4-2 oxidation state 1058 carbonyl halides

Ever since the early days of cluster chemistry, we have been fascinated by these homopolyatomic (multinuclear) units [M ] built firom like atoms of one element just like Plato was fascinated by polyhedra. They are observed most commonly with the first half of the 4d or 5d elements ( early transition elements M) in low oxidation states when halide ligands are involved or, roughly, with the second half with carbonyl or comparable ligands. 

We are now close to being in a position to examine the effects on CO stretching parameter, for the isoelectronic isostructural series of first row carbonyl halides considered, of changes in metal, degree of substitution, coordination site, and halide. Along the sequence Cr(0), Mn(I), Fe(II), both 5 a and 2n populations fall. In other words, a higher oxidation state of the metal renders it at once a better a-acceptor and a poorer jr-donor perhaps a predictable result. Numerically the a -effect is the greater, but both are in the same direction, and parameters... 

For practical purposes the field of metal-metal bonds and metal atom clusters can be divided into two broad areas. (1) Those compounds with the metal atoms in formal oxidation states of zero or close to it, including negative ones. For the most part these are polynuclear metal carbonyls, or very similar compounds. In these compounds the M-M bonds are usually long, weak and of order one. (2) Compounds with the metal atoms in low to medium positive oxidation states, and ligands of the same kinds normally found in classical Werner complexes, e.g., halide, sulfate, phosphate, carboxylate or thiocyanate ions, water, amines and phosphines. Compounds of this type include metal-metal bonds of orders ranging from about 1/2 to 4.0. 

Complexes in the 0 oxidation state may be obtained either by reduction of halides such as RuCl2(PPh3)3 with Na or Zn in the presence of CO or other ligands such as RNC, or by reaction of metal carbonyls with phosphines. Reactions of polynuclear carbonyls such as Ru3(CO)12 (Section 18-F-13) with phosphines tend to preserve the cluster structure. 

More recently, methods based on the use of mild reductants, able to transfer a single electron to the polyhaloalkyl halide, have been described. Various metals or their derivatives have been employed ruthenium, platinum and their complexes in low oxidation state, iron" and its carbonyl complexes, or tetrakis(triphenylphosphane)palladium. Sodium arcncsul-finate, sodium dithionite" and various oxidants have also been used. Other examples of polyhaloalkyl halide additions to simple alkenes are summarized in Table 1. Typical examples are the formation of diiodide 6, chloro iodide 7, and iodo steroid 8. ... 

Re carbonyl halides in higher oxidation states are rare. The paramagnetic di-[ReX4(CO)2] (X = Cl, Br) resnlts from oxidative addition of X2 to (15) or [Re(CO)3Br3] . hi... 

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