I.B.M. introduced its least expensive work station and other new products yesterday in an effort to move more aggressively into the rapidly growing market for network computing systems for scientific and commercial users. With its new Powerstation 220 machine, the world's largest computer maker is playing catch-up at the low end of the work station market, analysts said. The low-cost, single-chip version of the RS/6000 work station family had been delayed for technical reasons and is still more expensive than similar machines from Sun Microsystems, Digital Equipment and Hewlett-Packard. Indeed, a black-and-white version of the machine without a disk drive is priced starting at $6,345. Similar low-end machines from Sun, Digital and Hewlett-Packard cost less than $5,000. But I.B.M. executives said the $9,995 price for a fully equipped color version of the Powerstation 220 was competitive with those of similar work stations from other companies.