Molecular shapes and orbitals Flashcards
chemical bond
link between 2 atoms
forms if resulting arrangement of the nuclei and their e- has lower energy than total energy of separate atoms (i.e. more energetically favourable)
name of group 6 elements
chalcogens
types of bonding
- ionic
- covalent
- metallic
types of bonding models
- valence bond theory (localised)
- molecular orbital theory (delocalised)
why does hypervalence exist?
due to free d-orbitals
dative bond
one atom donating a lone pair of e- in one orbital into a vacant orbital on another atom
resonance
e- in molecules aren’t always localised to specific bonds
Lewis structure preducts 1 double bond and 2 single bonds (actually both lengths are equal + charges = shared)
used to explain that actual structure is a hybrid of all possible forms
charge = delocalised
which position would double bonds occupy - axial or equatorial?
equatorial - minimises space and repulsion
which occupy more space - electronegative sub. or electropositive sub.?
bonded pairs to electropositive substituents
stereochemically inactive lone pair
occurs if l.p. is in an s-orbital and is evenly distributed around the molecule (∴ has no impact on shape)
possible structures with 5 valence e-
- trigonal bipyramidal - more likely
- square based pyramid
fluxional
isomers that can undergo dynamic interchanges between atoms in symmetry-equivalent positions
which position do lone pairs always occupy?
equatorial
Berry pseudorotation
the process which scrambles axial and equatorial ligands on a trigonal bipyramidal
how to prevent flux between molecules?
reaction @ low temp. (freeze it out)
product = only thermodynamic one (might not be most kinetically stable one)
VSEPR - single/multiple bonds
indistinguishable
multiple bond treated as single bond of higher e- density
electronegativity + effect of bond polarity on BP/LP in orbital arrangements
more polar bonds concentrate e- density at one atom or the other
also influence e- repulsion
e.g. e- density closer to F than Cl (more electronegative) - make it easier to push bonding pairs together to maximise BP-LP repulsion
order of repulsion power
LP-LP (strongest)
LP - double bond
LP - single bond
double bond-double bond
double bond-single bond
single bond-single bond (weakest)
rotation by 180°
C2 rotation
rotation by 120°
C3 rotation
rotation by 60°
C4 rotation
rotation by 90°
C6 rotation
transition metal
metal in an element in which the valence e- = d electrons
“d-block” or “transitional”
elements with partially full or full d-shells
Haemoglobin
O2 transport
contains dicationic Fe2+ metal centre surrounded by nitrogen-based ligand (= haem ligand)
complex binds via l.p. on oxygen to form saturated coordination Fe2+ complex
CO and CN- are good ligands for Hb and compete for Fe2+ site (∴ toxic)
ligand
group or atom that gives metal centre pair of e-, normally available via l.p.
metal ion = Lewis acid (acceptor)
ligand = Lewis base (donator)
monodentate ligand
only binds to metal through 1 l.p. of electrons
polydentate ligand
binds to metal via 2 or more lone pairs
shape of transition metals
d-block elements have 9 orbitals available - VSEPR doesn’t work
shape dictated by no. of coordinated atoms around metal
chiral molecules
non-superimposable mirror images (related by enantiomers)
coordination isomers
ligands attached to each of the 2 metals are different
linkage isomers
involve ambidentate ligands
M-NCS or M-SCN
polymerisation isomers
same forumla, different molar mass (same empirical formula)
stability constant and hard/soft acids + bases
if Kf is large = ligand binds more tightly to metal centre than H2O
if Kf is small = ligand binds less tightly to metal centre than H2O
chelate effect
complex containing one or more 5/6 membered chelate rings is more stable (stability constant = larger) than one without chelate ring
due to thermodynamics
choose polydentate ligand over monodentate
larger log(β) value
chelate effect + ring size
decreases as ring size increases
chance of “dangling arm” binding to metal becomes smaller
4 membered ring = 5-membered ring > 6-membered ring > 7-membered ring
early transition metals (Ti, V, Cr) + halides
complexing strength = F-»_space; Cl- > Br- > I-
later transition metals + halides
complexing strength = I-»_space; Br- > Cl- > F-
hard cations
high density charge
less polarisable
form more stable complexes with hard ligands
soft cations
low density charge
more polarisable
form more stable complexes with soft ligands
hard acids
prefer to complex with hard bases
soft acids
prefer to complex with soft bases
do hard-hard or soft-soft interactions have greater electrostatic/ionic contribution?
hard-hard
do hard-hard or soft-soft interactions have greater orbital/covalent interactions (overlap)?
soft-soft
trend in hardness down a group
decreases
orbitals get larger + more diffuse
electronegativity decreases
influence of electronegativity on hardness/softness of an element
more electronegative = harder (less polarisable)
influence of size on hardness/softness of an element
smaller element/ion = harder
