FvCr2(CO)4L2(Fv = Fulvalene; L = PMe2Ph, PMe3), Metal−Metal-Bonded Compounds Which Undergo Spontaneous Thermal Homolysis to Their Biradical Isomers. Relevant Chemistry of the Compounds FvCr2(CO)4L2X2(X = H, Cl, Br, I)

Two new (fulvalene)dichromium carbonyl dihydrides, FvCr 2 (CO) 4 L 2 H 2 (L = PMe 2 Ph ( 2b ), PMe 3 ( 2c )) were prepared by phosphine substitution of FvCr 2 (CO) 6 H 2 ( 2a ) and studied by variable-temperature 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR. Compound 2b assumes a 77:23 and 2c an 85:15 cis / trans isomeric ratio at −50 °C, while low-temperature NMR spectra of 2b present evidence for the meso and dl forms of the cis, cis isomer. Hydride hydrogen atom abstraction from 2b and 2c by Ph 3 C •, as well as hydrogenation of isoprene and 1,3,5-hexatriene by 2b and 2c, result in the formation of FvCr 2 (CO) 4 L 2 (L = PMe 2 Ph ( 6a ), PMe 3 ( 6b )). Variable-temperature IR and 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR studies demonstrate that both 6a and 6b maintain facile equilibria between the trans isomers of the chromium−chromium-bonded species and the novel biradical species L(CO) 2 Cr(μ-Fv)Cr(CO) 2 L at room temperature in solution. Room-temperature reactions of 6a with alkyl halides reflect the radical character of this compound, halogen atom abstractions leading to the new dihalide compounds FvCr 2 (CO) 4 (PMe 2 Ph) 2 X 2 (X = I ( 3b ), Br ( 4 ), Cl ( 5 )). Compounds 3b, 4, and 5 were obtained alternatively in the hydrogen−halogen atom exchange reactions of 2b with the appropriate alkyl halides. These reactions are shown to proceed via intermediate formation of the corresponding mixed hydrido halide compounds cis -FvCr 2 (CO) 4 (PMe 2 Ph) 2 HX (containing cis -halide halves). FvCr 2 (CO) 6 I 2 ( 3a ) was prepared analogously from 2a and ethyl iodoacetate and was also identified as the product of iodine atom abstraction from ethyl iodoacetate by FvCr 2 (CO) 6 . The reaction of FvCr 2 (CO) 6 I 2 with PMe 2 Ph provided access to cis -(CO) 2 (PMe 2 Ph)ICr(μ-Fv)[ trans -Cr(CO) 2 (PMe 2 Ph) 2 ]I ( 7 ); the reaction was shown to proceed via consecutive formation of the intermediates (CO) 3 ICr(μ-Fv)[Cr(CO) 3 PMe 2 Ph]I and cis -(CO) 2 (PMe 2 Ph)ICr(μ-Fv)[Cr(CO) 3 PMe 2 Ph]I.