Four-electron donors

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Bis(dimethylamino)methylborane thus acts as a four-electron donor. The reaction takes place in dioxane at RT and the red, crystalline complex is sublimable at 10 torr/20 C in 2-4% yield. 

Some iminoboranes dimerize during reaction with a transition-metal complex to diazadiboretidines. These four-membered rings then act as four-electron donors in different complexes ... 

Dihydro-lH-l,5,2-azasilaboroles derive from the 2,5-dihydro-lH-l,2-aza-boroles ( 6.5.3.3) by substitution of the carbon neighboring N by a silicon atom. They may act as four-electron donors using electron density from the C=C double bond and the N atom. The B atom behaves as an acceptor center. Two pathways are known for complex synthesis reaction with a generated transition-metal complex fragment and reaction with metal atoms by the metal-vapor synthesis method. 

Dihydro-l,3-diborolenes are accessible as pentaorgano derivatives " and serve as ideal precursors of the diborolyl radieal. The formation of the radical and its interaction with a metal center is formally described as a metal oxidation. The resulting 2,3-dihydro-1,3-diborolyI ligand is either a three-electron donor or an anionic four-electron donor. l,3,4,5-Tetraethyl-2-methyl-2,3-dihydro-l,3-diborole reacts with [T -CpNi(CO)2] or (T7 -Cp>2Ni in hot mesitylene to yield a sandwich complex and a triple-decked complex ... 

Bis-ylides (Scheme 31) may function as four-electron donors and become geminally diaurated. The donor capacity of ylides is generally higher than that of most other ligands, and even powerful donors like tertiary phosphines can thus be readily replaced by non-stabilized ylides. Ylides may therefore be used as auxiliary ligands which are retained, while other components of the coordination sphere are subject to substitution. 

On the other hand, in the complex [Ni(CO)2cht] the cht ligand is a four-electron donor, so two double bonds are functioning as electron pair donors and the structure is... 

The benzene molecule is attached as a phenyl group, between two metal centers (structure I), and as a "benzene four-electron donor, three-center coordinating group in structures II, VI, and X. The C-C distance in all these molecules is 1.42 A, and the dihedral angle between the planes containing the osmium atoms and the mean plane of the benzyne in the three complexes is remarkably constant at 69 3°. 

The photoreduction of eight electron-acceptor- and four electron-donor-substituted nitrobenzenes has been studied and quantum yields for either starting material disappearance or product formation have been reported 7). Photolysis of 4-nitrobenzonitrile and 4-nitrotoluene in air-saturated solutions was completely quenched and thus a triplet multiplicity of the reacting excited state was derived 7). 

When this occurs, the electron contribution of the ligand is increased by two. In die neutral atom scheme, it is now a three-electron donor and by the oxidation state method, a four-electron donor. The complex Rh CO C has two bridging chloro ligands ... 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

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