Germanium and Tin Analogues of Alkynes and Their Reduction Products

The reduction of terphenylgermanium(II) or terphenyltin(II) chlorides with alkali metals was
investigated. Treatment of Ar‘GeCl or Ar*GeCl (Ar‘ = C6H3-2,6-Dipp2, Dipp = C6H3-2,6-Pri2; Ar* = C6H3-2,6-Trip2, Trip = C6H2-2,4,6-Pri3) with lithium, sodium, or potassium afforded the neutral alkyne analogues
Ar‘GeGeAr‘, 1, Ar*GeGeAr*, 2, the singly reduced radical species NaAr*GeGeAr*, 3, or KAr‘GeGeAr‘, 4,
or the doubly reduced compounds Li2Ar‘GeGeAr‘, 5, Na2Ar*GeGeAr*, 6, or K2Ar*GeGeAr*, 7. Similarly,
reduction of Ar‘SnCl or Ar*SnCl afforded the neutral Ar‘SnSnAr‘, 8, or Ar*SnSnAr*, 9, the radical anions
[(THF)3Na{Ar*SnSnAr*}], 10, [K(THF)6][Ar‘SnSnAr‘], 11, [K(THF)6][Ar*SnSnAr*], 12, [K(18-crown-6)(THF)2]
[Ar*SnSnAr*], 13, or the doubly reduced Na2Ar*SnSnAr*, 14, K2Ar‘SnSnAr‘, 15, or K2Ar*SnSnAr*, 16. The
compounds were characterized by UV−vis, 1H and 13C NMR or EPR spectroscopy. The X-ray crystal
structures of all compounds were determined except those of 2 and 9. The neutral 1 and 8 displayed
planar, trans-bent CMMC (M = Ge and Sn) cores with M−M−C angles of 128.67(8) and 125.24(7)°,
respectively. The M−M bond lengths, 2.2850(6) and 2.6675(4)Å, indicated considerable multiple character
and a bond order approaching two. Single and double reduction of the neutral species resulted in the
narrowing of the M−M−C angles by ca. 12−32° and changes in the Ge−Ge and Sn−Sn bond lengths.
One-electron reduction afforded a slight (ca. 0.03−0.05Å) lengthening of the Ge−Ge bonds in the case of
germanium species 3 and 4 and a greater lengthening (ca. 0.13−0.15Å) for the Sn−Sn bonds in the tin
compounds 10−13. The addition of another electron yielded salts of the formal dianions [Ar‘MMAr‘]2- and
[Ar*MMAr*]2- which are isoelectronic to the corresponding doubly bonded, neutral arsenic and antimony
derivatives. All the dianion salts were obtained as contact ion triples with two alkali metal cations complexed
between aryl rings. The Ge−Ge bonds in the dianions of 5−7 were longer, whereas the Sn−Sn distances
in the dianions in 14, 15, and 16 were shorter than those in the monoanions. Unusually, the Li2Ar‘GeGeAr‘
salt, 5, displayed a longer Ge−Ge bond (by ca. 0.06Å) than those of its Na+ or K+ analogue salts which
was attributed to the greater polarizing power of Li+. It was concluded that the M−M bond lengths in 3−7
and 10−16 are dependent on several factors that include M−M−C angle, Coulombic repulsion, alkali metal
cation size, and the character of the molecular energy levels. The M−M bonding in the neutral compounds
was accounted for in terms of a second-order Jahn−Teller mixing of σ*- and a π-orbital which afforded
bond orders near two for the neutral compounds, 1, 2, 8, and 9. Calculations on MeMMMe (M = Ge or Sn)
model species showed that the LUMO corresponded to an orbital that had n+ lone pair character. The
slight Ge−Ge bond length increase upon one-electron reduction is consistent with these results, and the
further bond lengthening upon double reduction is consistent with increased Coulombic repulsion. The
greater Sn−Sn bond length increase seen for one-electron reduction of the tin species is probably due to
the increased p-character of orbitals comprising the Sn−Sn σ-bond when the Sn−Sn−C angle is decreased
by ca. 30°. Upon further reduction, the slight decrease in the Sn−Sn bond is probably a result of the reduced
importance of Coulombic repulsion due to the larger size of tin and a widening of the Sn−Sn−C angles
which may shorten the Sn−Sn σ-bond.