Introduction
Homonuclear metal-metal bonds has always attracted chemists due to their
enormous applications.1 One such landmark advancement
in this area was triggered by the discovery of first Zn(I) – Zn(I) σ
bonded complex
[(η 5-Cp*)2-Zn2]
(Cp* = C5Me5) by Carmona and
coworkers.2 This seminal discovery has paved the way
for isolation of various Zn(I) – Zn(I) bonded complexes involving
various ligands.3 Recent years has also witnessed
examples of multiply bonded Zn-Zn complexes.4,5
Bare Zn2 dimer, characterized by UV spectroscopy,
appears to be a van der Waals complex with a Zn-Zn distance of 4.19
Å.6 In principle, the nature of bond in
Zn2 and Be2 is almost similar as
Be2 has also been characterized in gas phase and
described as van der Waals complex with Be-Be distance of 2.45
Å.7-9 The close analogy between the
Zn2 and Be2 dimer is their similarity in
valence electronic configuration (Be =
2s22p0; Zn =
3d104s24p0) and
exhibit typical Morse-van der Waals type
potential.10,11 Thus, it appears that Zn will prefer
similar chemistry as that of Be to a large extent. Brea and coworkers
investigated an interesting phenomenon of single electron intramolecular
Be-Be bond in 1,8-diBeX-naphthalene radical anion derivatives (Scheme
1).12 Their calculations reveal that the extra
electron forms the Be-Be bond with dramatic shortening of the Be-Be bond
with respect to the neutral molecule. Both –BeX and –ZnX are acidic in
nature and there is similarity in valence electron configuration of Be
and Zn, we therefore, expect that Zn will exhibit similar intramolecular
one electron bond formation. It should be noted that the distance
between the H atoms of naphthalene attached to C1 and C8 atoms is almost
similar (2.47 Å) to the distance between the H atoms attached to C1 and
C2 atoms of benzene (Scheme 1). The choice of C1 and C8 disubstituted
BeX in the work of Brea and coworkers12 is obvious as
these two positions are chemically most reactive.13However, all positions in benzene are chemically equivalent. Thus, one
can envision that similar intramolecular one electron Be-Be and Zn-Zn
bonds formation will be possible in substituted benzene derivatives. We
therefore, have undertaken a detailed theoretical study on the possible
existence of intramolecular one-electron Be-Be and Zn-Zn bonds in
1,2-diBeX-benzene and 1,2-diZnX-benzene radical anion derivatives
(Scheme 1). The probable questions that are likely to be answered in the
contribution are – How strong can this one electron bond be? What will
be the effect of X group? Are these radical anions experimentally
realizable?