Multiple Bonding in Group 15

Like the group 14 compounds, catenation is not difficult with group 15.

Reduction of the monochloride can lead to a single bond:

2 Me2ECl + 2Na Me2E-EMe2 + 2 NaCl

This works best with bismuth and antimony, but is generalized over the entire group 15.

In the case of arsenic, elimination of HCl is possible:

Me2AsCl + Me2AsH Me2As-AsMe2 + HCl

Pyridine Analogues

Analogues to pyridine for group 15 molecules have been around long enough to have two trivial names:

Phosphabenzene, arsabenzene, stibabenzene, bismuthabenzene

Phosphinine, arsinine, stibinine, bismuthinine

They are made by methathesis with a tin alkyl.

Substituted aryl rings are also known.

SnBu2

+ EX3 X2SnBu2 +

EX

E

+ HX

Pyridine AnaloguesBecause they have a different basicity than pyridine, these ring systems can be found as both and 6 bonded ligands:

Mo(CO)3P

Mo(CO)5

P

Phosphorous and arsenic see the most employment as heteroatoms, due to their stronger bonds and greater participation in the aromatic system (which is weaker than nitrogen in pyridine).

This trend in stability is rounded out by the Bi analogue being unstable.

Pyrrole Analogues

Pyrrole analogues with heavier members of group 15 are also known.

These analogues can be dealkylated and form Cp-like anions. The trend in system participation is the same as seen for the pyridine derivatives: decreasing down the group.

These ligands, including pyrrole, have been shown to 5 coordinate to transition metal centres after the fashion of Cp.

ER + NaH RH + E Na+

Multiple Bonding in Group 15

There were only two published structures of diarsenesMes*AsAsCH(SiMe3)2 and [As{C(SiMe3)3}]2 by Cowley and co-workers.

J. Am.Chem. Soc. 1983, 105, 5506.J. Chem. Soc., Dalton Trans. 1985, 383.

A Bi=Bi unit

Deep purple crystalsBi–Bi = 2.8206(8) Å, Bi–Bi–C = 100.5(2)° (Bi–Bi single bond = 2.990(2) Å in Ph2Bi–BiPh2)propose three orthogonal 6p orbitals without significant hybridization leads to a bond angle of approx. 90° Stable as a crystal even in air for several hours.

Tokitoh, N.; Arai, Y.; Okazaki, R.; Nagase, S. Science 1997, 277, 78.Extended to include Sb analogues Sb=Sb 2.642(1) Å in:J. Am. Chem. Soc. 1998, 120, 433.

Double Bonds in Group 15

Using large, protective ligands, Powers has made a series of compounds RECl2 and reduced them with magnesium to give single and double bonds:

LiAr + ECl3 ArECl2 + LiCl

J. Am. Chem. Soc. 1999, 121(14), 3363.

Group 15 E=ENotably, the ligand either has Me (this example) or iPr groups on the ortho-position on the flanking rings:

ArECl2 + Mg ArE=EAr + MgCl2

Group 15 E=EDipnictenes are intensely colored - range from yellow or orange yellow to deep red-purple

Two bands more intense * and less intense n-*

The * band shifts to longer wavelength as the group is descended - consistent with a weakening of the bond

Mechanism

The mechanism isn’t clear:

REX2

+ Mg

+ Mg

- MgX

REX(MgX)

R(X)E

- MgXR(X)E-E(X)R

+ REX2

-MgX

+ REX2, -Xor dimerization

Mechanism (cont’d)

REX(MgX)

- MgX2 "ER"

+ REX2

-MgX2

R(X)E-E(X)R + Mg

-MgX2RE=ER

dimerization

possible reaarangementto other products

Bonding

Aryl

Aryl

E E

Ar

Ar

Note that the angle at the pnictogen atom in dipnictenes narrows from ca.. 113.6(2)in PhNNPh to 102.8(9)in Mes*PPMes* and 100.5(2)in Bi compounds.

We expect the s-orbital character of the lone pairs also increases with increasing atomic number such that at Sb or Bi the lone pairs have mostly s-orbital character.

Accordingly, the and bonds comprising the Sb=Sb and Bi=Bi double bonds are mostly composed from p-orbitals.

Multiple Bonding in Group 15

Bi-Bi bond = 2.8769(5) Å in (CO)5W(BiR)2 (R = CH2SiMe3)

Compare to Bi=Bi = 2.82-2.83 Å

Organometallics 2003, 22, 2919.

Multiple Bonding in Group 15

Bi-Bi-C bond angles in 1 are 109.3(2) and 109.54(18) - greaterBi-Bi bond distance in 1 is 3.1442(7) Å - lengthenedBi-Zr bond lengths (2.9903(10) and 3.0044(11) Å) may onlybe compared with 3.190 Å, the sum of the bismuth and zirconium covalent radii

J. AM. CHEM. SOC. 2005, 127, 7672

unprecedented opportunity to examine a dibismuthene “before” and “after” coordination

Multiple Bonding in Group 15Bond options

(1) Cp2Zr is a 14-electron fragment while (CO)5W is a 16-

electron moiety

(2) electron-rich Cp ligands tend to donate electron density to transition metals, whereas the strong -acid (electron accepting) CO ligands decrease the electron density at the metal.