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Metallacyclobutanes form a class of compounds that has received considerable interest, mainly because of their (presumed or proven) intermediacy in several catalytic processes, such as olefin metathesis [']. Many synthetic approaches have been developed to prepare metallacyclobutanes. In our group, di-Grignard reagents are the startingpoint, and it has been shown that 1,3-di-Grignard reagents are excellent synthons for the preparation of metallacyclobutanesL2I.Recently, (1 &naphthalenediyl)magnesium (1) has been prepared and structurally chara~terizedf~]. The 1,3-relationship between the organomagnesium functions called for an investigation of the suitability of 1 as a reagent for the formation of four-membered metallacycles (Scheme 1).Scheme 1 1 2 w cyclic valence angles in the four-membered ring of 2 cause large deviations for the carbon-centered angles from the ideal 120". For that reason, the formation of dimeric compounds 4, i. e. compounds containing a 1,5-dimetallacyclooctane unit, could not be excluded beforehand. In Scheme 2, known representatives of both monomeric (3) and dimeric (4) species are listed. The only known transition metal compound is dimeric 4b; in view of the preferred linear coordination geometry of mercury(II), this is not surprising ["]. Special interest in the resulting 1,s-naphthalenediyl transition metal compounds 2 stems from the obvious strain that is imposed on the metal-carbon bonds; the small endoFrom the X-ray crystal structure analyses of 3b and 3f the severe strain in these compounds can be deduced; the naphthalene moieties are strongly distorted to accommodate the four-membered ring. Although bond lengths as well as angles deviate from the ideal values, no significant deviations from planarity have been observed. In the analogous transition metal compounds 2 (Scheme l), which are the topic of the present investigation, the metals have tetra-

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