Pentacoordinated metals

Bimolecular substitution and oxidation reactions of 17-electron pentacoordinate metal carbonyl radicals. A. Poe, Transition Met. Chem. (Weinheim, Ger.), 1982,7, 65-69 (41). 

British Biotech has described co-crystal structures of both BB-3497 and actinonin bound in the active site of E. coli PDF [24]. The metal centre (Ni ) in both complexes adopts a pentacoordinate geometry, bound by the two oxygen atoms of the hydroxamate along with Cys-90, His-132 and His-136. This coordination pattern is consistent with the mechanism of de-formylation proposed by Becker et al. [56] and Jain et al. [67], in which a pentacoordinated metal centre stabilises the transition state during hydrolysis of the formamide bond. When compared to the co-crystal structure of a substrate hydrolysis product, Met-Ala-Ser, it is clear that the side chains of these two inhibitors bind into the active site pockets similarly to the substrate [56]. 

The dianion of 6,6 -bis(2,2 -diphenyl-2-sulfanylethyl)-2,2 -bipyridine (104) gives a chloro-iron (III) complex in which the pentacoordinated metal is in the 5=3/2 state. ... 

Figure 13.6. Valence orbitals of a pentacoordinated metal A (longer gray bracket) is analogous to Asu- The splitting of the upper levels (shorter gray bracket) is a measure of the trans effect.

Aluminium-based catalysts with tetraphenylporphinato [32,35,38,81], Schiff s base [37-40] and calix[4]arene ligands [41] are characterised by the appearance of a non-associated, isolated pentacoordinate metal atom forming an active bond Mt X with nucleophilic substituent X. As stated above, epoxide polymerisation with these catalysts involves the rearward attack of the nucleophilic substituent on the coordinating epoxide molecule [scheme (1)]. In order to explain this, a mechanism involving the simultaneous participation of two catalyst molecules in the initiation and propagation reaction has been proposed [40,41,62,82]. According to this mechanism, the rearward attack of the nucleophilic substituent on the coordinating monomer is carried out by the six-coordinate aluminium species. These species can appear as neutral epoxide... 

The photochemical cis-trans isomerization of bisphosphine-substituted carbyne complexes [Eq. (57), Section IV,B], which was studied in our group, was postulated to proceed via pentacoordinate metal ketenyl intermediates (97). Support for this postulate was found in the low-temperature characterization of ketenyl complex 208 on irradiation of complexes 207 in the presence of PMe, [Eq. (164)]. In the presence of phenylacetylene the ketenyl alkyne tungsten complex W(/ -PhCCO)CI(PhC2H)(CO)(PMe3)2 forms at low temperatures. 

Molybdenum hexacarbonyl is a highly toxic substance. It is listed in Toxic Substance Control Act. Acute symptoms include headache, dizziness, nausea, vomiting, and fever. Because of its low vapor pressure, the risk of exposure of this compound, however, by inhalation is lower than that from the volatile tetra- or pentacoordinated metal carbonyls. Ingestion of the compound can cause death. Absorption of its solution through skin may cause severe poisoning, manifesting the same effects as those from other routes of exposure. 

Figure 8 Two ligands with two (a) and three (b) bi- (dark blue) or tridentate (orange) metal coordination sites and possible structures for M2(ligl)2 and M3(lig2)2. Different structures provide a different coordination environment around the metal cations head-to-tail isomers of M2(ligl)2, for example, would require a pentacoordinate metal ion, one head-to-head metal ion with octahedral coordination geometry, and one with tetrahedral coordination geometry. (Reproduced from Ref. 6. John WUey Sons, Inc., 2009.)...

Comparison between various coordination numbers. The metal-carbon monoxide bonds in tetracoordinated and pentacoordinated metal carbonyls appear to be less stable than the metal-carbon monoxide bonds in the corresponding hexacoordinated metal carbonyls. For example, [Co(CO)4] reacts with allyl halides at room temperature to produce directly the 77-allyl derivative, C3H5Co(CO)3 the presumed pentacoordinate intermediate a-allyl derivative C3H5Co(CO)4 is not isolable, losing carbon monoxide rapidly even at room temperature 110). In contrast to this behavior. 

More recently, Auf der Heyde and Burgi have analyzed pentacoordinate metal complexes by comparing experimental... 

Figure 12 The Sn2 (a) and Berry (b) pseudo-rotation distortions that dominate the TBP to SQP transition in pentacoordinate metal complexes...

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