Neutral binary

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

Several explosive species in which three or more azido groups are bonded to tellurium have been well characterized.59 The salt [Te(N3)3][SbF6] is formed from the reaction of Te4[SbF6]2 with potassium azide in liquid sulfur dioxide. The neutral binary tellurium azide Te(N3)4 is prepared by the reaction of TeF4 (in CFC13)60 or TeF6 (in CH3CN)61 with trimethylsilyl azide. 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

The development of G. N. Lewis s octet rule for the s/p-block elements was strongly influenced by the stoichiometric ratios of atoms found in the common compounds and elemental forms (CH4, CCI4, CO2, CI2, etc.). Let us therefore begin analogously by examining the formulas of the common neutral binary chloride, oxide, and alkyl compounds of transition metals. (Here we substitute alkyl groups for hydrogen because only a small number of binary metal hydrides have been well characterized.)... 

Neutral Binary Carbonyls of Group VIII Elements... 

Table 8.1 compiles the most common stable homonuclear neutral binary carbonyls. They can be prepared by direct carbonylation of pure metals or made from available metallic precursors by reductive carbonylation. However, most of them are commercially available. Moreover, heteronuclear carbonyl compounds are known, and they are usually prepared by reaction between homonuclear binary carbonyls or carbonyl-derivative complexes. 

Table 8.1 Most common stable neutral binary metal carbonyls ...

Catalysts prepared from iridium neutral binary carbonyl compounds and several supports have been studied extensively. Small Ir (x = 4, 6) clusters supported on several oxides and caged in zeolite, and their characterization by EXAFS, have been prepared [159, 179, 180, 194-196]. The nuclearity of the resulting metallic clusters has been related with their catalytic behavior in olefin hydrogenation reactions [197]. This reaction is structure insensitive, which means that the rate of the reac-hon does not depend on the size of the metallic particle. Usually, the metallic parhcles are larger than 1 nm and consequently they have bulk-like metallic behavior. However, if the size of the particles is small enough to lose their bulk-like metallic behavior, the rate of the catalytic reaction can depend on the size of the metal cluster frame used as catalyst. 

Lower yields of Na3[Mn(CO)4] were obtained from the direct reduction of Mn2(CO)10 in Na-HMPA, because the neutral dimer underwent slow disproportionation in this medium to form [Mn(HMPA)Jt][Mn(CO)5]2, in contrast to Re2(CO)10, which showed no tendency to react with HMPA at room temperature. Of all neutral binary carbonyl dimers known, Re2(CO)10 appears to be the most resistant toward Lewis base-promoted disproportionation reactions. The slightly lower yields of Na3[Re(CO)4], compared to those of Na3[Mn(CO)4], may have arisen from the fact that Re2(CO)10 does not cleanly reduce to [Re(CO)5] in HMPA or other solvents (22). It should... 

Subsequent to Stock s work, many additional hydrides have been prepared and characterized, including a number by R. Schaeffer and associates, and today about 35 neutral binary boranes (including isomers) are known (Table 5-2). Many derivatives containing organic or halo substituents have been synthesized, and thousands of other species have been generated via insertion of metal or nonmetal heteroatoms into borane frameworks as described in Section 5-6. The polyhedral borane anions (see below) add yet another dimension to the remarkably rich area of polybo-rane chemistry. 

In addition to the neutral, binary chalcogen compounds, there are ionic and ternary ones. Examples of the former are the P7S ion31 and the Asi0Te3" ion.32... 

Subsequently Bartlett and colleagues [72,73] made detailed studies of the oxidant strengths of the anions AgF4 and NiF -, of the neutral binary fluorides AgF3 and NiF4, derived from the anions by action of Lewis acids in HF, and of cationic... 

This index is provided so that the reader might locate information about a particular complex of interest. It is organized as follows The first section contains the neutral binary complexes, followed by charged dimers, and then by larger complexes in the last section. Within each section, the complexes are ordered by the type of molecules contained. The order is as follows XH, YH2, ZH3, carbonyl, carboxyl, imine, amide, nitrile, alkyne, alkene, alkane, others. (X refers to any halogen F, Cl, Br, I Y to O, S, etc. and Z to N, P, etc.) Any alkylated or similar substitutions follow immediately after their parent group. 

We have seen that SiN, and (SN) can be prepared from S4N4. Other neutral binary sulfides may be obtained from it as well. When S.,N., is heated underpressure in a solution of CS, containing sulfur, SjN, is formed. This molecule has a half-chair conformation (Fig. 16.31). Others such as S5N< and S, N, have also been reported. 

The occurrence of stable neutral. binary carbonyls is restricted to the central area of the d block (Table 19.3), where there are low-lying vacant metal orbitals to accept o-donated lone-pairs and also filled d orbitals for jr back donation. Outside this area carbonyls are either very unstable (e.g, Cu, Ag, p. 1199), or anionic, or require additional ligands besides CO for stabilization. As with boranes and carboranes (p, 181), CO can be replaced by isoelectronic equivalents such as 2e , H , 2H or L. Mean bond dissociation energies Z>(M-CO)/kJ mol increase in the sequence Cr(CO)fi 109, MoCCO>6 151. W(CO)6 176, and in the sequence Mn2(CO)so lOO, Fe(CO)s 121, Co2(CO)g 138. NiCCO)4 147. 

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