Important Classes of Metal Carbonyls

CAUTION neutral homoleptic metal carbonyls are volatile, lipophilic and highly toxic. 

The ability of carbon monoxide to stabilize low oxidation states is most impressively demonstrated by the synthesis and isolation of complexes wherein the metal adopts formally negative oxidation states, of which the d10 complexes [MCCO) 4 (M IV = Cr, Mo, W) represent the (current) extreme. 

Alkali metal salts of carbonyl metallates are often pyrophoric (spontaneously flammable in air) however, in many cases they may be isolated more conveniently as salts of large organic cations, e.g. [N(PPh3) ] (PPN ) and [AsPh,]H 

Chemistry involving transformations of the cyclopentadienyl (Cp) ligand itself will be discussed in Chapter 7 however, much of the chemistry of cyclopentadienyl complexes involves the ligand acting purely as a spectator. Thus there arises a broad area of chemistry involving complexes with both carbonyl and cyclopentadienyl ligands. The most common classes of such compounds (Table 3.4) are the mononuclear complexes 

The nitrosonium cation [NO]+ is isoelectronic with CO and accordingly many mixed nitrosyl-carbonyl complexes are known. For electron counting purposes, the neutral molecule is considered to act as a 3 (or occasionally 1) VE donor. Thus various series of isoelectronic complexes can be envisaged (Table 3.5). The majority of synthetic routes to nitrosyl-carbonyl complexes involve (i) photochemical CO substitution or metal-metal bond cleavage by NO (ii) electrophilic attack by nitrosonium salts, e.g. [NO]BF4 or nitrosyl halides (e.g. C1NO) upon electron- 

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