wherein M is an alkali metal, e.g. lithium, sodium, or potassium, M??? is an alkaline earth metal such as Mg, Ca or Sr, each R represents an alkyl radical containing from 1 to 8 carbon atoms, each R??? radical represents a substituent selected from those consisting of alkyl radicals containing from 1 to 8 carbon atoms (i.e. R radicals) and aryl radicals of the benzene series containing from 6 to 9