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Inorganic Chemistry > Organometallics [Andy Johnson] > Flashcards

Organometallics [Andy Johnson] Flashcards

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1
Q

early transition metals - groups 3,4

A

labile - anion and cation exchange easily

high Zeff

strongly electrophilic and oxophilic

few redox reactions

polar and v. reactive M-C bonds

preference for “hard” σ-donors (non-polarisable)

weak complexation of π-acceptors

typical catalysis = polymerisation

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2
Q

middle transition metals - groups 5-7

A

ligands bound strongly

strong, not v. reactive M-C bonds

preference for σ-donor/π-acceptor combinations

typical catalysis = alkene and alkyne metathesis

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3
Q

late transition metals - groups 8 - 11

A

easy ligand association/dissociation

weak - not v. reactive M-C bonds

even weaker/reactive M-O/M-N bonds

preference for s-donor/weak π-acceptor ligands

typical catalysis = hydroformylation

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4
Q

role of catalyst

A

lowers activation barrier

by introducing a new reaction pathway

** doesn’t change thermodynamics **

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5
Q

why can sigma, pi and delta bonds be treated separately?

A

they overlap between different classes of orbitals (i.e. net overlap = 0)

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6
Q

criteria for strong orbital interactions

A

correct symmetry

spatial overlap - must occupy same region in space

similar energy

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7
Q

why can’t pi bonding occur in M-C alkyl bonds?

A

all available orbitals on C are involved in C-H/C-R bonding

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8
Q

why can’t pi bonding occur in M-C aryl bonds?

A

it would disrupt the aromaticity of the phenyl ring

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9
Q

what is the most stable state in an 18 e- system?

A

18 e- involved in bonding orbital

0 e- involved in anti-bonding orbital

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10
Q

L ligand

A

both electrons provided by ligand

= dative covalent bond

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11
Q

X ligand

A

one electron provided by ligand and by metal

= normal covalent bond

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12
Q

Z ligand

A

both electrons provided by metal

= dative covalent bond

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13
Q

metallo

A

[metal separate from ring]

metal “on” rather than “in”

e.g. ferrocene

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14
Q

metalla

A

metal “in” ring

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15
Q

oxidation state (in terms of X ligands)

A

(number of X ligands) + (charge on complex)

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16
Q

bridging ligands

A

halides often bridge between 2 metal centres using a lone pair

= LX

X = sigma bond
L = donor bond

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17
Q

metal-metal bonds

A

[doesn’t add to oxidation state]

assign one of the electrons of the electron pair in a single to each metal centre

double, triple, quadruple -> 2,3,4 respectively

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18
Q

NO molecules

A

radical

BENT - acts as a 1 electron X ligand (l.p. on nitrogen not used)

LINEAR - l.p. now can be used (LX)

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19
Q

electronic effects

A

late TM d8 -> square planar

d10 -> trigonal

-as Z increases, d-shell is stabilised
-occupied dx2 orbital (perpendicular to plane) no longer involved in ligand bonding

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20
Q

steric effects

A

early TM have less d-electrons
-> must achieve 18 e- count via larger ligands

if ligands = too bulky, low-electron count complexes are formed

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21
Q

back-bonding

A

ligand donates sigma/non-bonding e- to metal while accepting e- density through overlap of metal t2g orbital and ligand π*

^ ligand = both sigma donor and π acceptor

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22
Q

what happens as the amount of pi e- density donated into pi* ligand increases?

A

more backhanding/electron density in ligand anti bonding orbital

weakens CO bond

bond lengthens

lowers CO stretching frequency

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23
Q

why is there an increase in CO frequency/bond order across the periodic table group?

A

d orbital energies decrease/increase in electron density on M

energies of dπ and π* orbitals separate

back bonding becomes worse

24
Q

non-classical carbonyls

A

major interaction = σ-donation from CO 5σ orbital (anti-bonding, l.p. on CO) to metal

stretching frequency > free CO

25
Q

relationship between wave number and bond length

A

wave number is proportional to 1/bond length

26
Q

how many electrons does CO contribute?

A

2e- whether terminal or bridging

27
Q

how can we fix a molecule which is coordinatively unsaturated?

A

M-M bonds

28
Q

what type of ligand are phosphine ligands?

