electron count oxidation state coordination number
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Electron Count Oxidation State Coordination Number. Basic tools for understanding structure and reactivity. Doing them should be “automatic”. Not always unambiguous Þ don’t just follow the rules, understand them!. The basis of counting electrons. - PowerPoint PPT PresentationTRANSCRIPT
04/22/23 Counting Electrons 1
Electron CountOxidation State
Coordination Number• Basic tools for understanding
structure and reactivity.• Doing them should be “automatic”.• Not always unambiguous don’t just
follow the rules, understand them!
04/22/23 Counting Electrons 2
• Every element has a certain number of valence orbitals:1 (1s) for H4 (ns, 3np) for main group elements9 (ns, 3np, 5(n-1)d) for transition metals
The basis of counting electrons
pxs py pz
dxzdxy dx2-y2dyz dz2
04/22/23 Counting Electrons 3
• Every orbital wants to be “used", i.e. contribute to binding an electron pair.
• Therefore, every element wants to be surroundedby 2/8/18 electrons.
• The strength of the preference for electron-precise structures depends on the position of the element in the periodic table.
The basis of counting electrons
04/22/23 Counting Electrons 4
• Too few electrons:An empty orbital makes the compound very electrophilic,i.e. susceptible to attack by nucleophiles.
• Too many electrons:There are fewer covalent bonds than one would think (not enough orbitals available). An ionic model is required to explain part of the bonding. The "extra" bonds are relatively weak.
• Metal-centered (unshared) electron pairs:Metal orbitals are fairly high in energy. A metal atom with a lone pair is a strong -donor (nucleophile) and susceptible to electrophilic attack.
The basis of counting electrons
04/22/23 Counting Electrons 5
H2
Every H has 2 e. OK
CH4
H has 2 e, C 8. OK
NH3
N has 8 e. Nucleophile! OK
Use a localized (valence-bond) modelto count electrons
H H
C
H
H
H
H
N
H
H
H
04/22/23 Counting Electrons 6
C2H4
C has 8 e. OK
singlet CH2
C has only 6 e, and an empty pz orbital: extremely reactive ("singlet carbene"). Unstable. Sensitive to nucleophiles and electrophiles.
triplet CH2
C has only 6 e, is a "biradical" and extremely reactive ("triplet carbene"), but not especially for nucleophiles or electrophiles.
C
H
H
C
H
H
C
H
H
C
H
H
04/22/23 Counting Electrons 7
CH3+
C has only 6 e, and an empty pz orbital: extremely reactive. Unstable. Sensitive to nucleophiles.
CH3-
C has 8 e, but a lone pair. Sensitive to electrophiles.
Cl-
Cl has 8 e, 4 lone pairs. OK Somewhat sensitive to electrophiles.
C
H
H
H
C
H
H
H
Cl
04/22/23 Counting Electrons 8
BH3
B has only 6 e, not stable as monomer,forms B2H6:
B2H6
B has 8 e, all H's 2 (including the bridgingH!). 2-electron-3-center bonds! OK
AlCl3
Al has only 6 e, not stable as monomer,forms Al2Cl6:
Al2Cl6
Al has 8 e, all Cl's too (including thebridging Cl!). Regular2-electron-2-center bonds! OK
B
H
H
H
B
H
H
B
H
H
H
H
Al
Cl
Cl
Cl
Al
Cl
Cl
Cl
Al
ClCl
Cl
04/22/23 Counting Electrons 9
2 MeAlCl2 Me2Al2Cl4
2-electron-3-center bonds are a stopgap!
H3B·NH3
N-B: donor-acceptor bond (nucleophile NH3 has attacked electrophile BH3).
Organometallic chemists are "sloppy" and write . Writing or would be more correct (although the latter does not reflect the “real” charge distribution).
Al
Cl
Cl
Cl
Al
MeCl
Me
Al
Cl
Cl
Al
Cl
Cl
Me
Me
B
H
H
H
N
H
H
H
B NH
H
H
H
H
H B NH
H
H
H
H
H B NH
H
H
H
H
H
04/22/23 Counting Electrons 10
PCl5
P would have 10 e, but only has 4 valence orbitals, so it cannot form more than 4 “net” P-Cl bonds.You can describe the bonding using ionic structures (hyperconjugation).Easy dissociation in PCl3 en Cl2.
HF2-
Write as FH·F-, mainly ion-dipole interaction.
?
P
Cl
Cl
Cl
Cl
Cl
P
Cl
Cl
Cl
Cl
Cl
?F H F F H F
04/22/23 Counting Electrons 11
1. Number of valence electrons(from periodic table)
2. Correct for charge, if any(only if it belongs to that atom!)
3. Count 1 e for every covalent bond to another atom4. Count 2 e for every dative bond from another
atom5. Add
How do you count?
04/22/23 Counting Electrons 12
B = 3- = 14H = 4tot = 8OK
Examples: counting electronsBH4
- B HH
H
H
H2CO C OH
H
C = 41=O= 22H = 2tot = 8OK
Pd =10- = 13Cl = 31NH3 = 2tot =16could have additional 2 e(Pd-Cl -bond?)
