Six-electron donors

A benzannulation reaction yielding the naphthoquinone 61 could also be performed with the ruthenium carborane-stabilised carbene 60 and 1-hexyne [56] (Scheme 36). The ruthenium carbene unit can be regarded as an 18-electron fragment containing a formal Ru(II) centre coordinated to a dianionic six-electron-donor cobaltacarborane ligand. 

The coordination chemistry of ancillary amidinate ligands with a pyridine functionality has been described. Magnesium, aluminum, zirconium, and lanthanum complexes have been prepared in which the amidinate anions act as tridentate, six-electron-donor ligands Amidinate ligands containing quinolyl substituents were constructed in the coordination sphere of lanthanide... 

Borazine is isoelectronic and isostructural with benzene and may act as a six-electron donor in complex chemistry. In contrast to arene ligands of arene-transi-tion-metal complexes, coordinated borazines lose their planarity and are slightly puckered . Nevertheless, the B atoms show interactions with metal atoms. 

In the complex [Cr(CO)3cht], the cht ligand functions as a six-electron donor because three CO ligands donate six electrons and 12 are needed by the chromium atom to reach a total of 36. Therefore, this complex has the structure... 

Cyclotieptainene is a six-electron donor that can form complexes similar to those of benzene but differing in the localization of the it electrons in C7H . The alternation in bond length in the free cyclohcpiairiene is retained in the complexes (Fig. 15.43). Furthermore, in CjH Mo(CO), the double bonds are located trans to the carbonyl groups, providing an essentially octahedral environment for the metal atom-... 

The a--N,/i2-N, 7 2-C=N (six electron) donor mode is represented by Fe2(CO)6(Bul-DAB) (26) in Table 4.119 The structure indicates that Bu -DAB donates two electrons via one N atom of the o--bonded N=C group (N=C 1.260(5) A) and four electrons via the other N=C group which has a bond length of 1.397(4) A. In this case this N=C group denotes two electrons through the bridging N atom and two electrons by t]2-bonding to the second Fe atom. 

The preparation, properties and chemistry of complexes containing 2 -C—N -bonded (six-electron) R-DAB groups have been extensively reviewed before.112 The structure of Fe2 (CO)6 (Bu -DAB) (26 Table 4)119 has been mentioned previously and the six electron-donor mode has been discussed formally (Section 13.5.3.3). 

An interesting point is that up till now it has been very difficult to prepare complexes containing the six electron donor mode with other substituents on the rf -bonded C=N unit. The only exception is MnCo(CO)6(Prc-DAB Me,Me ) with a cyclopropyl substituent on the N atoms.137 No doubt steric reasons are important but it should be noted that in the case of the eight electron donor mode for R-DAB R, R" stable complexes could also be obtained for R = R" = Me.158... 

In addition to compounds containing six electron donor R-DAB ligands compounds with bonding modes have also been formed. 

Figure 6),113 an almost planar /u3-TeM3 skeleton in which the central tellurium atom acts as a six-electron donor. The complex was formed from the reaction between CpMn(CO)2(THF) and H2Te and was obtained in the presence of a by-product formulated as Te2[Mn(CO)2Cp]3. 

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