Metallate anion, penta-carbonyl

As a result of the unusually strong reductive character of the penta-carbonyl metalates(-II) [M(CO)5]2 (M = Cr, Mo, W), and the consequent ready oxidizability of the dinuclear anions [M2(CO)10]2 and [HM2(CO)10], only relatively lately have we been able to publish an interpreted IR spectrum of Na2[Cr(CO)5] (31). There is no doubt, however, that the [M(CO)5]2 anions have trigonal bipyramidal structures as the observed intensities and frequency differences of the CO bands are inconsistent with a C4v symmetry of a tetragonal pyramid. 

Their strong reducing nature is a special characteristic of the penta-carbonyl metalates(-II) and decacaibonyl dimetalates(—I) of chromium, molybdenum, and tungsten that distinguishes them from the other carbonyl metalate anions of the 3d metals, e.g., [Co(CO)J. The oxidation of the mononuclear species with water... 

Although Ni(CO)4 was discovered many years ago, no neutral Ni2(CO)x compound has ever been synthesized in macroscopic amounts. However, several communications report ionic species such as [Ni2(CO)8l+, [Ni2(CO)7], and [Ni2(CO)6]+, where structures with one or two bridging carbonyls are proposed.2418 Plausible structures for neutral Ni2(CO)x (x = 5, 6, 7) have been investigated by theoretical methods, and decomposition temperatures well below room temperature have been predicted.2419,2420 Tetra-, penta-, and hexanuclear nickel carbonyl clusters have been investigated by means of molecular orbital theory. It is found that the neutral forms are more stable than the corresponding anionic forms but the anionic forms gain in stability as the nuclearity rises.2421 Nickel carbonyl cluster anions are manifold, and structural systematics have been reviewed.2422,2423 An example includes the anion [Ni9(CO)i6]2- with a close-packed two-layer metal core.2424... 

Finishing off this section we can report on various reactions of penta-fluorohalobenzenes with transition metal carbonyl anions ... 

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