Plumbylene

Plumbylenes (or plumbylidenes) are divalent organolead(II) analogues of carbenes, with the general chemical formula, R2Pb, where R denotes a substituent.

[2] Plumbylenes can generally be synthesized via the transmetallation of PbX2 (where X denotes halogen) with an organolithium (RLi) or Grignard reagent (RMgX).

[3] Adding an organolithium or Grignard reagent with a different organic substituent (i.e. R’Li/R’MgX) from RPbX leads to the synthesis of heteroleptic plumbylenes (RR’Pb).

[9] Consequently, plumbylenes exclusively have a singlet spin state due to the large singlet–triplet energy gap, and tend to exist in an equilibrium between monomeric and dimeric forms in solution.

[2] Plumbylenes are able to undergo dimerization in two ways: either through the formation of a Pb=Pb double bond to form a formal diplumbene, or through bridging halide interactions.

[7][11]These diplumbenes possess a trans-bent structure similar to that in lighter, non-carbon congeners (disilenes, digermylenes, distannylenes).

The two common intramolecular modes are resonance from a lone pair on the atom directly attached to the lead or by coordination from a Lewis base elsewhere in the molecule.

[21] For example, Group 15 or 16 elements directly adjacent to Pb donate a lone pair in manner similar to their stabilizing effect on Fisher carbenes.

38 kcal mol−1; this was supported by X-ray crystal structures showing the favourable positioning of said B–H bonds in proximity of Pb.

[26] In contrast, the reaction between stannylenes and Me3NO produces the corresponding distannoxane (from oxidation of Sn(II) to Sn(IV)) instead of the Lewis adduct, which can be attributed to tin being a period above Pb, experiencing the inert pair effect to a lesser degree and hence having a higher susceptibility to oxidation.

[6] Insertions into lead-substituent bonds can also occur.27 In the examples below, insertion is accompanied by intramolecular rearrangement to place more electron-donating heteroatoms next to the electron-deficient lead.27 Plumbylenes are known to undergo nucleophilic substitution with organometallic reagents to form transmetallated products.28 In an unusual example, the use of TlPF6, bearing the weakly coordinating anion PF6−, led to the formation of crystals of an oligonuclear lead compound with a chain structure upon work-up, highlighting the interesting reactivity of plumbylenes.28 In addition, plumbylenes can also undergo metathesis with group 13 E(CH3)3 (E = Al, Ga) compounds.

Plumbylenes, R 2 Pb
General synthesis of plumbylenes via transmetallation
Synthesis of various plumbylenes via transmetallation from [((CH 3 ) 3 Si) 2 N] 2 Pb
Synthesis of plumbylenes via reductive dehalogenation of tetravalent organolead compounds
Key valence orbitals of Pb in plumbylenes: the 6s lone pair and vacant 6p
Dimethyllead, (CH 3 ) 2 Pb, and diphenyllead, (C 6 H 5 ) 2 Pb
AIM molecular graph of diphenyllead. Green points indicate (3, +3) critical points; orange, (3, -1); and purple, (3, -3).
Equilibrium between plumbylenes (monomer) and diplumbenes (dimer) in solution
Valence bonding diagram showing donation of 6s lone pairs of Pb into vacant 6p orbital of adjacent Pb in diplumbene
Equilibrium between monomers and halogen-bridged dimers in monohalogenated plumbylenes
(a) Valence orbital diagram showing the donation of a lone pair on heteroatom E to the vacant 6p orbital of adjacent Pb. (b) Examples of heteroatom-stabilized plumbylenes
Reaction of plumbylenes with various Lewis bases
Insertion reactions of R 2 Pb with R'–X (X = Br, I), R'–E–E–R' (E = S, Se) and S 8
Insertion of an alkylazide and an isocyanate into [(Si(CH 3 ) 3 ) 3 Si] 2 Pb
Transmetallation between a plumbylene and various organometallic reagents (top) and the formation of an unusual oligomeric chain compound (bottom)
Metathesis reaction between R 2 Pb and E(CH 3 ) 3 (E = Al, Ga). In this example, the plumbylene formed dimerizes into a diplumbene
Transmetallation reaction between R 2 Pb and R’ 2 Pb
Synergic σ-donor-σ-acceptor interactions of PbCl 2 with Pt(PCy 3 ) 2