Advances in Organometallic Chemistry (eBook)

4 Compounds with E5 Fragments

As for E4, the E5 group is well represented by P and As examples (Table XXXVII). The planar [E5]− ring is isoelectronic to Cp− and forms similar complexes with transition metals (169–172). Thus, one Cp− ring of metallocene complexes can be replaced with the E5− unit, and analogues to the CpM(CO)3 molecules are known. Triple-decker sandwich complexes such as 170 are well established. This geometry has been studied by extended Hückel calculations,475–477 and electron counts of 26 to 34 electrons were found to be stable, depending on the degree of interaction between the metal fragments and the central ring. The M–M interaction through the rings was found to be partly responsible for stabilizing the electron-deficient electron counts.477 The [CpMo]2As5 compound, which shows two long As–As distances, may be better written as [CpMo]2(As3)(As2). The asymmetry in the As–As bonding was attributed to a Jahn–Teller-type distortion. Additional metal fragments can add to the E–E bonds as in 173, or insert into them as in 174–176.

Two examples of metallated E5R5 compounds are known and have similar coordination geometries (177, 178). The major difference between the arrangements in 177 and 178 is the additional coordination of the middle PR unit in 177. As with E3R3 and E4R4 units, there is a preference for the terminal E atoms to bridge between metal centers.

5 Compounds with E6 Fragments

Planar six-membered rings from triple-decker complexes (179) for P and As (Table XXXVIII) with metals including V, Nb, Cr, Mo, and W. These complexes have been examined by extended Hückel calculations, as have the E5 analogues.476 Unlike the E5 system, no compounds with only one metal moiety attached to a planar E6 group have been described. The compound [CpTi]2P6 has titanium atoms on opposite sides of a P6 ring, but each Ti atom is only bonded to three of the P atoms and the result is a cubane-like structure (180). Various isomeric forms of P6 have been examined by SCF MO calculations.216

Complexes 181–183 can be derived from As6 trigonal prisms in which varying degrees of bond breaking between the As atoms have occurred. These complex molecules can also be viewed as derived from other cluster frameworks. For example, the framework for 182 can also be derived from an As4MoCo trigonal prism capped on its faces by two As atoms and one Co fragment. It has been compared to 95+, which has a similar framework. Both have 22 skeletal electrons.494 Compound 183 has been described as a distorted sphenocorona structure with some similarity to Au11 cluster compounds containing one Au atom in an interstitial site. Quite different geometries of the P6 unit have been noted for [(Cp1B)2Th]2P6 (184), which is based on a tricyclic main group fragment, whereas in 185 the compound described previously as 131 is redrawn to emphasize the P6 fragment, which can be viewed as arising from the 73− ion with removal of the apical P atom.

Metallated organic derivatives of both P6 and As6 units have been reported. A very intriguing molecule is based on a nearly planar 6tBu6 ring with a nickel atom located at its center (186). This arrangement suggested an unusually high electron count at Ni,491 but calculations later concluded that the compound is effectively a 16-electron species at Ni.490 Compound 186 has a similar orbital structure to [In{Mn(CO)4}5]−, in which an In atom sits at the center of a nearly planar pentagon of Mn atoms. The 6iPr6 chain has been observed bound to Fe2(CO)4 (187), which is similar to complexes of the other PxRx (x = 3–5) units. Again, the terminal P atoms show the tendency toward bridging two metal centers. The two centermost P atoms are also coordinated. The more unusual P3{P(SiMe3)2}{PP(SiMe3)2} ligand has been isolated bound to a Cp2Zr fragment (188). The cyclic As6Ph6 ring coordinates in a chair conformation to Mo(CO)4 as shown in 189.

6 Compounds with E7 Fragments

The 73− ions (190) are known for P, As, and Sb (but not Bi), and they form the basis for a number of metallated derivatives (Table XXXIX). For ease of discussion of the 190 structure, we will refer to the E atoms in the triangular unit as basal, those that are only two-coordinate as bridging, and the remaining P atom as apical. The bridging E atoms can be readily seen from the diagram to be relatively highly charged. Functionalization by cationic (R+, R2P+) species occurs here: P7R3 (191), P7(PR2)3 (192), P7(PR2)(P2R2) (193). These derivatives are mentioned because they have also been metallated. Similar to the alkylation reaction, 73− forms the metallated derivative P7(Fp)3 (194) with the metals attached to the bridging P atoms. The compounds 190–193 may serve as simple donor ligands to 16-electron metal fragments. This donation may occur via the basal atoms, the bridging atoms, or both (195, 196). 191 can also function as a chelating ligand with two bridging atoms attached to a single metal center (197). The PR2-derived compound also may act as a donor ligand at the basal sites and/or at the PR2 groups as illustrated in 198, or it may chelate using one PR2 function and one bridging P atom (199). Molecules having one, two, or three metal atoms chelated as in 199 have been...