Five- and Six-co-ordinate Complexes

Ligand exchange reactions in octahedral carbonyl complexes often involve five-coordinate intermediates (see Chapter 1) and these species may be fluxional on the time-scale of the reaction. Square-pyramidal [W(CO)4(PPhs)] formed in the reaction of [W(CO)4(tmpa)l (tmpa= VWW -tetramethyl-l,3-diaminopropane) with PPhg is fluxional, but the most stable isomer probably contains an equatorially bonded 

Protonation of cA-[M(CO)2(P P)al (M=Mo, P—P=dmpe or dppe M=W, P—P=dmpe) gives the seven-co-ordinate hydrido-cations [MH(CO)2(P—Pla] which on deprotonation with KOBu produce isomeric /r j-[M(CO)2(P—P)2l- These transformations constitute the use of seven-co-ordinate intermediates in the isomerization of six-co-ordinate complexes. The rra/w-complexes [M(CO)2(P—P)2] revert under first-order kinetics to the more stable cw-isomers with rates (W Mo) which are unaffected by excess base or one atmosphere of carbon monoxide and these data are consistent with earlier results supporting a chelate arm-off arm-on mechanism with a five-co-ordinate intermediate (see Vol. 5). 

The six-co-ordinate carbene complexes [M(CO)4LJ (M=Cr, Mo, or W, L= CNMeCH=CHNMe, CNMeNMeCH=CH, CCHMeCH=NNMe. or tOSlMe-CeH4NMe) undergo cis- trans isomerization on photolysis and the reverse process occurs thermally and irreversibly by an intramolecular mechanism. Kinetic parameters for trans- cis isomerization of the bis(imidazole-ylide) complex [Mo- 

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R2S = thioxan or Et2S) in solution was also examined. Five- and six-co-ordinated complexes [Pt(thioxan) ] (n = 5 or 6) are formed in solution by treatment of [Pt(thioxan)J with excess thioxan the complexes for n = 6 were isolated as nitrate and picrate salts. ... 

Bromo(bispentafiuorophenyl)thallium reacts with the sodium or thallium(i) salts of the j8-diketones, (MeCO)gCH2, CFa-CO-CHa-COMe, (CF, CO)3-CH2,PhCOCHj-COMe,and (PhCO)sCH, to form four-co-ordinate dionato complexes, e.g. (C,F5)sTl[(MeCO),CH], and with the sodium salt of 8-hydroxyquinoline to form a dinuclear complex with bridging ligands. Reaction of these with triphenylphosphine oxide and triphenylarsine oxide, and with 2,2 -bipyridyl and phenanthrene, yields a number of five- and six-co-ordinate complexes. ... 

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