The invention features protected linker compounds which comprise at one terminus a protected amino group and at another other terminus a phosphorous activating group, such as a phosphoramidite group. These protected linker compounds are introduced chemically at the 5′-end of oligonucleotides for the purpose of preparing 5′-amino modified oligonucleotides. After deprotection, the thereby introduced amino group then allows further modification (e.g. attachment of dyes) or immobilization (on surfaces or beads) of the oligonucleotide. Specifically, the presented amino protecting group is designed to provide such protected linker compounds in a solid form, which facilitates efficient purification by precipitation or crystallization and aliquoting for distribution and storage.