Organometallics [Andy Johnson] Flashcards
early transition metals - groups 3,4
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
middle transition metals - groups 5-7
ligands bound strongly
strong, not v. reactive M-C bonds
preference for σ-donor/π-acceptor combinations
typical catalysis = alkene and alkyne metathesis
late transition metals - groups 8 - 11
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
role of catalyst
lowers activation barrier
by introducing a new reaction pathway
** doesn’t change thermodynamics **
why can sigma, pi and delta bonds be treated separately?
they overlap between different classes of orbitals (i.e. net overlap = 0)
criteria for strong orbital interactions
correct symmetry
spatial overlap - must occupy same region in space
similar energy
why can’t pi bonding occur in M-C alkyl bonds?
all available orbitals on C are involved in C-H/C-R bonding
why can’t pi bonding occur in M-C aryl bonds?
it would disrupt the aromaticity of the phenyl ring
what is the most stable state in an 18 e- system?
18 e- involved in bonding orbital
0 e- involved in anti-bonding orbital
L ligand
both electrons provided by ligand
= dative covalent bond
X ligand
one electron provided by ligand and by metal
= normal covalent bond
Z ligand
both electrons provided by metal
= dative covalent bond
metallo
[metal separate from ring]
metal “on” rather than “in”
e.g. ferrocene
metalla
metal “in” ring
oxidation state (in terms of X ligands)
(number of X ligands) + (charge on complex)
bridging ligands
halides often bridge between 2 metal centres using a lone pair
= LX
X = sigma bond
L = donor bond
metal-metal bonds
[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
NO molecules
radical
BENT - acts as a 1 electron X ligand (l.p. on nitrogen not used)
LINEAR - l.p. now can be used (LX)
electronic effects
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
steric effects
early TM have less d-electrons
-> must achieve 18 e- count via larger ligands
if ligands = too bulky, low-electron count complexes are formed
back-bonding
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
what happens as the amount of pi e- density donated into pi* ligand increases?
more backhanding/electron density in ligand anti bonding orbital
weakens CO bond
bond lengthens
lowers CO stretching frequency
why is there an increase in CO frequency/bond order across the periodic table group?
d orbital energies decrease/increase in electron density on M
energies of dπ and π* orbitals separate
back bonding becomes worse
non-classical carbonyls
major interaction = σ-donation from CO 5σ orbital (anti-bonding, l.p. on CO) to metal
stretching frequency > free CO
