Four-carbon-donor Ligands

Two isomers of the compound [Fea(CO)6(CoHij)] have been investigated. In (22a) the allene trimer co-ordinates one Fe(CO)3 group through a 1,3-diene system and the other Fe(CO)a group through a w-allyl system and 

The complex [Fe(7r-C4H6)(w-C6H6)(GeCl2Me)] (24) is described as an acute sandwich , with an angle of 17.6° between the planes of the two hydrocarbon ligands. The iron atom has a distorted octahedral co-ordination, 

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Table 2 collects representative chemical shift values, primarily from the more recent literature. It can be seen, for example, that the tabulated values for linear sUver(I) coordinated to two carbon atoms (AgC2 ) span a range of ca 100 ppm, from roughly 550 to 650 ppm. In stark contrast, the chemical shifts of the recently reported square-planar silver(III) ions with four carbon-donor-atom ligands span a range from 2000 to >2500 ppm . These new values expand the previously known range of ca 1500 ppm, primarily for Ag(I), by an additional 1000 ppm The dominant contribution to the chemical shifts of Ag is the paramagnetic term in the treatment of Ramsey... 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

Section IV describes reactions which produce four-electron donor alkyne ligands by coupling two single carbon based ligands (CR, CO, CNR). Section VIII,A describes syntheses of i72-vinyl complexes (t 2-CR=CR2 ) by nucleophilic addition to four-electron donor alkyne ligands. In the products of the coupling reactions and in the reagents of the r 2-vinyl syntheses, the role of alkyne 7t donation is crucial. 

Bisalkyne d4 monomers, with N = 3 by symmetry, exhibit proton and carbon chemical shifts at higher fields than those of monoalkynes with N = 4. The proton chemical shift of 10.45 ppm for Mo(PhC=CH)2-(S2CNEt2)2 (52) falls nicely between the four-electron donor Mo(CO)-(PhC=CH)(S2CNEt2)2 case (12.6 ppm) and the two-electron donor (7r-C5H5)2Mo(HC=CH) case [7.68 ppm (Table II)]. Additional data for bisalkyne complexes, including pyrrole-N-carbodithioate derivatives, support a correlation of H chemical shifts with alkyne ttj donation, with three-electron donors typically near 10.0 0.5 ppm. Similar H values are found for cyclopentadienyl bisalkyne complexes with terminal alkyne ligands. Chemical shifts between 8.5 and 10.5 ppm characterize all the neutral and cationic bisalkynes listed in Table V except for [CpMo-(RC=CH)2(MeCN)]+ where one isomer has S near 11 ppm for the acetylenic proton (72). 

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