The reaction of (η5-C5H5)Co(PPh3)2 (1) with 1,3-bis(isopropyl)imidazol-2-ylidene (ImiPr2, 17) leads to the formation of (η5-C5H5)Co(PPh3)(ImiPr2) (5) in 69% yield. N-Heterocyclic carbene 17 also undergoes reaction with (η5-C5H5)Co(CO)2 (9) to give (η5-C5H5)Co(ImiPr2)(CO) (6) in 30% yield. The barrier to rotation about the Co−Ccarbene bond in 6 has been determined by variable-temperature 1H NMR spectroscopy (13.6 kcal/mol) and by computation (13.3 kcal/mol). Complex 5 undergoes reaction with PhSSPh to give the paramagnetic thiolato complex (η5-C5H5)Co(ImiPr2)(SPh) (7), which is oxidized to the metallosulfone complex (η5-C5H5)Co(ImiPr2)(SO2Ph) (8). The solid-state structures of 5−8 were determined by X-ray crystallography. The structural and dynamic properties of 6, (η5-C5H5)Co(ImMe2)(CO) (ImMe2 = 1,3-dimethylimidazol-2-ylidene), and (η5-C5H5)Co(ImAr2)(CO) (ImAr2 = 1,3-dimesityl-2-ylidene) were examined by quantum chemistry calculations.

The first productive reactions of a characterized metallacyclobutene complex with alkenes are reported. Thus, the metallacyclobutene complex (eta5-C5H5)(PPh3)Co[kappa2-(C,C)-C(SO2Ph) C(Si(CH3)3)CH(CO2CH2CH3)] (2) undergoes reaction with alkenes to give 1,4-diene complexes with a high degree of regio- and stereoselectivity. A mechanism is proposed in which the metallacyclobutene generates a cyclic vinylcarbene intermediate that undergoes [4 + 2]-cycloaddition reactions with activated alkenes. A model of the vinylcarbene intermediate has been examined using quantum mechanical methods.

The solid state structures of the cobalt−alkyne complexes (η 5 -C 5 H 5 )(PPh 3 )Co{η 2 -(R 3 Si)CC(SO 2 Ar)} (3-TMS, R = Me, Ar = C 6 H 5 ; 3-TIPS, R = CH(CH 3 ) 2 , Ar = p-C 6 H 4 CH 3 ) and the noncoordinated alkyne (Me 3 Si)CC(SO 2 Ph) (6-TMS) have been characterized by X-ray crystallography and, in the case of 3-TMS, 6-TMS, and (η 5 -C 5 H 5 )Co{η 2 -(Me 3 Si)CC(SO 2 Ph)} (5-TMS-calc), by B97D/Def2-TZVPP computational analysis. The phosphine-dissociated complex 5-TMS-calc is determined to be a ground state singlet. Analysis of bond angle and distance metrics, calculated NMR chemical shift data, and molecular orbital analysis provide strong evidence for a four-electron-donor alkyne ligand in 5-TMS-calc. The degree of asymmetry in metal−alkyne bonding, as defined by the Gladysz alkyne-slippage parameter, is dramatically reduced in 5-TMS-calc relative to that in the precursor complex 3-TMS-calc.

The metallacyclobutene complex (η 5 -C 5 H 5 )(PPh 3 )Co-{κ 2 (C 1 ,C 3 )-C(SO 2 Ph)dC(SiMe 3 )CH(CO 2 Et) undergoes a regioand stereoselectiVe reaction with both nitrosobenzene and 2-methyl-2-nitrosopropane to giVe ring-expanded metallacycles with an η 2 (N,O)-hydroxylamido ligand. These transformations represent a new synthetic route toward late-metal η 2 (N,O)hydroxylamido complexes.