Hexaisopropoxyditungsten and dodecaisopropoxytetratungsten: W2(O-i-Pr)6 and W4(O-i-Pr)12. 2. Studies of cluster dynamics and the equilibrium between the 12-electron cluster and two metal-metal triple bonds. A symmetry-allowed [.pi.s2 + .pi.s2] cycloaddition reaction and comparisons with the chemistry of cyclobutadiene