the zwitterionic resonance formulas 2A and 2 C and best described by formula 2 B ([Nil = mi(CO)3]).
This type of coordination of a 1,3-diphosphacyclobutadiene to complex fragments is hitherto unparalleled and documents the influence of the donor substituents NR, on the coordinating properties of phosphaalkyne 1. Complex 2 forms so rapidly that not even IR and NMR spectroscopy can detect intermediates. Thus in principle several reaction paths are possible. Scheme 1 depicts the most probable .. f€ e m .. e [tFr,N-CEP-Ni(CO),] -[rFr,N=C=P-Ni(CO),] IA IB routes. Along route a, the mononuclear complex I postulated as primary product undergoes a [2 + 21-cycloaddition with another molecule of 1 to form the 1,3-diphosphacyclobutadiene derivative 11, which reacts with additional [Ni(CO),] to give the end product 2. We favor route b in which the zwitterion IB that best represents the bonding situation in I forms intermediate I11 with [Ni(CO),]. This intermediate reacts with another molecule of 1 to yield the complex 2. These proposals take account of the following 1 n rn 2 Scheme 1. Proposed routes for the formation of 2. reaction of 1, but workup introduced considerable losses. The isolable yield was only 46% Spectroscopic data of 2: IR (KBr), pentane: i (CO) = 2063 (w), 2046 (s), 1991 (vs, br.), 1986 (vs, br.); ' H N M R (300MHz, CDCI,, 2 5 T , TMS): 6 = 1.09(d,3J(H,H) = 6.50Hz,6H;CH3),1.33(d,'J(H,H) = 6.94Hz,6H;CH3), 3.29 (sept, 3J(H,H) = 6.94 Hz, 1 H; CH), 4.45 (dsept, '4H.H) = 6.50 Hz, 4J(P,H) = 2.68Hz, I H ; CH); "P NMR (36.44MHz, CDCI,, 25 "C, 85% H'PO,): 6 = 106.3 (d. 'J(P,P) = 9.0 Hz), 94.2 (d, 'J(P,P) = 9.0 Hz); I3C NMR (75.43 MHz, CDCI,, 25 "C, TMS): 6 = 19.9 (s; CH,), 20.4 (d, 4J(P,C) = 14.8Hz; CH,), 51.6 (s; CH), 56.7 (d, 'J(P,C) = 18.2 Hz; CH), 197 7 (d, *J(P,C) = 3.4 Hz; CO), 210.2 (dd, 'J(P,C) = 3.8 and 56.1 Hz, sp'-C).