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ampies of anti addition prior to 2 and others of its type. 16 The NMR data for 2 could have easily lead to the wrong assignment. It seems reasonable to conclude that the normal mode of attack by electrophilic acetylenes on tj'-QHs rings in stable metal complexes will be such as to afford the anti product, at least where other bulky ligands are coordinated to the metal.The structure of 2 provides major confirmation of the postulate15 of hyperconjugation in 1 whereby the electrons of the Pt-Cp bond are delocalized with those of the diene unit. This causes the Pt-Cp bond distance to be longer than the Pt-CH3 one. Since such delocalization is not possible for 2, the differences observed in the NMR and structural trans influences between the methyl and Cp groups of 1 should not be observed for the methyl and norbornadiene groups of 2. This is indeed the case, for the bond distances of these latter ligands to platinum are the same in 2. This was also evident in the NMR of 2, where, although different resonances for the vinyl protons trans to the two ligands were resolved, the /(PtH(vinyl)) coupling constants were identical.16 Com-paring the two structures, the Pt-CH3 distances in 1 and 2 are indistinguishable [1, 2.068 (8) and 2, 2.054 ( 14) Á; 0.9 ] whereas the other Pt-C(sp1 23 4) distances are quite different [Pt-Cp, 2.151 (8) and Pt-C7F6H5, 2.054 (10) A; 7.6er]. Finally, the COD ligand in 2 is twisted in a manner similar to that in 1, further substantiating the claim that the differences in the platinum-carbon(sp3) bonds in 1 are not due to steric effects. This comparison of 1 and 2 is satisfyingly consistent with the postulate of hyperconjugative effects in 1.Registry No. 1, 56200-09-0; 2, 56200-13-6; CF3C=CCF3, 692-50-2. Supplementary Material Available: Listing of structure amplitudes (8 pages). Ordering information is given on any current masthead page.

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The reactions of organoaluminum phosphides with a variety of transition-metal carbonyl complexes containing labile ligands have been investigated. The reaction of Cr(CO)sNMe3 with (Me3SiCH2)2AlPPh2 in benzene solution leads to the formation of Cr(CO)5[PPh2Al(CH2SiMe2)2-NMe3], a fully characterized new compound. An X-ray structural study has identified discrete isolated molecules of Cr(CO)5[PPh2Al(CH2SiMe3)2-NMe3], separated by normal van der Waals distances, in the monoclinic crystal space group P2i/n with a = 11.839 (4) Á, b = 18.517 (5) Á, c = 16.158 (4) Á, ß = 90.32 (2)°, and p(calcd) = 1.20 g/cm3 for Z = 4 with molecular weight 637.82. Diffraction data were collected with a Syntex P2, diffractometer, and the structure was refined to RF = 6.4% for all 4948 reflections. There are no abnormally short intermolecular contacts. The unusual features identified in the investigation are the long bond distances for Al-P of 2.485 (1) Á and Cr-P of 2.482 (1) Á. The Al-N bond seems to be normal. The geometry about the tetrahedrally coordinated phosphorus atom is decidedly irregular. Similarly, the aluminum atom has a rather distorted tetrahedral environment. The reaction of Cr(CO)5 [PPh2Al(CH2SiMe3)2-NMe2] with anhydrous HBr leads to the formation of Cr(CO)5PPh2H, Br3AlNMe3, and SiMe4. A likely path for this reaction involves the initial cleavage of the long P-Al bond. In attempts to find other preparative reactions to compounds with a CfcrP-Al bond sequence, the related reactions of Cr(CO)5L (L = CO, CH3CN, THF) with R2AlPPh2 (R = Me, Et) were studied but the desired compounds were not formed. Available data suggest that the labile ligand on chromium was attacked by the aluminum-phosphorus reagent.

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