Synthesis, structure and reactions of a dihapto-formaldehyde complex, Os(η2-CH2O)(CO)2(PPh3)2

Abstract Os(η 2 -CH 2 O)(CO) 2 (PPh 3 ) 2 results from direct reaction of formaldehyde with the zerovalent complex, Os(CO) 2 (PPh 3 ) 3 . The structure of Os(η 2 -CH 2 O)(CO) 2 (PPh 3 ) 2 was determined by X-ray crystallography. The crystals are triclinic, space group P 1 , a 18.739(2), b 11.157(1), c 9.986(1) A, α 116.70(1), β 93.20(1), γ 107.93(1)°, V 1727.69 A 3 , Z = 2, D m 1.55(2), D c 1.57 g cm −3 . Refinement of atomic parameters was by full-matrix least-squares methods, employing anisotropic thermal parameters for all non-hydrogen atoms except for the carbon atoms of the phenyl rings. The formaldehyde hydrogen atoms were located from difference electron density maps, other hydrogens were included in calculated positions. Final residuals were R = 0.047 and R w = 0.061 for 3508 unique observed reflections measured on an automatic diffractometer. The complex itself is monomeric, although interstices in the crystal lattice are occupied by hydrogen-bonded water dimers which fulfil a purely space-filling role. The osmium is bonded to two mutually trans triphenylphosphines, two carbonyls, and the η 5 -formaldehyde, in an arrangement which is best described as distorted octahedral. The geometry of the coordinated formaldehyde is characterised by OsO 2.039(7), OsC 2.186(8) and CO 1.584(11) A. The OsP bonds are equivalent at 2.372(2) and 2.378(2) A but the OsCO bond trans to the formaldehyde carbon 1.931(7) A is longer than that trans to the formaldehyde oxygen 1.907(7) A. Os(η 2 -CH 2 O)(CO) 2 (PPh 3 ) 3 has proved to be a useful synthetic precursor for stable formyl, hydroxymethyl, methoxymethyl, and halomethyl (CH 2 X, X  Cl, Br, I) complexes. The compounds Os(CHO)H(CO) 2 (PPh 3 ) 2 , Os(CH 2 OH)H(CO) 2 (PPh 3 ) 2 , Os(CH 2 OMe)Cl(CO) 2 (PPh 3 ) 2 and Os(CH 2 Cl)Cl(CO) 2 (PPh 3 ) 2 are illustrative of the many compounds which have been characterised. A general synthetic route to neutral formyl osmium complexes, Os(CHO)X(CO) 2 (PPh 3 ) 2 (X = halide or alkyl) has been developed from reaction of the cations [OsX(CO) 3 (PPh 3 ) 2 ] + with BH 4 − . Acetaldehyde also reacts with Os(CO) 2 (PPh 3 ) 3 forming Os(η 1 -C[O]CH 3 )H(CO) 2 (PPh 3 ) 2 . No reaction was observed with benzaldehyde, and trichloroacetaldehyde affords the cation, [OsCl(CO) 2 (PPh 3 ) 3 ] + .