Roger H. Cayton

Electronic structure of piano-stool dimers—VII. Interrelationships among the electronic structure, reactivity and conformational preferences of hydrocarbyl-bridged piano-stool dimerst☆

Abstract The electronic structure of a number of hydrocarbyl-bridged piano-stool dimers has been calculated by using the non-empirical Fenske-Hall molecular orbital method. It is shown that there is a close connection between a number of electronic structural factors (e.g. the number of available metal-based orbitals, the number and energies of available orbitals on the hydrocarbyl bridge, and the electronic competition between the hydrocarbyl bridge and the ancillary ligands) and the reactivities and conformational preferences of the dimers. Examples are chosen from both low-valent dimers with π-acid ancillary ligands and high-valent dimers with predominantly σ-donor ligands.

Further studies of the reactivity of cyclopentadienyl, substitutedcyclopentadienyl and indenyl ligands withM2Cl2(NMe2)4(M = Mo or W). Crystal and molecular structuresofMo2(C9H7)2(NMe2)4 andW2(C5H4Me)2(NMe2)4

Metathetic reactions employing n1,2-M n 2 nCl n 2 n(NMe n 2 n) n 4 n compounds n(M = Mo or W) and the sodium or lithium salts of ncyclopentadienyl (Cp), methylcyclopentadienyl, indenyl n(C n 9 nH n 7 n) and bis(methyl)bis(cyclopentadienyl)silyl, nMe n 2 nSi(C n 5 nH n 4 n) n 2 n n 2- n n, anions carried out in hydrocarbon or ether solvents have led to nthe isolation of orange, hydrocarbon-soluble, air-sensitive crystalline nsolids of formula n1,2-M n 2 nR n 2 n(NMe n 2 n) n 4 n. In solution nthe NMR data are consistent with C n 2 n symmetry and nrestricted rotation about the M–N bonds. The complexes where nR = C n 5 nH n 5 n show only one CH resonance nin their 1 nH NMR spectra consistent with ring whizzing. The nsolid-state and molecular structures of nMo n 2 n(C n 9 nH n 7 n) n 2 n(NMe n 2 n) n n 4 n and nW n 2 n(C n 5 nH n 4 nMe) n 2 n(NMe n 2 n n) n 4 n reveal that the indenyl and C n 5 nH n 4 nMe nligands are η n 3 n-bonded to the MM n 6+ n nmoiety with three M–C distances within the range 2.35 to 2.52 nA and two distinctly larger, ca. 2.75–2.80 A. nThe M–M distances, 2.252(1) A (M = Mo) and n2.345(1) A (M = W) and M–N distances, 1.96 to n1.99 A, are similar to those for n1,2-M n 2 nR n 2 n(NMe n 2 n) n 4 n compounds, nwhere R = alkyl or aryl. The linked Cp complex nW n 2 n[(C n 5 nH n 4 n) n 2 nSiMe n 2 n] n(NMe n 2 n) n 4 n was found to show similar structural nfeatures although the molecular structure suffered from disorder. The ndata reported, together with previous results, show that ncyclopentadienyl ligands compete favorably with the NMe n 2 n nligand as σ n 2 nπ n 2 n donors, nη n 3 n-C n 5 n. The η n 5 n-C n 5 n nmode is not favoured (on electronic grounds) because this would require ndisruption of the M–M triple bond in the presence of four nMe n 2 nN σ n 2 nπ n 2 n donors.