A Strong Metal-to-Metal Interaction in an Edge-Sharing Bioctahedral Compound that Leads to a Very Short Tungsten–Tungsten Double Bond

An ionic edge-sharing
bioctahedral (ESBO) species has been prepared having a tetramethylated
bicyclic guanidinate with two fused six-membered rings characterized
by a fairly flat N–C­(N)–N skeleton and abbreviated as
TMhpp. The anion has two WIV atoms bridged by two oxo groups;
the metal atoms are also spanned by two bridging guanidinate ligands,
and each has two monodentate chlorine atoms. The complex formulated
as (H2TMhpp)2[W­(μ-O)­(μ-TMhpp)­Cl2]2 has the shortest W–W distance (2.3318(8)
Å) of any species with a σ2π2 electronic configuration. The anion and cations are connected by
hydrogen bonds. To unambiguously ascertain the existence of the double-bonded
W2(μ-O)2 entity, density functional theory
calculations and natural bond orbital analyses were done on an analogous
but hypothetical species with a W2(μ-OH)2 core having trivalent tungsten atoms and a possible σ2π2δ2 electronic configuration.
The calculations decidedly support the presence of tungsten–oxo
instead of tungsten–hydroxo groups and thus the existence of
the double-bonded W2(μ-O)2 core. The strong
bonding interaction between metal atoms is a clear indication that
under certain circumstances the two octahedra in ESBO species do not
behave as the sum of two mononuclear compounds.