Water-soluble N,N-bis-(2-hydroxyethyl)-glycine/metal chelates represented by the formula: ##STR1## wherein R is H or alkyl (1-12C); M is Ti or Zr; k is a number in the range between 0 and 1, p is a number in the range between 1 and 2, and k+p=2 (in particular embodiments: k=1 and p=1; k=0 and p=2; and k=0.5 and p=1.5). The chelates can be used as cross-linking agents in hydraulic fracturing fluids and in gels that are used for selectively plugging permeable zones in subterranean formations or for plugging subterranean leaks.