sigma-Bond Metathesis of Alkanes on a Silica-Supported Tantalum(V) Alkyl Alkylidene Complex: First Evidence for Alkane Cross-Metathesis We are grateful to Dr. L. Lefort (LCOMS) and Dr. B. Maunders (BP Chemicals) for fruitful discussions. We also wish to thank the C.N.R.S., C.P.E. Lyon, and BP Chemicals for financial support