A

L type

29
Q

2 important parameters of PR3 ligands

A

[electronic] - donor and acceptor strength

-> change R groups

[sterics]

size of sub. -> Tolman Cone Angle

30
Q

what do phosphine ligands primarily function as?

A

Lewis bases

interact with metals as σ donor ligands

31
Q

phosphine ligands - electronics

A

e- density on P is low when R group contains EWG

e- density on P is high when R group contains EDG

32
Q

why can’t NR3 participate in back-bonding?

A

strong σ-donors BUT have no accessible molecular orbitals available with correct symmetry

33
Q

PR3, AsR3, SbR3

A

descending 15 σ* orbitals which bond R groups to P, As and Sb -> get lower in energy

makes it more available

34
Q

why does M-P bond lengths increase and P-R bond lengths decrease upon oxidation?

A

oxidation decreases ability of metal to backbone -> removes e- density from metal
=> would cause M-P bond length to increase

σ*P-R orbitals are used in back-bonding -> stops elongating with less back-bonding

35
Q

CO stretching frequency trend

A

more e- density on metal

= more back bonding onto CO

= greater occupancy of π* CO

= more electron density on M

= lower CO stretching frequency

36
Q

what does wider cone angles indicate?

A

greater steric congestion

= more labile phosphine ligand

37
Q

how does V(CO) change as the electron-donating ability of the phosphine ligand increases?

A

strongly donating = more e- rich metal

better at CO back-bonding

= lower V(CO) due to decreased C-O bond order

38
Q

C-C π to empty metal orbital (σ)

A

e- density moves from bonding orbital to empty metal orbital

depopulation of π orbital = C-C bond length increasing

M-C = SHORTER

39
Q

occupied metal d -> empty C-C π*

A

e- density moves from full M orbital to anti-bonding C-C orbital

population of π* orbital leads to C-C length increases

M-C = SHORTER

40
Q

factors which favour π-back bonding

A

[make M e- rich]

strong donor ligands on M

negative charge on M

low oxn state on M

41
Q

how does back bonding affect alkene reactivity?

A

back bonding reduces δ+ charge

reduces reactivity to nucleophiles

42
Q

how do compounds relieve strain?

A

hybridisation

sp2 -> sp3 = less strain

43
Q

metal alkynes

A

more electronegative

encourage back bonding

bind more strongly to metals

44
Q

activation

A

cleavage of organic bonds by metal centres

produces more reactive form of organic species

45
Q

oxidative addition - favoured conditions

A

favoured with more electron rich metal centres

-> low oxidation states
-> negative charge
-> ED CO-ligands

46
Q

PPh3 vs P(OPh)3

A

[PPh3]
- stronger σ donor
- more e- density on metal
- strong Co-H

[P(OPh)3]
- better π-acceptor (electronegative group)
- more back bonding
-less e- density on metal
- Co-H = weaker and more acidic

47
Q

M-C bond enthalpy

A

decreases with atomic number

increases within a TM triad

48
Q

sigma bond metathesis compared to oxidative addition/reductive elimination

A

sigma bond metathesis -> no change in oxidation state

oxidative addition/reductive elimination -> 2 electron change in oxidation state

49
Q

problems for C-H activation - THERMODYNAMICS

A

ΔS - disfavoured entropically (2 -> 1 molecules)

ΔH - must be sufficiently exothermic to overcome loss of entropy

50
Q

H-H vs CH3-H

A

H-H and CH3-H have similar strengths
-> weaker M-alkyl bonds disfavour C(sp3)-H activation

51
Q

Ph-H vs CH3-H

A

energy required to break the stronger Ph-H is outweighed by strong M-Ph and M-H bonds in the product

-> Ph-H bond activation much easier than sp3-C-H activation

52
Q

what is a negative ΔS of activation indicative of?

A

associative mechanism

53
Q

what is a positive ΔS of activation indicative of?

A

dissociative mechanism

54
Q

what are Fischer carbenes direct analogues of?

A

reactivity of carboxylic acids

both have electrophilic carbon centres with leaving groups

55
Q

sterically crowded early TM alkyls

A

undergo alpha-elimination readily to give nucleophilic carbenes and alkanes

56
Q

what promotes carbene formation?

A

increasing steric strain in metal complex

Inorganic Chemistry (15 decks)

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