Compounds with unbridged dative metal-metal bonds of formula (R3P)(2)(OC)(3)OsW(CO)(5) and related complexes

Complexes of formula (R3P)(2)(OC)(3)OsW(CO)(5) and similar complexes have b een prepared from W(CO)(5)(THF) and Os(CO)(3)(PR3)(2) in CH2Cl2/hexane at r oom temperature. The analogous Cr complexes could not he synthesized except for [MeC(CH2O)(3)P](2)(OC)(3)OsCr(CO)(5) and (Me2PCH2CH2PMe2)(OC)(3)OsCr(C O)(5), that is, with sterically undemanding P substituents. Tungsten compou nds with P ligands with cone angles greater than similar to 125 degrees cou ld also not be prepared. The crystal structures of (Me2PCH2CH2PMe2)(OC)(3)O sM(CO)(5) (M = Cr, W), [MeC(CH2O)(3)P](2)(OC)(3)OsW(CO)(5) (2), (OC)(3)(Me3 P)(2)OsW(CO)(5) (3), [MeC(CH2O)(3)P] (OC)(3)(Me3P)OsW-(CO)(5) (4), and [(Me O)(3)P] (OC)(3)(Me3P)OsW(CO)(5) (5) reveal that all have unbridged OsM bond s that are considerably longer than the corresponding OsM bond in (Me3P)(OC )(4)OsM(CO)(5). The P ligands]nave an axial, radial, arrangement except in 3, where the PMe3 ligands have a trans, diradial orientation. In 4 and 5 th e phosphite ligands are in the site trans to the OsW bond even though they have smaller cone angles than the PMe3 ligand. Solution NMR data indicates that for 2 and 3 both the ax,rad and dirad isomers are present; for 4 both ax,rad forms are present, whereas for 5 only the solid state form is found. There was no evidence in solution for the dirad forms of 4 and 5. (The C-1 3 NMR spectra of the compounds also indicated that (2)J(PC) becomes zero fo r PMC angles of about 103 degrees.) The unusual site preference of the P li gands in these molecules is interpreted in terms of steric effects and an e lectronic preference for a good pi -acceptor ligand to adopt the position t rans to the dative metal - metal bond. Complexes 2, 3, and 5 react with PPh 3 in CH2Cl2 at room temperature over 2 days to give W(CO)(5)(PPh3) and Os(C O)(3)(PR3)(PR'(3)) in the order 3 > 5 > 2.