Das Dimethylphosphanomethanid-Ion als Brückenligand - Struktur von μ-Dimethylphosphido, μ-Dimethylphosphanomethanidotetrakis(trimethylphosphan)dicobalt / The Dimethylphosphanomethanide Ion as a Bridging Ligand - Structure of μ-Dimethylphosphido, μ-Dimethylphosphanomethanidotetrakis(trimethylphosphane)dicobalt