The acyl anion complex [(CO)2ReC(O)CH2CH2(η5-C5H4)]-Li+ (7) in which a two-carbon tether links the cyclopentadienyl ring to the acyl carbon was synthesized by attachment of a 2-lithioethyl side chain to the cyclopentadiene ring of CpRe(CO)3 followed by intramolecular attack of the lithium reagent on a carbonyl group. Alkylation of 7 with (CH3)3O+BF4 - occurred at oxygen to give the methoxycarbene complex (CO)2ReC(OCH3)CH2CH2(η5-C5H4)(9), which was shown by X-ray crystallography to have significant strain associated with the tethered ring. Protonation of 7 gave a mixture of hydroxycarbene complex (CO)2ReC(OH)CH2CH2(η5-C5H4) (2) and the metal acyl hydride complex trans-(CO)2HReC(O)CH2CH2(η5-C5H4) (3). The unusual observation of the metal acyl hydride is attributed to the two-carbon tether introducing strain into the three-legged piano stool geometry of 2 but leaving the four-legged piano stool geometry of 3 relatively unstrained. In agreement with this hypothesis, no strain was apparent in the X-ray structure of the three-carbon-tethered methoxycarbene complex (CO)2ReC(OCH3)CH2CH2CH2(η5-C5H4) (17) and only the hydroxycarbene tautomer (CO)2ReC(OH)CH2CH2CH2(η5-C5H4) (18) was observed at equilibrium. A two-carbon tether did not introduce sufficient strain into the aminocarbene complex (CO)2ReC[NH(CH3)]CH2CH2(η5-C5H4) (19) to allow observation of its iminoacyl hydride tautomer.

Treatment of zirconaaziridines Cp2Zr-η2-[N(R1)CH(R2)](THF) with carbodiimides results in the insertion of the carbodiimide into the Zr−C bonds. The insertion of bis(trimethylsilyl)carbodiimide is reversible, which becomes significant at high THF concentrations. Kinetic data indicate that the THF ligand must dissociate prior to carbodiimide insertion. Protic cleavage of the organic fragment from zirconium results in formation of α-amino amidines.

The para-aminobiphenyl compound [(η(6)-C(6)H(5))(C(6)H(4)-4-NH(2))]Cr(CO)(3) (1) has an arene-phenyl dihedral angle of 38.01(6)°, as determined by single-crystal X-ray crystallography, and 34.7(11)°, as determined by DFT calculations. It undergoes haptotropic rearrangement at 140 °C in solution to form [(η(6)-C(6)H(4)-4-NH(2))(C(6)H(5))]Cr(CO)(3) (2), even though previous reports have suggested that such rearrangements should not be observed in compounds with arene-phenyl dihedral angles greater than 22°. NMR analysis gave a rate constant of k = 5.0 × 10(-5) s(-1) for the rearrangement of 1 to 2. The ortho-substituted analog [(η(6)-C(6)H(5))(C(6)H(4)-2-NH(2))]Cr(CO)(3) (3) has an arene-phenyl dihedral angle of 67.70(7)°, as determined by single-crystal X-ray crystallography, and 51.9(10)°, as determined by DFT calculations. Surprisingly, even though it displays a more extreme canting of arene rings, 3 rearranges to [(η(6)-C(6)H(4)-2-NH(2))(C(6)H(5))]Cr(CO)(3) (4) at 140 °C in solution with a rate constant of k = 2.6 × 10(-4) s(-1). This approximately five-fold rate enhancement likely results from the ortho-amino group providing intramolecular stabilization for intermediates formed during the rearrangement.

The amphiphilic rhenium carbene complex Cp(CO)2-Re=CDCHzCH2CMe3 undergoes stereospecific addition of HCl to produce a single diastereomer of cis-Cp(C0)zClReCHDCHz-CHzCMe3,I but the absolute stereochemistry of the process could not be determined in part because rotation about the Re=C bond is expected to be fast.2 In the course of synthesizing a rotationally restricted rhenium carbene complex having a twocarbon link between the cyclopentadienyl ligand and the carbene carbon atom, we discovered an equilibrium between the hydroxycarbene complex (C0)2Re=C(0H)CH2CH2(q5-CsH4) (1) and the isomeric metal acyl hydride complex trans-