Metal−Metal Multiply Bonded Complexes of Technetium. 5. Tris and Tetrakis(formamidinato) Complexes of Ditechnetium

Compounds of the type
Tc2Cl4(PR3)4
(PR3 = PEt3, PMe2Ph,
PMePh2) react with the molten formamidines
HDPhF
(HDPhF = diphenylformamidine) and HDTolF (HDTolF =
di-p-tolylformamidine) to produce mixtures of
tris-
and tetrakis-bridged formamidinate complexes of ditechnetium. The
displacement of chloride and phosphine by
[DPhF]- was accompanied by the oxidation of the dimetal
core to produce the mixed-valent complexes
Tc2(DPhF)3Cl2 (1) and
Tc2(DPhF)4Cl (2) in modest
yield. The solid-state structures of the di-p-tolyl
analog of 1,
Tc2(DTolF)3Cl2 (1a), and
Tc2(DPhF)4Cl·C7H8
(2·C7H8) have been determined by
single crystal X-ray diffraction studies and
are described in detail. The structure of 1a consists
of three formamidinate ligands spanning the two technetium
atoms. The two chloride ligands, which complete the coordination
sphere, are bound equatorially at distances of
2.357(1) and 2.346(2) Å from the metals. Though
possessing no crystallographic symmetry, 1a
approximates
C2v symmetry. The
metal−metal bond length of 2.0937(6) Å ranks among the shortest
reported for technetium
and is indicative of a Tc−Tc multiple bond. Compound
2 crystallizes with the Tc atoms colinear with
a
crystallographic 4-fold axis. The four bridging formamidinate
ligands are arranged in a lantern geometry about
the dimetal unit. The chloride is bonded in an axial position at a
distance of 2.450(4) Å. The Tc−Tc bond
length of 2.119(2) Å is also consistent with the presence of a
high order Tc−Tc bond. The electronic structures
of 1 and 2 were investigated by means of
SCF−Xα−SW molecular orbital calculations using the model
compounds
Tc2(HNCHNH)3Cl2 and
Tc2(HNCHNH)4Cl. The results support
the presence of a σ2π4δ2δ*
ground state
configuration giving rise to a formal bond order of 3.5. The LUMO
in both cases is a low-lying π* orbital. The
formamidinate complexes 1 and 2 have been further
characterized by IR spectroscopy and cyclic voltammetry.
The crystallographic parameters for 1a and
2·C7H8 are as follows: 
Tc2(DTolF)3Cl2
(1a), monoclinic space group
P21/n (No. 14) with a =
16.185(2) Å, b = 15.637(2) Å, c =
17.812(1) Å, β = 110.142(5)°, V =
4232.3(6) Å3
and Z = 4;
Tc2(DPhF)4Cl·C7H8
(2·C7H8), tetragonal space group
P4/ncc (No. 130) with a =
15.245(2) Å, c =
21.832(3) Å, V = 5074.1(9) Å3 and
Z = 4.