Methyl–hydride metathesis between [Zr(cp)2Me2] and [HAl(μ3-NBut)]4: molecular structures of [Me1−xHxAl(μ 3-NBut)]4 (x = 0, 0.78 or 1) and [(cp)2ZrMe(μ-H)]2 (cp = η5-C5H5) - Simon Bott (Journal of the Chemical Society, Dalton Transactions, 1997)