Transcript Group 14 - University of Ottawa

Orbital energies in Group 14
the radii of ns and np (n = 3–6)
orbitals of the heavier congeners Si–
Pb differ considerably - orbital mixing
in these elements is more difficult
the valence s electrons become
increasingly lone pair in character
This is the rationalization for
decreased multiple bonding.
Can one make a Group 14 triple bond?
stable alkyne congeners RMMR (M = Si–Pb) – (1) greater
steric requirements for the R group since each element has
only one substituent, (2) scarcity of suitable precursors that
could be smoothly converted to stable RMMR molecules.
Remember Gallium
i
i
Pr
i
two sp 2 hybridized Ga(I)R fragments
Pr
i
Pr
donor bond
Pr
R
i
Pr
i
??
Ga
i
i
Pr
Pr
Ga
i
R
Pr
i
Pr
Pr
donor bond
p-p bond
i
Pr
i
Pr
Structure of Si2H2
Microwave spectrum of SiH4 plasma at –196℃ indicated
an unusual structure for Si2H2
The energetic array of E2H2
H
E
E
H
H
E
E
H
H
E
E
H
H
E
E
H
s2p1p1 configuration indicates
low tendency to hybridize
Theory
Possible interaction modes of two SiH units
electronic and steric effects of substituents are very important
Electropositive silyl groups stabilize disilynes. Therefore, proposed bulky silyl
groups, such as SiTbt3 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl)
Transient Si-Si triple bond
Inserts into C-C bonds – ligand design will play a role in
advancing this area
Organometallics 2000, 19, 2724-2729
Metal- E Triple Bonds?
Metal- E Triple Bonds?
Loss of CO and NaCl
Mo–Ge–C interligand angle
of 172.2(2)°.
Mo–Ge bond length of
2.271(1) Å (ca. 2.65 Å for
single)
J. Am. Chem. Soc. 2000, 122, 650-656
Metal- E Triple Bonds?
J. Am. Chem. Soc. 2000, 122, 650-656
Synthesis of a W-Ge triple bond?
Exploits the thermal elimination of N2 from trans[W(dppe)2(N2)2]
Formal oxidative addition of Ge-X bond and
“reorganize” the electrons
Angew. Chem, Int. Ed. 2000, 39, 2778.
Structure of the Germylyne
Complex
Trans configuration of the
Ge/Cl
W-Ge bond 2.302(1) Å
(single bond length 2.4932.681 Å)
Angle at Ge = 172.2(2) deg.
Theory suggests a similar s
donor to carbyne and
comparable p acceptor
How would one make RMMR (M = Si–Pb)?
Reduction of Sn(Cl)Ar* (Ar*
= C6H3-2,6-Trip)
Leads to single and double
reduced compounds not
neutral!
R-M-M angles range from
93-107 deg.
It was found that the more
soluble, neutral Ar*MMAr* (M =
Ge or Sn) species could also be
obtained as red or green crystals
once the monoanion salts had
been removed.
Single-bonded valence isomer
of neutral
The first neutral RMMR
Pb–Pb bond length, 3.1881(1) Å
trans-bent CPbPbC with Pb–Pb–
C angle, 94.26(4)°
Pb-Pb in diplumbanes usually in
the range 2.85–2.95 Å.
isolated as amber-green
dichroic crystals in ca. 10%
yield by this route.
Owing to the near 90° Pb–Pb–
C angle, the structure of
Ar*PbPbAr* corresponds to a
diplumbylene (rather than a
diplumbyne species)
Modifying the ligand
Take off the para group on
the flanking aromatic rings
Bonding Models
Triple bond
At 90° undoes the two dative
interactions
This leads to a single p-bond when the transbending is 90°- WHAT?
Bonding Models
Another MO model:
mixing of M–M s* and p
levels to give a molecular
orbital that basically
nonbonding. Stabilizes the
original p orbital but
weakens the p bond! M–M
bond reduced.
Also models how a triple bond can be transformed into a sbond with lone pairs at metal when bending the geometry
through 90°.
The orbital mixing is possible since the energy levels are closer
to each other in the heavier elements as a result of weaker M–
M bonds
Two reviews on multiple bonding:
Power, J. Chem. Soc., Dalton Trans., 1998, 2939
Power, Chem. Commun., 2003, 2091
Si-Si triple bonds
Disilyne: emerald-green crystals (73%) and
stable up to 127°C.
SiSi triple-bond length of 2.0622(9) Å (SiSi
double-bond 2.14 Å and average Si-Si singlebond length of 2.34 Å) trans-bent with a bond
angle of 137.44(4)°
Sterically protected by extremely bulky
substituent groups. Also electropositive (recall
early slide)
the two Si-Si p bonds are not equivalent
Sekiguchi et al Science 2004, 305,1755