The reduction of [Co(III)(tmsalen)py(2)](+)ClO(4)(-), where tmsalen = 4,4',7,7'-tetramethylsalen, with NaBH(4)/PdCl(2) in alkaline methanolic solution, followed by the oxidative addition of CH(2)ClI, leads to the expected trans organometallic dimeric species 1, [CH(2)ClCo(tmsalen)](2), provided that the product is recovered from the reaction mixture immediately after the completion of the reaction. If 1 is left for longer time in contact with the reaction mixture, the intramolecular reaction of the axial chloromethyl group with the equatorial chelate leads to the formation of the monocationic complex 2, containing a seven-membered ring. In this complex the novel tetradentate ligand coordinates Co in a cis fashion, the other two positions being occupied by one py and one water molecule. The resulting complex is chiral, even if the reaction product is a racemic compound. The unidentate ligands of 2 have been exchanged quantitatively for N-MeIm, and the resulting complex 3 still maintains the beta cis geometry. Therefore, 2 may be considered the precursor of a new class of organocobalt derivatives with a folded tetradentate ligand and two adjacent exchangeable sites. On the basis of the geometry of the tetradentate Schiff bases in complexes, where they adopt a planar geometry, it was suggested that there is a significant electron density delocalization involving the metal center over the two chemically equivalent moieties of Co(chel). Comparison of the geometry of the planar salicylaldiminate (sal) moiety with that of the cyclized methoxy-iminate one (imi) in 2 and 3 strongly supports that the delocalization, still present in sal, is essentially either lost or strongly reduced in imi.

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Novel cobalt( 111) complexes of tetradentate chelating agents (chel) and monodentate tertiary phosphines of the type [ColIJ(chel) PR3]+CIO4and [ColI1(chel) PR,( H20)]+CIO4-are reported. A five-co-ordinate square pyramidal structure is suggested for both types of compound in non-complexing solvents. The thermodynamic rrans-effect of PR, is indicated by the stoicheiometric formation constants of [CoIII(chel) (PR,) (NO,)]O which can be correlated with u" Taft constants of the phosphines. The trend of the trans-effect of L in [CoIII(chel)- Trieste, Trieste, ItalyA FIVE-CO-ORDINATE cobalt ( 111) complex [CH3CorIr(chel)]0 [chel = the tetradentate NN'-ethylenebis(acety1acet-0neiminato)dianion (bae) (I) and a methyl group in the axial position] has been reported previously.1,2 A square pyramidal crystal structure was assigned to the compound on the basis of an X-ray diffraction study., On the basis of physicochemical properties, five-coordinate structure was also assumed for other cobalt (111) complexes of general formula RCo(che1) l p 2 p 4 * (R is an organic group and chel = NN'-ethylenebis (salicylideneiminato) (salen) (11) ; NN'-ethylenebis-(7,7'-dimethylsalicylideneiminato) (7 ,7'-Me,-salen) (111) ; o-phenylenebis(salicylideneiminat0) (saloph) (IV) . Reversible changes in optical spectrum of organocobinamides were also explained by the equilibrium, R-CoIII corrin (H,O) R-CoIII corrin + H20 implying a five-co-ordinate approximately square pyramidal species or a very weak CoI11-OH2 bond in ~o l u t i o n . ~ Five-co-ordinate species are thought to be intermediates in S x l substitution reactions 8-10 and can be relevant in the biochemical function of vitamin B,, complexes.Here we report some novel apparently five-co-ordinate complexes with apical tertiary phosphines as ligands [PR,Co(chel)]+ and [PR,Co(chel) (H20)]+. The thennodynamic trans-effect of PR, is correlated with the CT*

Results of the accurate crystal structure determination of NO(2)Cbl.2LiCl (1), NO(2)Cbl.NaCl (2), NCSCbl (3) and NCSeCbl (4), based on synchrotron diffraction data collected at 100 K, are described. The nitro group in (1) was found to be disordered with two orientations that differ by a rotation of approximately 60 degrees about the Co-NO(2) bond, whereas in (2) the nitro group has only one orientation. The first X-ray structural determination of a cobalamin with a Co-Se bond is reported. Comparison of the axial distances indicates that SeCN has a bond length of 2.384 (3) A and that the trans influence on the Co-N bond is only slightly greater than that of SCN. The crystals of the thiocyanate cobalamin contain both the S- and N-bonded coordination isomers in a 3:2 ratio. The structural features of the Co-S bond in cobalamins are discussed. The crystal chemistry of cobalamins is discussed in terms of packing of roughly spherical molecules. The unit-cell parameters can be used to group the cobalamins' crystal structures in different arrays intermediate between distorted hexagonal close packing and primitive hexagonal arrangements. The structural features of cobalamins, and of cobaloximes that have the same axial fragment as the cobalamins, are reviewed and discussed in terms of the cis influence of the equatorial ligand.

Treatment of R = -CH(2)X (X = halogen) derivatives of the type [RCo(III)(LNH-py)(HLNH-py)](+), where HLNH-py = 2-(2-pyridyl-ethyl)amino-3-butanone oxime and LNH-py its conjugated base with diluted NaOH, afforded a new complex containing a three-membered ring by a pathway involving the intramolecular nucleophilic addition of an equatorial nitrogen donor to the axial carbon. The X-ray analysis reveals a highly distorted structure. The C-Co-N angle is acute (42.8 degrees ) with the distortion of the coordination sphere concentrated in the Co-C axial and Co-N bonds, which move away from the pseudo-octahedral positions in the CH(2)X parent complex to form the C-N bond of the metallocycle. Kinetic studies of the formation of this novel complex starting from [(XCH(2)Co(III)(LNH-py)(HLNH-py)](+) (X = Cl,Br,I) showed that the metallocycle formation rates increase in the order Cl < Br < I. Kinetic data are consistent with a mechanism involving an intermediate species resulting from the deprotonation of an amine equatorial nitrogen in a rapid preequilibrium, followed by the slow step of the ring closure.