ADDITION-REACTIONS OF THE UNSATURATED CLUSTER ANION [RE4(MU-H)3(MU-3-H)2(CO)12]- WITH CO, PPH3, AND MECN AND CHARACTERIZATION OF AN UNSTABLE ADDUCT WITH WATER - X-RAY CRYSTAL-STRUCTURE OF THE TETRAETHYLAMMONIUM SALT OF THE DERIVATIVE [RE4(MU-H)4(MU-3-H)(CO)12(PPH)3]-