Metallacyclobutanes are an important
class of organometallic intermediates,
due to their role in olefin metathesis. They can have either planar
or puckered rings associated with characteristic chemical and physical
properties. Metathesis active metallacyclobutanes have short M–C<sub>α/α′</sub> and M···C<sub>β</sub> distances, long C<sub>α/α′</sub>–C<sub>β</sub> bond length, and isotropic <sup>13</sup>C chemical
shifts for both early d<sup>0</sup> and late d<sup>4</sup> transition
metal compounds for the α- and β-carbons appearing at
ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show
metathesis activity have <sup>13</sup>C chemical shifts of the α-
and β-carbons at typically 40 and 30 ppm, respectively, for
d<sup>0</sup> systems, with upfield shifts to ca. −30 ppm for
the α-carbon of metallacycles with higher d<sup><i>n</i></sup> electron counts (<i>n</i> = 2 and 6). Measurements
of the chemical shift tensor by solid-state NMR combined with an orbital
(natural chemical shift, NCS) analysis of its principal components
(δ<sub>11</sub> ≥ δ<sub>22</sub> ≥ δ<sub>33</sub>) with two-component calculations show that the specific
chemical shift of metathesis active metallacyclobutanes originates
from a low-lying empty orbital lying in the plane of the metallacyclobutane
with local π*(M–C<sub>α/α′</sub>)
character. Thus, in the metathesis active metallacyclobutanes, the
α-carbons retain some residual alkylidene character, while their
β-carbon is shielded, especially in the direction perpendicular
to the ring. Overall, the chemical shift tensors directly provide
information on the predictive value about the ability of metallacyclobutanes
to be olefin metathesis intermediates