Cl Ru ClMe3P
PMe3
CH2
Ru = 82Cl = 22PMe3= 41CH2 = 2tot =16could have additional 2 e
PdCl3(NH3)-
Cl Pd NH3Cl
Cl-
04/22/23 Counting Electrons 13
Counting is not always trivial
Pd =102- = 23Cl = 31CH2= 1tot =16could have additional 2 e
Cl PdCl
Cl
NH3
Cl PdCl
Cl
NH3
-
is
04/22/23 Counting Electrons 14
• Odd electron counts are rare.• In reactions you nearly always go from even to
even (or odd to odd), and from n to n-2, n or n+2.• Electrons don’t just “appear” or “disappear”.• The optimal count is 2/8/18 e. 16 e also occurs
frequently, other counts are much more rare.
Remember, when counting:
04/22/23 Counting Electrons 15
Most elements have a clear preference for certain oxidation states. These are determined by (a.o.) electronegativity and the number of valence electrons:
Li: nearly always +1.Has only 1 valence electron, so cannot go higher. Is very electropositive, so doesn’t want to go lower.
Cl: nearly always -1.Already has 7 valence electrons, so cannot go lower.Is very electronegative, so doesn’t want to go higher.
Oxidation States
04/22/23 Counting Electrons 16
1. Start with the formal charge on the metal2. Ignore dative bonds3. Ignore bonds between atoms of the same element
(this one is a bit silly)4. Assign every covalent electron pair to the most
electronegative element in the bond: this produces + and – charges (usually + at the metal)
5. Add
Calculating theformal oxidation state
04/22/23 Counting Electrons 17
charge C =04C-Cl: C+-Cl- =+4tot =+4
Examples: oxidation statesCCl4
COCl2 charge C= 02C-Cl: C+-Cl- =+21C=O: C2+-O2- =+2tot =+4
AlCl4- charge Al= -14Al-Cl: Al+-Cl- = +4tot = +3
MnO4-
charge Mn = -14Mn=O: Mn2+-O2- = +8tot = +7
PdCl42- charge Pd= -24Pd-Cl: Pd+-Cl- = +4tot = +2
04/22/23 Counting Electrons 18
charge C = 03C-Cl: C+-Cl- =+3tot =+3trivalent carbon ?
Examples: oxidation states
charge Mg = 04Mg-Me: Mg+-Me- = +4tot = +4impossible, Mg has only 2 valence electrons!
charge Pt = -23Pt-Cl: Pt+-Cl- = +3tot = +1univalent Pt ?
C2Cl6Pt2Cl64-
MgMe4
04/22/23 Counting Electrons 19
Oxidation states are formal.However, they do give an indication whether a
structure or composition is reasonable (apart from the M-M complication).
The significance ofan oxidation state ?
04/22/23 Counting Electrons 20
For group n or n+10:– never >+n or <-n (except group 11: frequently +2 of +3)– usually even for n even, odd for n odd– usually 0 for metals– usually +n for very electropositive metals– usually 0-3 for 1st-row transition metals of groups 6-11, often
higher for 2nd and 3rd row– electronegative ligands (F,O) stabilize higher oxidation states, -
acceptor ligands (CO) stabilize lower oxidation states– oxidation states usually change from m to m-2, m or m+2 in
reactions
Acceptable oxidation states
04/22/23 Counting Electrons 21
Simply the number of atoms directly bonded to the atom you are interested in, regardless of bond orders etc.
CH4: 4C2H4: 3C2H2: 2AlCl4
-: 4Me4Zn2-: 4OsO4: 4
Coordination number
B2H6: 4 (B)1 (terminal H)2 (bridging H)
04/22/23 Counting Electrons 22
For complexes with -system ligands, the whole ligand is usually counted as 1:
Cyclopentadienyl groups are sometimes counted as 3,because a single Cp group can replace 3 individual ligands:
Coordination Number
Cl PdCl
Cl-
ZrClCl C.N. 4
H
CoCOOC
COCO
CoOC CO
04/22/23 Counting Electrons 23
The most common coordination numbers for organometallic compounds are:
2-6 for main group metals4-6 for transition metalsCoordination numbers >6 are relatively rare. So are
very low coordination numbers (<4) together with a “too-low” electron count.
Coordination Number
04/22/23 Counting Electrons 24
C.N. "Normal" geometry2 linear or bent3 planar trigonal, pyramidal, "T-shaped"4 square planar, tetrahedral5 square pyramid, trigonal bipyramid6 octahedron
Coordination number and coordination geometry
04/22/23 Counting Electrons 25
Could WH6(PMe3)3 be ?Count W: 18 VE (OK), oxidation state 6 (OK), coordination number 9 (very high). Possible.
Protonation gives WH7(PMe3)3+.
Could that be ?Count W: 18 VE (OK), oxidation state 8 (too high), coordination number 10 (extremely high). W+ must form 7 covalent bonds using only 5 electrons. That will not work!
Illustration:protonation of WH6(PMe3)3
W PMe3Me3PMe3P HH H
HHH
W PMe3Me3PMe3P HH H
HHH
H
+
04/22/23 Counting Electrons 26
Give electron count and oxidation state for the following compounds. Draw conclusions about their (in)stability.
Me2Mg Pd(PMe3)4 MeReO3
ZnCl4 Pd(PMe3)3 OsO3(NPh)ZrCl4 ZnMe4
2- OsO4(pyridine)Co(CO)4
- Mn(CO)5- Cr(CO)6
V(CO)6- V(CO)6 Zr(CO)6
4+
PdCl(PMe3)3 RhCl2(PMe3)2
Ni(PMe3)Cl4 Ni(PMe3)Cl3 Ni(PMe3)2Cl2
Exercises
Cl PdMe3P
PMe3
BMe3
-