SYNTHESIS AND STUDY OF THE (PH(3)P)(2)PT COMPLEXES OF 3 MEMBERS OF A SERIES OF HIGHLY PYRAMIDALIZED ALKENES

The (Ph(3)P)(2)Pt complexes (3) of three members of a homologous serie s of tricyclo[3.3.n.0(3,7)]-alk-3(7)-enes (2a-c) have been prepared by allowing these highly pyramidalized alkenes to displace ethylene from (Ph(3)P)(2)PtC2H4. The X-ray crystal structures of 3a-c show that pyr amidalization angles at the carbons bonded to platinum increase from a n average of phi = 48.3 degrees in 3c to phi = 55.1 degrees in 3b to 6 2.3 degrees in 3a. In addition, as predicted computationally, the leng ths of the bonds between the carbons attached to platinum increase and the C-Pt bond lengths decrease from 3c to 3a. The H-1, C-13, P-31, an d Pt-195 NMR spectra of 3a-c provide information about how pyramidaliz ation at these carbons affects the electronic structures of the comple xes. The changes in the C-13, P-31, and Pt-195 chemical shifts and als o the changes in the C-13-P-31 and P-31-P-31 coupling constants are co nsistent with the expected increase in back-bonding from the HOMO of ( Ph(3)P)(2)Pt into the empty, pi LUMO of the alkene as pyramidalizatio n increases from 3e to 3a. The increase in the one-bond C-13-Pt-195 co upling constant from 3c to 3a is found to be linearly related to the d ecrease in the P-31-Pt-195 coupling constant. This finding indicates t hat, with increasing pyramidalization, donation from the pi HOMO of th e alkene into the 6s orbital of platinum also increases but at the exp ense of donation into platinum 6s from the in-phase combination of pho sphine lone pair orbitals.