The oxidative electron-transfer properties of several superphane complexes consisting of cyclopentadienylcobalt cyclobutadiene moieties linked by either three (compounds 1 and 4) or five (compounds 2 and 5) bridging methylene groups have been studied by experimental and theoretical methods. In both cases two separate one-electron oxidations are found. The mixed-valent monocations of the (−CH2−)5-bridged complexes are valence-trapped with very weak interactions between metal centers. The (−CH2−)3 complexes, however, have strong interactions between the two molecular halves. The intervalence transfer (IT) band of 1 + has characteristics of both class II (localized) and class III (delocalized) behavior, but the IR spectra of carboxy-labeled 4 + clearly establish trapped valence for the monocations of the propano-bridged systems. Photoelectron spectra and ab initio calculations at the UHF level show that, in the ground electronic state, 1 + has a half-filled orbital (i.e., electron spin) that is essentially localized in one Co d xy , orbital, but that the charges on the two metals are unequal owing to inductive electronic effects which give unequal electron flow from the ligands to the two metal centers. Calculations and IR (carbonyl) spectral shifts suggest about a 70:30 charge ratio between the two metal centers in 1 + and 4 +, whereas both spin and charge localization is virtually complete in the pentano-bridged complexes 2 + and 5 +. The intervalence transition in 1 + is proposed to proceed through a “hole”-transfer process mediated by a π-cyclobutadiene MO, ultimately involving a through-bond transannular mechanism.

Striking differences in the reactions of alkoxycarbene and thiocarbene complexes of chromium and tungsten are observed. Thus, (β-imino)ethoxycarbene complexes 10a−e, generated in situ from [(OC)5WC(OEt)CH2R] (7a−c; R = n-Pr, Me, c-C7H7) and imidoyl chlorides R1ClCNCHR2R3 (9a−f; R1 = t-Bu, Ph, 2-furyl; R2 = H, Me; R3 = Me, Et, Ph), undergo a metalla(di-π-methane) rearrangement to (N-enamino)ethoxycarbene complexes 12a−e, while the corresponding (β-imino)thiocarbene complexes 11a−l, derived from [(OC)5MC(SEt)CH2R] (8a−e; M = W, Cr; R = n-Pr, Me, c-C7H7, c-C6H7Fe(CO)3) and imidoyl chlorides under similar conditions, form pyrroles 16a−h and 17k,l by α-cyclization. On the basis of the calculated DFT/BP86 potential energy surfaces of the particular reaction channels it is shown that (β-imino)alkoxycarbene compounds 10 prefer a metalla(di-π-methane) rearrangement due to the kinetic stability of the (N-enamino)ethoxycarbene products, while formation of pyrroles is not favored due to the presence of high energetic stationary structures in the α-cyclization pathway. For (β-imino)thiocarbene compounds 11, on the other hand, rearranged products are kinetically unstable, and α-cyclization reactions are strongly favored on thermodynamic grounds.