Tungsten hexacarbonyl reacts with alkali or tetraalkylammonium halides or pseudohalides MIX (X = halogen or NCS) with elimination of one CO ligand, giving compounds of the type MI[W(CO),X] 121. However, tetraethylammonium azide [31 and W(CO)6 (molar ratio 1 : 15) in diethylene glycol dimethyl ether (diglyme) at 80 "C afford yellow, diamagnetic tetraethylammoniuin isocyaiiatopentacarbonyltungstate(O), [N(C2H5)jl[W(CO).jNCO]. The anion [W(CO)5NCO]-is also directly accessible from W(CO)6 and potassium cyanate in diglyme at 100 "C and can be precipitated by tetraphenylarsonium chloride in ethano1,'water as a lemon-yellow, crystalline salt, [As(CgH5)4] [W(CO),NCO], m.p. 157 O C (decamp.), soluble in ethanol, acetone, and tetrahydrofuran. This complex corresponds in analysis and infrared spectrum to the product obtainable from W(CO)6, azide, and A s ( C ~H ~) ~C ~
; conductivity and molecular-weight determinations indicate the salt to be extensively dissociated (Mexp. = 377). In accord with symmetry, the infrared spectrum of the anion [W(CO)5NCO]-(in acetone) contains three CO bands, at 2067CO bands, at (w), 1927(st)(st), and 1868(m) cm-1, and pseudoasymmetric and symmetric NCO stretching vibrations at 2236(m) and 1323(m) cm-. 1 141. The positions of the vaSNCO and 'J,NCO vibrations prove the anion [W(CO)jNCO]-to be an isocyanate complex containing a W-NCO bond. Reaction of W(CO)6 with azide probably occurs by nucleophilic attack of azide on a CO ligand, loss of nitrogen, and rearrangement to the stable isocyanate, analogously to the Curtius azide degradation. Several reactions of metal carbonyls involving nucleophilic attack on carbon monoxide bound in a complex have been discovered recentlyr5l. The first such reaction of W(CO)6 observed was that with carbanions [61.