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
iPr
iPr
iPr
iPr
iPr
iPr
Ga
iPr
iPr
iPr
iPr
iPr
iPr
Ga
??
R
R
p-p bond
donor bond
donor bond
two sp2 hybridized Ga(I)R fragments
Structure of Si2H2
Microwave spectrum of SiH4 plasma at –196℃ indicated an unusual structure for Si2H2
The energetic array of E2H2
E E
H
H
E E
H
E E
H
H
E E
H
H
H
s2p1p1 configuration indicates low tendency to hybridize
TheoryPossible 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 bondInserts 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?
J. Am. Chem. Soc. 2000, 122, 650-656
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)
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 2.493-2.681 Å)
Angle at Ge = 172.2(2) deg.
Theory suggests a similar donor to carbyne and comparable 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.
Single-bonded valence isomer of neutral
It was found that the more soluble, neutral Ar*MMAr* (M =Ge or Sn) species could also be obtained as red or green crystalsonce the monoanion salts had been removed.
The first neutral RMMR
isolated as amber-greendichroic crystals in ca. 10% yield by this route.
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.
Owing to the near 90° Pb–Pb–C angle, the structure of Ar*PbPbAr* corresponds to adiplumbylene (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 -bond when the trans-bending is 90°- WHAT?
Bonding ModelsAnother MO model:
mixing of M–M * and levels to give a molecular orbital that basically nonbonding. Stabilizes the original orbital but weakens the bond! M–M bond reduced.
Also models how a triple bond can be transformed into a -bond 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 single-bond 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 bonds are not equivalent
Sekiguchi et al Science 2004, 305,1755
