Homogeneous catalysis has been re-from the "chiral pool". However, the re-terns. The second section highlights the sponsible for many major recent devel-markable achievements in this area have most significant, homogeneously cataopments in synthetic organic chemistry. been based on an empirical approach. lyzed reactions involving complexes The combined use of organometallic This article attempts to bridge the gap with nitrogen ligands. Foremost among and coordination chemistry has pro-between the synthetic and the coordinathem are the asymmetric aspects of hyduced a number of new and powerful tion chemist. The first section discusses drogenation (particularly those involvsynthetic methods for important classes the rates of formation and dissociation ing boranes as reducing agents). hyof compounds in general and for opti-of complexes with nitrogen donors, their drosilylation, cyclopropanations, Dielscally active substances in particular. For relationship to the rates of product for-Alder reactions. aldol condensations, this purpose. complexes with optically mation, and presents the factors which alkylation of aldehydes, conjugate addiactive ligands have been used, most of induce homolytic cleavage of M-C tion reactions, Grignard cross-coupling them coordinating through phosphorus.bonds. It also provides a summary of the reactions, allylic alkylations, oxidation More recent developments have high-main types of organometallic complexes reactions, olefin epoxidations. and dilighted the use of "nitrogen-donors", formed by metal centers coordinated to hydroxylation of olefins. particularly as they are easily obtained nitrogen donors and their reactivity pat-\

Chatt and Veuanxi. 4735 955. Olefin Co-ordination Compounds. Part VI.* Dime Complexes of Rhodium(~).

on a wide range of couplings has shown that consistent results are often obtained if A W is regarded as being constant within a series of similar compounds. The terms involved in A E are singlet-to-triplet excitation

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The compounds (Ph,P),NiX, (X = C1, Br, I, NO,, and SCN) have been prepared and their properties studied. The chloro-and iodo-, and presumably also the bromo-and nitrato-complexes, have tetrahedral structure, and pefF.'s of about 3 B.M. The thiocyanato-complex is diamagnetic and has tram-square planar structure.The formation of tetrahedral nickel(I1) complexes is discussed in terms of ligand-field theory.CONTRARY to general belief, tetrahedral complexes of nickel(r1) are rather uncommon. This structure has been assigned to many such compounds, e.g., bisacetylacetonenickel (I) (peE. = 3.2 B.M.) ,l, dehydrated bis-salicylaldehydenickel(I1) (paramagneti~),~ bis(diethy1-3 : 3' : 5 : 5'-tetramethylpyrromethene-4 : 4'-dicarboxy1ate)nickel (111) (pes. = 3.2 B.M.),4 and bistriethylphosphinedinitratonickel, (Et,P) 2Ni(N0,) (peE. = 3.10 B.M.) .5 Me;C=ONo unanibiguous evidence, however, has been advanced to show that any of these is tetrahedral. Bisacetylacetonenickel is trimeric in the solid state,6 and the fact that it is monomeric in pentyl benzoate is not very significant because the latter is a donor solvent. Dehydrated bis-salicylaldehydenickel has an X-ray powder pattern different from that of bis-salicylaldehydecopper, which is said to have square planar ~t r u c t u r e ,~ and very similar to that of the dehydrated zinc complex, the structure of which is not known. Moreover, the nickel and the zinc complex appeared to be incompletely dehydrated?The pyrromethene complex (111) has been ascribed tetrahedral structure on the basis of its paramagnetism, and the fact that calculations of van der Waals radii for the molecule show serious overlap of the a-methyl group^.^ No molecular-weight determination, however, has been reported, and the presence of other potential donor groups in the molecule casts doubt on the validity of the conclusions.Bistriethylphosphinedinitratonickel, (Et,P) ,Ni(NO,) 2, has always been considered most probably to be tetrahedral. It is monomeric in benzene and has a high electric dipole moment (8.85 D).* The magnetic moment in solutions of non-donor solvents, however, has not been measured, and Nyholm has pointed out that stereochemical arrangements other than tetrahedral can also be postulated for such a comp0und.~110At the beginning of the present investigation the only well-established cases of nickel(I1)

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Compounds of the types [diene PdX,], [(diene*OR),Pd,X,], and [ (diene*OR) , $-toluidine PdCl] have been prepared. Their properties indicate that they have similar structures to those of their platinous analogues. Generally, the compounds are more deeply coloured, more easily formed, more reactive, and less stable than the corresponding platinous compounds. The alkoxy-derivatives are the first strictly organometallic derivatives of palladium to be reported.IN Part IV we recorded the preparation of a number of chelated platinous complexes of the type [diene PtCl,] and their conversion into alkoxy-derivatives of the type [ (diene*OR),Pt,Cl,]. It was interesting to see whether palladium(11) would give similar derivatives and how they compared with their platinum analogues.Stable diene derivatives were obtained from the cyclic diolefins cycloocta-1 : 5-diene, C,H,,, and dicyclopentadiene, ClOH1,, but not from dipentene, C,,H,,, which reduced palladous salts to the metal. The compounds prepared are listed in the Table . Diene complexes of palladium( 11) atzd their alkoxy-derivatives.