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The new bis(methimazolyl)silane PhSiH(mt) 2 (mt = methimazolyl), obtained from methimazole (Hmt) and phenyldichlorosilane, reacts with [Ru(η 4 -C 8 H 12 )(η 6 -C 8 H 10 )] in refluxing tetrahydrofuran to provide the cis and trans isomers of [Ru{κ 3 Si,S,S 0 -SiPh(mt) 2 }]. The structurally characterized trans isomer crystallizes directly from THF but on dissolution in CH 2 Cl 2 converts exclusively to the cis isomer.The dihydrobis(methimazolyl)borate ligand, [H 2 B(mt) 2 ] -, coordinates to a diverse variety of metal centers and in doing so displays a geometrically derived proclivity toward tridentate coordination through two sulfur donors and one threecenter, two-electron B-H-metal interaction (Chart 1; A, κ 3 H,S,S 0 ). 1,2 This behavior is of particular relevance to the mechanism by which such complexes of the platinum group metals may evolve, via B-H activation, into dibuttressed metallaboratranes which feature a dative MfB bond (B, κ 3 B,S,S 0 ). 3,4 It has also been noted that rhodium complexes of the bis(methimazolyl)methane ligand adopt geometries that bring one C-H bond of the bridgehead methylene into close ("pregostic") proximity with the rhodium center, 1a while bis(imidazolyl)triselenane, when coordinated to ruthenium, does so via one bridgehead selenoether and two nitrogen donors. 5 In addition to facilitating the formation of MfB interactions, methimazolyl bridges have also been found to support bimetallic assemblies, 1b including recent examples of dative PdfSn, PdfSi, and PtfSi bonds in the complexes [PdSn(μ-mt) 4 Cl 2 ](PdfSn) and [MSi(μ-mt) 4 Cl 2 ](MfSi) (M = Pd, Pt). 6 It has even been observed that in some cases the tris(methimazolyl)borate ligand adopts the κ 3 H,S,S 0 coordination mode in preference to the more common κ 3 S,S 0 ,S 00

The complex Ru((1−6η)-C8H8)((1−4-η)-C8H8) (2), which is made in two steps from Ru(acac)3, shows three fluxional processes:  (1) equilibration of the protons and carbon atoms of the (1−4-η)-C8H8 ring, which appear as a singlet even at −100 °C; (2) role reversal (exchange of hapticity) of the two rings; (3) a 1,5-shift within the (1−6-η)-C8H8 ring. Processes 2 and 3 cause the 1H and 13C resonances of C8H8 to appear as a broad singlet at room temperature, though the (1−6-η)-ring is static at ca. −60 °C. The free energies of activation ΔG ⧧ 235 for processes 2 and 3 have been estimated from magnetization transfer experiments to be 12.8 ± 0.2 and 13.5 ± 0.2 kcal mol-1, respectively. Complex 2 reacts with ligands L to give adducts Ru(L)((1−4-η)-C8H8)((1,2,5,6-η)-C8H8) (L = CO (4), CN-t-Bu (5), P(OMe)3 (6), PMe3 (7), and PEt3 (8)). The (1,2,5,6-η)-rings in these compounds are static at room temperature and do not exchange with the fluxional (1−4-η)-C8H8 rings. The (1−4-η)-C8H8 ring in 4 undergoes the usual 1,2-shift, the free energy of activation ΔG ⧧ 259 of 13.7 ± 0.1 kcal mol-1 being considerably higher than that of 2 or of Ru(CO)3((1−4-η)-C8H8). The X-ray structures of 2, 4, 5, and 7 are reported.

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