Phosphaalkenes, Metallo-l,2-dihydrophosphetes, X-Ray The metallo-phosphaalkenes (/75-C5Me5)(CO)2M -P = C (N R 2)2 (4a,b, 5) result from the reac tion of the corresponding bromo compounds (/75-C5Me5)(CO)2MBr with Me3Si-P = C (N R 2)2. The complexes 4a and 5a are cleanly converted into the l-metallo-l,2-dihydrophosphetes (//5-C5Me5)(CO)2M -PCH(E)-C(E) = C -NM e2 (lO a -c) (M = Fe, Ru; E = C 0 2Me, CN) by treatment with equimolar amounts of dimethyl fumarate or fumarodinitrile, respectively. The products 4a,b, 5 and lO a -c have been characterized by elemental analysis as well as by spec troscopic data (IR, 'H, 13C, 3IP NM R, MS). The molecular structures o f 4a and 10a have been determined by single crystal X-ray analysis.

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Reaction of equimolar amounts of thiuronium salt [ (Me2N)2C-AS-C(NMe2)=C(C02Me)-&I(C02Me) (6). Compounds 3, SMe]I (1) with LiAs(SiMe& . 2 THF (2) afforded arsaalkene 4, and 6 were characterized by means of spectroscopy (IR, Me,SiAs=C(NMe,), (3). Combination of 3 with (qs-C5Me5)-'H, 13C(1H], 2ySi(1H] NMR and mass spectrometry). The mo-(CO),FeBr led to the formation of (qs-C5Me5)(C0)2-lecular structures of 4 and 6 were determined by X-ray dif-FeAs=C(NMe& (4). Reaction of the latter with dimethyl fraction analysis. fumarate gave the 1,2-dihydroarsete (q5-C5Me5)(CO),Fe-In contrast to the vast amount of information available on phosphaalkenes reports on the corresponding arsaalkenes have remained scarce so far. Accounts on arsaalkenes include species such as RAs=C (OSiMe3)(tBu) (R = alkyl, aryl, Me3Si)[2,3], MesAs=CPh2L4], PhAs=CH(NMe2)L5l, PhAs=C(tBu)N(R)SiMe, (R = Ph, p-T01)[~], Pr,SiAs=C-Ph2[7al, PhAs = C(Si-iPr3)Ph[7a], iPr3SiAs = cs1ozc-(=NRyN(R) (Is = 2,4,6-iPr3C6H2, R = Mes; c-C6H11)[7bl, CF3As=CF2[8], CF,AS=C(F)NR,[~], CF3As=C-(NR2)2 (R = Me, Et)L91, and the first metalloarsaalkenes (q'-CS€-15)(C0)2FeA~=C(OSiMe3)(R) (R = tBu, Mes, Mes*)[']. The gap of information is even more evident when the rich and well developed chemistry of phosphaalkynes['Ol is compared with that of Mes*C=As (Mes* = 2,4.6-fBu3C6H2), which to date is the only known kinetically stabilized and isolable arsaalkyne" ll. Results and DiscussionDuring the course of our studies of the chemistry of metalloarsaalkenes thiuronium salt [(Me2N)2CSMe]I (1)[12] was allowed to react with an equimolar amount of LiAs-(SiMe3)2 . 2 THF (2) [13] in the temperature range between -35 and 20"C, whereupon the color of the solution turned to orange. Orange Me3SiAs=C(NMe2)2 (3) was isolated as an oil in good yield after filtration and removal of solvent and A s ( S~M~~)~, which invariantly was formed as a byproduct. This approach to acyclic arsaalkenes is novel and parallels our synthesis of phosphaalkene Me,SiP=-C(NMe2)2 from 1 and LiP(SiMe3)2 . DME (DME = 1,2-dimetho~yethane)['~]. Arsaalkene 3 is sensitive to oxygen, moisture, and temperature. In keeping with this, attempted distillation of the crude product at a bath temperature of 65 "C and mbar was thwarted by decomposition. Solu-tions of 3 in A~( s i M e~)~, however, are surprisingly stable at 20°C in the absence of light. Upon combination of 3 with one equivalent of (q5-C5Me5)(CO)2FeBr in n-pentane a reaction occurred in the range between 0 and 20°C to give black microcrystalline metalloarsaalkene 4 in 44% yield. This condensation method has precedent in the synthesis of metallophosphaalkene ( T~' -C~M~~) ( C~)~F~-P = C ( N M~~)~ (5) from Me3-Sip= C(NMe2)2 and the bromoiron c0mplex['~1 and complements the only other route to such compounds involving (q5-C5H5)(C0)2FeAs(SiMe3)2 and carboxylic chlorides ['].( T~-C~M~~) ( C O )~F~-P = C ( N M~~)~ 5 Two singlets in the 'H-NMR spectrum of 3 at 6 = 0.47 and 2.64 are attributed to the protons of the trimethylsilyl and dimethylamino groups, respectively. In the '...

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