Ligand backbone alkylation of the complex [Cr(CO)4(dppm)] (dppm = bis(diphenylphosphino)methane) with alkyl iodides yields the C-substituted dppm ligand complexes [Cr(CO)4{Ph2PCH(R)PPh2}] (R = methyl, n-hexyl, benzyl). Activation of these complexes via one-electron oxidation with Ag[(Al(OC4F9)4] and CO removal with triethylaluminium, or (in the case of R = methyl) by in situ treatment of the free ligand with a chromium salt and modified methyl alumoxane (MMAO), leads to catalysts showing some selectivity for ethylene trimerization and tetramerization. NMR spectroscopic studies of the parent dppm or [Cr(CO)4(dppm)] compounds suggest that ligand deprotonation and decomplexation may be the cause of the surprisingly poor catalytic performance of these specific derivatives.