Synthesis and study of the (Ph3P)2Pt complexes of three members of a series of highly pyramidalized alkenes

Article abstract of DOI:10.1021/om00007a055

The (Ph₃P)₂Pt complexes (3) of three members of a homologous series 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 (Ph₃P)₂PtC₂H₄. The X-ray crystal structures of 3a-c show that pyramidalization angles at the carbons bonded to platinum increase from an average of φ = 48.3° in 3c to φ = 55.1° in 3b to 62.3° in 3a. In addition, as predicted computationally, the lengths of the bonds between the carbons attached to platinum increase and the C-Pt bond lengths decrease from 3c to 3a. The ¹H, ¹³C, ³¹P, and ¹⁹⁵Pt NMR spectra of 3a-c provide information about how pyramidalization at these carbons affects the electronic structures of the complexes. The changes in the ¹³C, ³¹P, and ¹⁹⁵Pt chemical shifts and also the changes in the ¹³C-³¹P and ³¹P-³¹P coupling constants are consistent with the expected increase in back-bonding from the HOMO of (Ph₃P)₂Pt into the empty, π* LUMO of the alkene as pyramidalization increases from 3c to 3a. The increase in the one-bond ¹³C-¹⁹⁵Pt coupling constant from 3c to 3a is found to be linearly related to the decrease in the ³¹P-¹⁹⁵Pt coupling constant. This finding indicates that, with increasing pyramidalization, donation from the π HOMO of the alkene into the 6s orbital of platinum also increases but at the expense of donation into platinum 6s from the in-phase combination of phosphine lone pair orbitals.