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

Structure of Si 2 H 2 Microwave spectrum of SiH 4 plasma at –196℃ indicated an unusual structure for Si 2 H 2

The energetic array of E 2 H 2 s 2 p 1 p 1 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 SiTbt 3 (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,

Metal- E Triple Bonds?

J. Am. Chem. Soc. 2000, 122, Loss of CO and NaCl Mo–Ge–C interligand angle of 172.2(2)°. Mo–Ge bond length of 2.271(1) Å (ca Å for single)

Metal- E Triple Bonds? J. Am. Chem. Soc. 2000, 122,

Synthesis of a W-Ge triple bond? Exploits the thermal elimination of N 2 from trans- [W(dppe) 2 (N 2 ) 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 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* = C 6 H 3 -2,6-Trip) Leads to single and double reduced compounds not neutral! R-M-M angles range from 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 crystals once the monoanion salts had been removed.

The first neutral RMMR isolated as amber-green dichroic crystals in ca. 10% yield by this route. Pb–Pb bond length, (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 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  -bond when the trans- bending is 90°- WHAT?

Bonding Models Another 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 (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 (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