One of the first examples of main group C-C activation was proposed to occur through a -alkyl migration mechanism. The thermolysis of tris(neo-pentyl)aluminium [Al{CH2C(Me)3}3] at high temperatures (200 °C) led to the reversible formation of iso-butene and trimethylaluminium, via sp 3 C-C bond activation. The release of iso-butene gas provides an entropic driving force for this reaction. A related reaction of the tris(neopentyl)stannyl cation [Sn{CH2C(Me)3}3] + has been proposed to occur during fragmentation in mass spectrometry measurements (Figure ). Related migration processes likely underpin rearrangements of strained polycyclic systems which incorporate main group elements. For example, the addition of aluminium nucleophiles to the -system of biphenylene and naphthalene creates strained bicyclic motifs that undergo skeletal rearrangements to break strong C-C bonds. Low-valent main group complexes have also been proposed to mediate C-C bond activation of arenes through a Büchner ring-expansion mechanism, this involves a stepwise [2+2] cycloaddition to form a nocaradiene-type intermediate followed by an electrocyclic ring-opening. Ring-opening of cyclopropanes by C-C bond activation has been reported using frustrated Lewis-pairs. 17 As part of an investigation of C-C bond activation at main group centres, we recently communicated an example of -alkyl migration at a Mg centre. The reaction of a -diketiminate stabilised magnesium hydride (1) with methylidene cyclopropane (3a) resulted in the formation of the ringopened but-4-en-1-ylmagnesium complex (Scheme 1). Here, we report a comprehensive investigation of this system. We expand the scope to a series of four-to six-membered methylidene cycloalkanes (3b-d) and show, for the first time, that analogous Zn complexes are capable of C-C bond activation.