction] - position depends on relative pKa of the 2 C-H involved -> stronger C-H acid will promote higher yield of metallation by lithium salt of weaker acid R'-Li + R''H -> R'-H + R''-Li
26
organolithium structures
2 valence electrons - electron-deficient compensated by formation of multi centre bonds (aggregation) tend to form oligomeric aggregates in solution + solids
27
what does the degree of association of organolithium compounds in solution depend on?
polarity coordinating ability of solvent
28
Li-CH3 in THF or Et2O
tetramer
29
Li-CH3 in TMEDA
dimer
30
Li-n-C4H9 in hexane
hexamer
31
Li-n-C4H9 in Et2O
tetramer
32
Li-t-Bu in hexane
tetramer
33
Li-C6H5 in THF or Et2O
dimer
34
Li-CH2C6H5 in THF or Et2O
monomer
35
what happens to the reactivity as the polar nature of Li-C increases?
reactivity increases
36
reactivity of organolithiums - metallation of CH, NH, OH
[i.e. as a base] EH + RLi -> ELi + RH EH = stronger acid than RH
37
reactivity of organolithiums - addition to multiple bonds
[i.e. as a nucleophile] uncompleted R-Li only read with conjugated dienes and styrene derivatives to provide polymers on addition of TMEDA -> nBuLi can initiate polymerisation of ethene
38
reactivity of organolithiums - reaction with transition metal/p-block halides
[produces salt] e.g. 4PhLi + SiCl4 -> SiPh4 + 4LiCl salt metathesis = thermodynamic driving force
39
heavier alkali metal organometallics
synthesis similar to alkyl lithiums e.g. metallation of R-X 2Na/K + RX -> MR + MX most convenient syntheses are by metathesis reactions of RO-M
40
polyhapto n^n interactions
organic anion with metal via >1 interaction
41
group 2 oxn states and general properties
[alkaline earth metals] +2 ionic radii smaller than group 1 in same period all elements (M0) highly electropositive/reducing
42
what happens to the reactivity of the M-C bond with increasing electropositivity of the metal?
becomes more reactive
43
Be
v. small and polarisable most toxic non-radioactive element max. coordination number = 4 organo derivatives tend to form 3-centre-2-electron bonds (divalent compounds)
44
Grignard reagents
R-X + Mg -> RMgX usually formed from alkyl/aryl halide and Mg in solvent such as diethyl ether (= exothermic) = oxidative addition reaction to the metal slow reaction to initiate - due to passivating oxide layer on Mg surface
45
Rieke method
= extremely reactive form of Mg MgCl2 + 2K ----> Mg + 2KCl -> (RX) RMgX finely divided (high SA) black powder
46
Grignard reagent structures - solid state
solvated by ether (4 coordinate Mg centres) less heavily solvated oligomers feature halide bridges
47
dialkylmagnesium compounds - synthesis
[redox transmetallation of Mg metal with diorganomercury compounds] Mg + R2Hg -> R2Mg + Hg
48
dialkylmagnesium - bonding
sp3 hybridised bridging methyl group 3-centre-2-electron bonds to sp3 metal
49
R2Mg + water/protic reagents
[eliminates alkanes] R2Mg + R'OH -> RMgOR' + RH
50
reactivity of organomagnesium compounds - alkylating/arylating for main group and transition metal halides
source of R- (from grignard) formation of magnesium dihalides = thermodynamic force e.g. SbCl3 + 3CH3MgX -> (CH3)3Sb + 3MgXCl Mg-C = less polar + reducing
51
organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba
far more reactive catalysts - sustainable, common, cheap (compared to Rhodium, Palladium, Platinum)
52
organometallic compounds of heavier alkaline earth metals - Ca, Sr, Ba - structural studies
less covalent than Mg analogues isolation of alkyl derivatives requires use of bulky alkyl ligands bent (rather than linear) - due to increased size and polarisability of metal bonding increasingly ionic as you descend group 2
53
group 13 organometallics
+3 oxn state stabilisation of lower oxn state (+1) as you go down group -> inert pair effect
54
R3B synthesis
[metathesis of a boron halide or trialkyl borate with polar source of carbanion] BF3.OEt2 + 3RMgX -> BR3 + 3MgFX/Et2O [hydroboration] 3R-HC=CH2 + BH3 -> B(CH2CH2R)3
55
R3B properties
monomeric organoboron alkoxides/amides display 2p(pi)-2p(pi) bonding stable to water due to low bond polarity oxidise readily burn in air strongly Lewis acidic (6 electron species) = use in many catalytic processes (presence of vacant p orbital)
56
R3Al lab prep
[metathesis with RLi or RMgX] AlCl3 + 3t-BuLi -> t-Bu3Al + 3LiCl [transmetallation] 2Al + 3HgPh2 -> 2AlPh3 + 3Hg **** 2:3 stoichiometry ****
57
structures of R3Al
>3 coordinate => due to electron deficiency unless R group = bulky smaller R groups dimerisation occurs via Al-C-Al 2e-3-centre bonds in condensed phase Al-C-Al bridging persists in non-donor solvents with fast Al-Me exchange (reaction tends to be labile)
58
R2AlX and RAlX2 prep.
