Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors

[Image: see text] 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(α/α′) and M···C(β) distances, long C(α/α′)–C(β) bond length, and isotropic (13)C chemical shifts for both early d(0) and late d(4) 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 (13)C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d(0) systems, with upfield shifts to ca. −30 ppm for the α-carbon of metallacycles with higher d(n) electron counts (n = 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(11) ≥ δ(22) ≥ δ(33)) 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(α/α′)) 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.