Intrinsic Bond Strengths of Multiple C−C, Si−Si, and C−Si Bonds Thanks are given to Prof. Y. Apeloig and Dr. M. Karni for the discussions. I would also like to thank Prof. H. Hopf and Prof. R. Herges for providing me with computing time.

Really a triple bond? According to quantum-chemical calculations of inverted Hessian matrices (compliance matrices), the strength of the C≡Si bond in I is only marginally stronger than the C=Si bond in silaethylene (II). Real-space force constants overestimate the C≡Si bond in I and should therefore be abandoned as bond strength descriptors.