Cover image: The structure of dihydrogen (yellow) bound to tungsten in W(H2)(CO)3[P(i-C3H7)3]2 (gray, C; red, O; magenta, P). Determined in 1984 by Kubas et al., this was the first well defined example of a stable three-center "sigma complex," the first step in breaking strong bonds such as H-H and C-H (lower equation). See the Perspective by Kubas on pages 6901–6907. Image courtesy of Gregory Kubas, Brian Scott, and Josh Smith (Los Alamos National Laboratory).