[redistribution of trialkyls/trihalides in correct stoichiometry] 2R3Al + AlCl3 -> 3R2AlCl R3Al + 2AlCl3 -> 3RAlCl2 -> reaction driven by labile nature of compounds in solution -> @rtp -> in non-donor solvents, compounds usually oliogemers formed by X-Al-X bridging
59
reactivity of R3Al
organoaluminium compounds = hard acids (6 valence e-) readily form adducts with bases hard donors
60
role of methylaluminoxane (MAO)
acts as a co-catalyst for alkene polymerisation
61
RnMX3n-1 M = Ga, In, Tl SUMMARY
prepared the same as R3Al halides RnMX3-n most readily prepared by redistribution reactions reactivity of R3Ga and R3In similar to R3Al
62
RnMX3n-1 M = Ga, In, Tl STRUCTURAL CHEMISTRY
don't show tendency to form dimers (metals have lower Lewis acidity) R2MX compounds -> larger ln can adopt coordination numbers >4 -> leads to coordination polymers in solid state
63
preparation of thallium (I) cyclopentadienyls
[v. toxic] Tl2SO4 + 2C5H6 + 2NaOH -> 2(C5H5)Tl + Na2SO4 + 2H2O
64
what type of ligands is best for lighter group members?
bulky ligands helps prevent disproportionation of monovalent metal (M0 -> M3+)
65
group 14 organometallics
lower polarity of M-C bond RnMX4-n electron precise/rich valence shell no tendency to associate through multicentre bonding via alkyl/aryl bridges reduced reactivity towards nucleophiles water/air stable
66
what are tetravalent group 14 organometallics used for?
[technological applications] silicones organotin compounds lead alkyls
67
Rochow process
high temp. direct reaction of RX/ArX with fluidised bed of Si presence of 10% metallic Cu as catalyst 2RCl + Si/Cu ----> R2SiCl2 300°C
68
organosilanes
Si-C bonds = very thermally stable (348 kJ mol-1) don't decompose before ca. 700°C R4Si compounds = highly resistant to chemical attack air/water stable because of low polarity of Si-C bonds
69
polyorganosiloxanes (silicones)
non-degradable; self-sealing high Mr polymers (Me2SiO)n propagated by Si-O-Si linkages v. thermally and chemically stable - due to strength of Si-C and C-H bonds + Si-O-Si
70
Si-O bond enthalpy
466 kJ mol-1
71
flexibility of Si-O-Si link
delocalisation of lone pairs on oxygen into available σ* orbitals on Si reduces directionality of Si-O bond + makes structure more flexible
72
heavier group 14 (Ge, Sn, Pb) organometallics
+4 stabilisation of lower oxn state as you go down group inert pair effect -> tendency of 6s2 electrons remained paired
73
organogermanium compounds
similar to silicon compounds - due to shielding effect of filled 3d shell much less common -> low terrestrial abundance of the element
74
R4Sn structure
tetrahedral - sp3 air/moisture stable
75
R3SnX
more electronegative groups increases Lewis acidity higher coordination derivatives result from coordination by bases in absence of external bases -> polymeric structures are common = larger = increased Lewis acidity of metal cation
76
organolead compounds
Pb(II) = most common Pb-C = sig. weaker than Sn-C bond = less thermally stable main application = Et4Pb (endothermic but kinetically stable) decomposes on heating by Pb-C homolysis as source of ethyl radicals petrol additive ("anti-knock agent") to prevent premature ignition in internal combustion engines
77
group 15 (P, As, Sb, Bi) organometallics
+3 and +5 stabilisation of +3 increases as you go down group -> inert pair effect (tendency of the ns2 electrons remain paired)
78
group 15 R3E compound shape
pyramidal with stereochemically active lone pair
79
σ-donor capacity of phosphine ligands
PH3 < PRH2 < PR2H < PR3 due to inductive effect of alkyl sub.
80
π-donor capacity of phosphine ligands
[depends on availability of low energy σ* orbitals] PF3 > PCl3 > P(OAr)3 > P(OR)3 > PAr3 > PAr2R > PR3
81
R5E shape
trigonal bipyramid in solid state EXCEPTION = pentaphenylstiborane, Ph5Sb = pyramidal
82
phosphazenes
family of compounds constructed from P-N linkages
83
phosphonitrilic dichloride (PNCl2)3
earliest reported inorganic heterocycle nPCl5 + nNH4Cl -> (NPCl2)3-10 + 4n HCl 130°C
84
phosphonitrilic dichloride (PNCl2)3 - structure
[cyclic trimer] planar six-membered ring all P-N bonds = identical (shorter than normal => suggested multiple/delocalisation)

Inorganic Chemistry (15 decks)

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