orbital energies in group 14

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